Antenna Toolkit 2nd Edition
Joseph J. Carr, K4IPV
OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI
Newnes An imprint of Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Avenue, Woburn, MA 01801-2041 A division of Reed Educational and Professional Publishing Ltd A member of the Reed Elsevier plc group First published 1997 Reprinted 1998 Second edition 2001 ß Joseph J. Carr 1997, 2001 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Rd, London, England W1P 0LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publishers. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 4947 X
Typeset by Keyword Typesetting Services Ltd Printed in Great Britain by
1. Radio signals on the move 2. Antenna basics 19
3. Wire, connection, grounds, and all that 4. Marconi and other unbalanced antennas
Civilization, the thing that makes humans different from the other animals on this planet, is the ability to pass knowledge down from generation to generation with each successive generation building on the knowledge of all generations that came before. It is this human knowledgebase which has allowed us to progress to the point we are at today and it is this knowledgebase that is now under ...
49 69 87
5. Doublets, dipoles, and other Hertzian antennas 6. Limited space antennas 7. Large loop antennas 8. Wire array antennas 9. Small loop antennas 10. Yagi beam antennas 11. Impedance matching 118
129 153 176 195 203 221
12. Simple antenna instrumentation and measurements 13. Getting a ‘good ground’ 237 Index 249
If you are interested in amateur radio, short-wave listening, scanner monitoring, or any other radio hobby, then you will probably need to know a few things about radio antennas. This book is intended for the radio enthusiast – whether ham operator, listening hobbyist, or radio science observer – who wants to build and use antennas for their particular requirements and location. All of the antennas in this book can be made from wire, even though it is possible to use other materials if you desire. These antennas have several advantages. One of the most attractive is that they can provide decent performance on the cheap. As one who has lived through the experience of being broke, I learned early to use bits of scrap wire to get on the air. My ﬁrst novice antenna back in the late 1950s was a real patched-together job – but it worked really well (or so I thought at the time!).
Another advantage of wire antennas is that they are usually quite easy to install. A couple of elevated supports (tree, roof, mast), a few meters of wire, a few bits of radio hardware, and you are in the business of putting up an antenna. As long as you select a safe location, then you should have little difﬁculty erecting that antenna. Finally, most high-frequency (HF) short-wave antennas are really easy to get working properly. One does not need to be a rocket scientist – or professional antenna rigger – to make most of these antennas perform as well as possible with only a little effort. There is quite a bit of detailed technical material to digest if you want to be a professional antenna engineer, but you can have good results if you follow a few simple guidelines.
_________________________ _________________________ SOFTWARE SUPPLEMENT TO THIS BOOK _________________________
At the time this book was conceived it was noted that the technology now exists to make Microsoft Windows-based antenna software available to readers along with the book. The software can be used to calculate the dimensions of the elements of most of the antennas in this book, as well as a few that are not. There are also some graphics in the software that show you a little bit about antenna hardware, antenna construction, and the like.
One might say that the business world is moving at a fast pace, a pace that is almost impossible to keep up with. This could be the result of the almighty Internet and the world of technology. In today s society, technology has revolutionized how we as consumers make decisions and procedures regarding the process of consumption. The Internet has made many vast improvements within our culture, both ...
_______________________________________________________ _______________________________________________________ ANTENNA SAFETY _______________________________________________________
Every time I write about antenna construction I talk a little bit about safety. The issue never seems too old or too stale. The reason is that there seem to be plenty of people out there who never get the word. Antenna erection does not have to be dangerous, but if you do it wrong it can be very hazardous. Antennas are deceptive because they are usually quite lightweight, and can easily be lifted. I have no trouble lifting my trap vertical and holding it aloft – on a windless day. But if even a little wind is blowing (and it almost always is), then the ‘sail area’ of the antenna makes it a lot ‘heavier’ (or so it seems).
Always use a buddy-system when erecting antennas. I have a bad back caused by not following my own advice. Another issue is electrical safety. Do not ever, ever, ever toss an antenna wire over the power lines. Ever. Period. Also, whatever type of antenna you put up, make sure that it is in a location where it cannot possibly fall over and hit the power line. The last issue is to be careful when digging to lay down radials. You really do not want to hit water lines, sewer lines, buried electrical service lines, or gas lines. I even know of one property where a long-distance oil pipeline runs beneath the surface. If you do not know where these lines are, try to guess by looking at the locations of the meters on the street, and the service entrance at the house. Hint: most surveyers’ plans (those map-like papers you get at settlement) show the location of the buried services. They should also be on maps held by the local government (although you might have to go to two or three ofﬁces!. The utility companies can also help.
__________________________ __________________________ A NOTE ABOUT UNITS AND PRACTICES __________________________
This book was written for an international readership, even though I am American. As a result, some of the material is written in terms of US standard practice. Wherever possible, I have included UK standard wire sizes and metric units. Metric units are not in common usage in the USA, but rather we still use the old English system of feet, yards, and inches. Although many Americans (including myself) wish the USA would convert to SI units, it is not likely in the near future. UK readers with a sense of
The electromagnetic spectrum is made up of six different types of waves. Radio waves, Microwaves, Infrared waves, Visible light, Ultravioletlight, X-rays and Gamma rays. The radio waves are used to transmit radio and television signals. The infrared waves are used to tell temperature of areas. Visible light is all the colors that we can see. Ultravioletlight can help things grow but to much can ...
viii ANTENNA TOOLKIT
history might recognize why this might be true – as you may recall from the George III unpleasantness, Americans do not like foreign rulers, so it is not likely that our measuring rulers will be marked in centimeters rather than inches.* For those who have not yet mastered the intricacies of converting between the two systems: 1 inch ¼ 2:54 centimeters (cm) ¼ 25:4 millimeters (mm) 1 foot ¼ 30:48 cm ¼ 0:3048 meter (m) 1m ¼ 39:37 inches ¼ 3:28 feet Joseph J. Carr
*I apologize for the bad play on words, but I could not help it.
Radio signals on the move
Anyone who does any listening to radio receivers at all – whether as a ham operator, a short-wave listener, or scanner enthusiast – notices rather quickly that radio signal propagation varies with time and something mysterious usually called ‘conditions.’ The rules of radio signal propagation are well known (the general outlines were understood in the late 1920s), and some predictions can be made (at least in general terms).
Listen to almost any band, and propagation changes can be seen. Today, one can ﬁnd propagation predictions in radio magazines, or make them yourself using any of several computer programs offered in radio magazine advertisements. Two very popular programs are any of several versions of IONCAP, and a Microsoft Windows program written by the Voice of America engineering staff called VOACAP. Some odd things occur on the air. For example, one of my favorite local AM broadcast stations broadcasts on 630 kHz. During the day, I get interference-free reception. But after the Sun goes down, the situation changes radically. Even though the station transmits the same power level, it fades into the background din as stations to the west and south of us start skipping into my area. The desired station still operates at the same power level, but is barely audible even though it is only 20 miles (30 km) away. Another easily seen example is the 3–30 MHz short-wave bands. Indeed, even those bands behave very differently from one another. The lowerfrequency bands are basically ground wave bands during the day, and become long-distance ‘sky wave’ bands at night (similar to the AM broadcast band (BCB)).
The S-band portion of the microwave band of the electromagnetic spectrum ranges from 2.0 to 4.0 GHz., crossing the boundary between UHF and SHF at 3.0 GHz. The S-band wavelength is around 10 cm. The maritime industry shares S-band radar with other entities permitted by the Federal Communications Commission (FCC) in this country and by the International Telecommunication Union (ITU) worldwide; and ...
Higher short-wave bands act just the opposite: during the
day they are long-distance ‘skip’ bands, but some time after sunset, become ground wave bands only. The very high-frequency/ultra high-frequency (VHF/UHF) scanner bands are somewhat more consistent than the lower-frequency bands. But even in those bands sporadic-E skip, meteor scatter, and a number of other phenomena cause propagation anomalies. In the scanner bands there are summer and winter differences in heavily vegetated regions that are attributed to the absorptive properties of the foliage. I believe I experienced that phenomenon using my 2 m ham radio rig in the simplex mode (repeater operation can obscure the effect due to antenna and location height).
___________________________________________ ___________________________________________ THE EARTH’S ATMOSPHERE ____________________________________________
Electromagnetic waves do not need an atmosphere in order to propagate, as you will undoubtedly realize from the fact that space vehicles can transmit radio signals back to Earth in a near vacuum. But when a radio wave does propagate in the Earth’s atmosphere, it interacts with the atmosphere, and its path of propagation is altered. A number of factors affect the interaction, but it is possible to break the atmosphere into several different regions according to their respective effects on radio signals. The atmosphere, which consists largely of oxygen (O2) and nitrogen (N2) gases, is broken into three major zones: the troposphere, stratosphere, and ionosphere (Figure 1.1).
The boundaries between these regions are not very well deﬁned, and change both diurnally (i.e. over the course of a day) and seasonally. The troposphere occupies the space between the Earth’s surface and an altitude of 6–11 km. The temperature of the air in the troposphere varies with altitude, becoming considerably lower at high altitude compared with ground temperature. For example, a þ108C surface temperature could reduce to À558C at the upper edges of the troposphere. The stratosphere begins at the upper boundary of the troposphere (6–11 km), and extends up to the ionosphere (%50 km).
Disclaimer The views expressed in this academic research paper are those of the authors and do not reflect the official policy or position of the US Government or the Department of Defense... iii Preface Interest in developing an asteroid defense system, intensified by the impact of comet Shoemaker-Levy 9 with Jupiter in 1994, continues to grow by leaps and bounds. Many major US publications such ...
The stratosphere is called an isothermal region because the temperature in this region is relatively constant despite altitude changes. The ionosphere begins at an altitude of about 50 km and extends up to 500 km or so. The ionosphere is a region of very thin atmosphere. Cosmic rays, electromagnetic radiation of various types (including ultraviolet light from the Sun), and atomic particle radiation from space (most of it from the Sun), has sufﬁcient energy to strip electrons away from the gas molecules of the atmosphere. The O2 and N2 molecules that lost electrons are called positive ions. Because the density of the air is so low at those altitudes, the ions and electrons can travel long distances before neutralizing each other
2 ANTENNA TOOLKIT
by recombining. Radio propagation on some bands varies markedly between daytime and night-time because the Sun keeps the level of ionization high during daylight hours, but the ionization begins to fall off rapidly after sunset, altering the radio propagation characteristics after dark. The ionization does not occur at lower altitudes because the air density is such that the positive ions and free electrons are numerous and close together, so recombination occurs rapidly.
RADIO SIGNALS ON THE MOVE 3
__________________________________________________ _________________________________________________ PROPAGATION PATHS __________________________________________________
There are four major propagation paths: surface wave, space wave, tropospheric, and ionospheric. The ionospheric path is important to medium-wave and HF propagation, but is not important to VHF, UHF, or microwave propagation. The space wave and surface wave are both ground waves, but behave differently. The surface wave travels in direct contact with the Earth’s surface, and it suffers a severe frequency-dependent attenuation due to absorption into the ground. The space wave is also a ground wave phenomenon, but is radiated from an antenna many wavelengths above the surface. No part of the space wave normally travels in contact with the surface; VHF, UHF, and microwave signals are usually space waves. There are, however, two components of the space wave in many cases: direct and reﬂected (Figure 1.2).
So What is an El Ni~no, Anyway? A non-technical description An El Ni~no is a temporary change in the climate of the Pacific ocean, in the region around the equator. You can see its effects in both the ocean and atmosphere, generally in Northern Hemisphere winter. Typically, the ocean surface warms up by a few degrees celsius. At the same time, the place where hefty thunderstorms occur on the ...
The ionosphere is the region of the Earth’s atmosphere that is between the stratosphere and outer space. The peculiar feature of the ionosphere is that molecules of atmospheric gases (O2 and N2) can be ionized by stripping away electrons under the inﬂuence of solar radiation and certain other sources of energy (see Figure 1.1).
In the ionosphere the air density is so low that positive ions can travel relatively long distances before recombining with electrons to form electrically neutral atoms. As a result, the ionosphere remains ionized for long periods of the day – even after sunset. At lower altitudes, however, air density is greater, and recombination thus occurs rapidly. At those altitudes, solar ionization diminishes to nearly zero imme-
4 ANTENNA TOOLKIT
diately after sunset or never achieves any signiﬁcant levels even at local noon. Ionization and recombination phenomena in the ionosphere add to the noise level experienced at VHF, UHF, and microwave frequencies. The properties of the ionosphere are therefore important at these frequencies because of the noise contribution. In addition, in satellite communications there are some transionospheric effects.
____________________________________ ___________________________________ GROUND WAVE PROPAGATION ____________________________________
The ground wave, naturally enough, travels along the ground, or at least in close proximity to it (Figure 1.3).
There are two basic forms of ground wave: space wave and surface wave. The space wave does not actually touch the ground. As a result, space wave attenuation with distance in clear weather is about the same as in free space (except above about 10 GHz, where absorption by H2O and O2 increases dramatically).
Of course, above the VHF region, weather conditions add attenuation not found in outer space. The surface wave is subject to the same attenuation factors as the space wave, but in addition it also suffers ground losses. These losses are due to ohmic resistive losses in the conductive earth. Surface wave attenuation is a function of frequency, and increases rapidly as frequency increases. For both of these forms of ground wave, communications is affected by the following factors: wavelength, height of both receive and transmit antennas, distance between antennas, and terrain and weather along the transmission path. Ground wave communications also suffer another difﬁculty, especially at VHF, UHF, and microwave frequencies. The space wave is like a surface wave, but is radiated many wavelengths above the surface. It is made up of
RADIO SIGNALS ON THE MOVE 5
two components (see Figure 1.2): direct and reﬂected waves. If both of these components arrive at the receive antenna they will add algebraically to either increase or decrease signal strength. There is nearly always a phase shift between the two components because the two signal paths have different lengths. In addition, there may be a 1808 ( radians) phase reversal at the point of reﬂection (especially if the incident signal is horizontally polarized).
Multipath phenomena exist because of interference between the direct and reﬂected components of the space wave. The form of multipath phenomenon that is, perhaps, most familiar to many readers (at least those old enough to be ‘pre-cable’) is ghosting in television reception. Some multipath events are transitory in nature (as when an aircraft ﬂies through the transmission path), while others are permanent (as when a large building or hill reﬂects the signal).
In mobile communications, multipath phenomena are responsible for reception dead zones and ‘picket fencing.’ A dead zone exists when destructive interference between direct and reﬂected (or multiple reﬂected) waves drastically reduces signal strengths. This problem is most often noticed at VHF and above when the vehicle is stopped; and the solution is to move the antenna a quarter wavelength. Picket fencing occurs as a mobile unit moves through successive dead zones and signal enhancement (or normal) zones, and sounds like a series of short noise bursts. At VHF, UHF, and microwave frequencies the space wave is limited to so-called ‘line of sight’ distances. The horizon is theoretically the limit of communications distance, but the radio horizon is actually about 15% further than the optical horizon. This phenomenon is due to refractive bending in the atmosphere around the curvature of the Earth, and makes the geometry of the situation look as if the Earth’s radius is four-thirds the actual radius. The surface wave travels in direct contact with the Earth’s surface, and it suffers a severe frequency-dependent attenuation due to absorption by the ground (Figure 1.3).
The zone between the end of the ground wave and where the sky wave touches down is called the skip zone, and is a region of little or no signal. Because of this phenomenon, I have seen situations on the 15 m band (21.390 MHz) where two stations 65 km apart (Baltimore, Maryland, and Fairfax, Virginia) could not hear each other, and their communications have to be relayed via a ham station in Lima, Peru! The surface wave extends to considerable heights above the ground level, although its intensity drops off rapidly at the upper end. The surface wave is subject to the same attenuation factors as the space wave, but in addition it also suffers ground losses. These losses are due to ohmic resistive losses in the conductive earth, and to the dielectric properties of the Earth. Horizontally polarized waves are not often used for surface wave communications because the Earth tends to short circuit the electrical (E) ﬁeld
6 ANTENNA TOOLKIT
component. On vertically polarized waves, however, the Earth offers electrical resistance to the E-ﬁeld and returns currents to following waves. The conductivity of the soil determines how much energy is returned.
_______________________________________ _______________________________________ IONOSPHERIC PROPAGATION _______________________________________
Now let us turn our attention to the phenomena of skip communications as seen in the short-wave bands, plus portions of the medium-wave and lower VHF regions. Ionospheric propagation is responsible for intercontinental broadcasting and communications. Long-distance radio transmission is carried out on the HF bands (3–30 MHz), also called the ‘short-wave’ bands. These frequencies are used because of the phenomenon called skip. Under this type of propagation the Earth’s ionosphere acts as if it is a ‘radio mirror,’ to reﬂect the signal back to Earth. This signal is called the sky wave. Although the actual phenomenon is based on refraction (not reﬂection, as is frequently believed) the appearance to the casual ground observer is that short-wave and low-VHF radio signals are reﬂected from the ionosphere as if it were a kind of radio mirror. The actual situation is a little different, but we will deal with that issue in a moment. The key lies in the fact that a seeming radio mirror is produced by ionization of the upper atmosphere. The upper portion of the atmosphere is called the ‘ionosphere’ because it tends to be easily ionized by solar and cosmic radiation phenomena. The reason for the ease with which that region (50–500 km above the surface) ionizes is that the air density is very low. Energy from the Sun strips away electrons from the outer shells of oxygen and nitrogen molecules, forming free electrons and positive ions. Because the air is so rariﬁed at those altitudes, these charged particles can travel great distances before recombining to form electrically neutral atoms again. As a result, the average ionization level remains high in that region. Several sources of energy will cause ionization of the upper atomosphere. Cosmic radiation from outer space causes some degree of ionization, but the majority of ionization is caused by solar energy. The role of cosmic radiation was ﬁrst noticed during World War II when military radar operators discovered that the distance at which their equipment could detect enemy aircraft was dependent upon whether or not the Milky Way was above the horizon (although it was theorized 10 years earlier).
Intergalactic radiation raised the background microwave noise level, thereby adversely affecting the signal-to-noise ratio. The ionosphere is divided for purposes of radio propagation studies into various layers that have different properties. These layers are only well deﬁned in textbooks, however, and even there we ﬁnd a variation in the height above the Earth’s surface where these layers are found. In addition,
RADIO SIGNALS ON THE MOVE 7
the real physical situation is such that layers do not have sharply deﬁned boundaries, but rather fade one into another. The division into layers is therefore somewhat arbitrary. These layers (shown earlier in Figure 1.1) are designated D, E, and F (with F being further subdivided into the F1 and F2 sublayers).
The D-layer is the lowest layer in the ionosphere, and exists from approximately 50 to 90 km above the Earth’s surface. This layer is not ionized as much as higher layers because all forms of solar energy that cause ionization are severely attenuated by the higher layers above the D-layer. Another reason is that the D-layer is much denser than the E- and F-layers, and that density of air molecules allows ions and electrons to recombine to form electroneutral atoms very quickly. The extent of D-layer ionization is roughly proportional to the height of the Sun above the horizon, so will achieve maximum intensity at midday. The D-layer exists mostly during the warmer months of the year because of both greater height of the sun above the horizon and the longer hours of daylight. The D-layer almost completely disappears after local sunset, although some observers have reported sporadic incidents of D-layer activity for a considerable time past sunset. The D-layer exhibits a large amount of absorption of medium-wave and short-wave signals, to such an extent that signals below 4–6 MHz are completely absorbed by the D-layer.
The E-layer exists at altitudes between approximately 100 and 125 km. Instead of acting as an attenuator it acts primarily as a reﬂector although signals do undergo a degree of attenuation. Like the D-layer, ionization in this region only exists during daylight hours, peaking around midday and falling rapidly after sunset. After nightfall the layer virtually disappears although there is some residual ionization there during the night-time hours. The distance that is generally accepted to be maximum that can be achieved using E-layer propogation is 2500 km, although it is generally much less than this and can be as little as 200 km. One interesting and exciting aspect of this region is a phenomenon called Es or sporadic E. When this occurs a layer or cloud of very intense ionization forms. This can reﬂect signals well into the VHH region of the radio spectrum. Although generally short lived, there can be openings on bands as high as 2 meters (144 MHz).
These may last as little as a few minutes, whilst long openings may last up to a couple of hours. The phenomenon also affects lower frequencies like the 10 meter and 6 meter amateur bands as
8 ANTENNA TOOLKIT
well as the VHF FM band. Sporadic E is most common in the summer months, peaking in June (in the northern hemisphere).
Distances of between 1000 and 2500 km can be reached using this mode of propagation.
The F-layer of the ionosphere is the region that is the principal cause of long-distance short-wave communications. This layer is located from about 150–500 km above the Earth’s surface. Unlike the lower layers, the air density in the F-layer is low enough that ionization levels remain high all day, and decay slowly after local sunset. Minimum levels are reached just prior to local sunrise. Propagation in the F-layer is capable of skip distances up to 4000 km in a single hop. During the day there are actually two identiﬁable, distinct sublayers in the F-layer region, and these are designated the ‘Fl’ and ‘F2’ layers. The F1 layer is found approximately 150–250 km above the Earth’s surface, while the F2 layer is above the F1 to the 450–500 km limit. Beginning at local sundown, however, the lower regions of the F1 layer begin to de-ionize due to recombination of positive ions and free electrons. At some time after local sunset the F1 and F2 layers have effectively merged to become a single reduced layer beginning at about 300 km. The height and degree of ionization of the F2 layer varies over the course of the day, with the season of the year, and with the 27 day cycle of the sun. The F2 layer begins to form shortly after local sunrise, and reaches a maximum shortly before noon. During the afternoon the F2 layer ionization begins to decay in an exponential manner until, for purposes of radio propagation, it disappears sometime after local sunset. There is some evidence that ionization in the F-layer does not completely disappear, but its importance to HF radio communication does disappear.
_______________ ______________ IONOSPHERIC VARIATION AND DISTURBANCES _______________
The ionosphere is an extremely dynamic region of the atmosphere, especially from a radio operator’s point of view, for it signiﬁcantly alters radio propagation. The dynamics of the ionosphere are conveniently divided into two general classes: regular variation and disturbances. We will now look at both types of ionospheric change.
There are several different forms of variation seen on a regular basis in the ionosphere: diurnal, 27 day (monthly), seasonal, and 11 year cycle.
RADIO SIGNALS ON THE MOVE 9
Diurnal (daily) variation
The Sun rises and falls in a 24 hour cycle, and because it is a principal source of ionization of the upper atmosphere, one can expect diurnal variation. During daylight hours the E- and D-levels exist, but these disappear at night. The height of the F2 layer increases until midday, and then decreases until evening, when it disappears or merges with other layers. As a result of higher absorption in the E- and D-layers, lower frequencies are not useful during daylight hours. On the other hand, the F-layers reﬂect higher frequencies during the day. In the 1–30 MHz region, higher frequencies (>11 MHz) are used during daylight hours and lower frequencies ( 1
Figure 1.8 shows a case where super-refraction occurs. The value of K can be substantially above one in cases where a hot body of land occurs next to a relatively cool body of water. This occurs off Baja, California, the Persian Gulf and Indian Ocean, parts of Australia and the North African coast. In those areas, there may be substantial amounts of super-refraction occurring, making directional antennas point in the wrong direction. Figure 1.9 shows a case of subrefraction. In this case, there is a relatively cold land mass next to relatively warm seas. In the Arctic and Antarctic this situation exists. The K factor will be substantially less than one in these cases. In fact, the signal may be lost to terrestrial communications after only a relatively short distance.
RADIO SIGNALS ON THE MOVE 17
Before looking at the various antennas we need to look at some of the basics of antenna systems. In this chapter you will learn some of these basics. And while they will not make you a red-hot professional antenna engineer they will set you up well enough to understand this book and others on amateur and hobbyist antennas. We will look at the matter of antenna radiation, antenna patterns, the symbols used to represent antennas, voltage standing wave ratio (VSWR), impedance, and various methods suitable for constructing wire antennas in the high-frequency (HF) and very high-frequency (VHF) regions of the spectrum.
________________________________________ ________________________________________ ANTENNA SYSTEM SYMBOLS _________________________________________
Figures 2.1 and 2.2 show the various symbols used to represent antennas and grounds. The reason why there are so many variants is that there are differences from country to country, as well as different practices within any one country (especially between technical publishers).
As for antenna symbols, I see the symbol in Figure 2.1C more often in the USA, but Figure 2.1B comes in for a close second. The supposedly correct symbol (endorsed by a professional society drawing standards committee) is that of Figure 2.1A – but it is only occasionally seen in the USA. The situation for grounds is a little different because some differences reﬂect different forms of ground (although some of the differences also represent national or publisher differences).
The ground in Figure 2.2A is usually found representing a true earth ground, i.e. the wire is connected to a
rod driven into the earth. The variant of Figure 2.2B usually represents a chassis ground inside a piece of equipment. The symbol in Figure 2.2C has two uses. One is to represent a common grounding point for different signals or different pieces of equipment. The other use is exactly the opposite: the triangle ground symbol often represents an isolated ground that has no direct electrical connection to the rest of the circuit, or with the earth. You will see this usage in medical devices. The grounds of Figures 2.2D and 2.2E are found mostly outside the USA. From the time of Hertz and Marconi to the present, one thing has remained constant in wireless communications: radio waves travel, as if by magic, from a transmitting antenna to a receiving antenna (Figure 2.3).
Whether the two antennas are across the garden from each other, across continents and oceans, or on the Earth and the Moon, if there is not a transmitting antenna and at least one receiving antenna in the system then no communications can take place. At one time, physicists believed that there must be some invisible medium for carrying the radio signal. But we now know that no such medium exists, yet radio waves travel even in outer space. Being electromagnetic waves, radio signals need no medium in order to propagate. If radio signals traveled only in the Earth’s atmosphere, then we could make some guesses about a medium for carrying the wave, but space communications demonstrates that the atmosphere is not necessary (although it does affect radio signal propagation).
20 ANTENNA TOOLKIT
Although there is no medium in which radio waves travel, it is useful to look at water waves for an analogy (even though imperfect).
In Figure 2.4 we see what happens when an object is dropped into a pool of water. A displacement takes place, which forms a leading wave that pushes out in concentric circles from the impact point. The situation in Figure 2.4 repre-
ANTENNA BASICS 21
sents a single pulse of energy, as if a transmitter ﬁred a single burst of energy. Real transmitters send out wave trains that are analogous to cyclically bobbing the object up and down so that it goes in and out of the water (Figure 2.5).
The result is a continuous stream of identical waves propagating out from the ‘transmitter’ impact point. If another object is ﬂoating on the surface, say a cork or toy boat, then it will be perturbed as the wave passes. This is analogous to the receiver antenna. The waves have an amplitude (‘A’), which corresponds to the signal strength. They also have a wavelength (j), which corresponds to the distance traveled by the wave in one complete up-and-down cycle. In radio work, the wavelength is measured in meters (m), except in the microwave region where centimeters (cm) and millimeters (mm) make more sense. Wavelength can be measured at any pair of points on the wave that are identical: two peaks, two troughs, two zero crossings, as convenient in any speciﬁc case. The number of cycles that pass a given point every second is the frequency of the wave. The classic measure of frequency was cycles per second (cps or c/s), but that was changed in 1960 by international consensus to the hertz (Hz), in honor of Heinrich Hertz. But since 1 Hz = 1 cps, there is no
22 ANTENNA TOOLKIT
practical difference. The hertz is too small a unit for most radio work (although many of our equations are written in terms of hertz).
For radio work the kilohertz (kHz) and megahertz (MHz) are used: 1 kHz ¼ 1000 Hz, and 1 MHz ¼ 1 000 000 Hz. Thus, a short-wave frequency of 9.75 MHz is 9750 kHz and 9 750 000 Hz. Wavelength and frequency are related to each other. The wavelength is the reciprocal of frequency, and vice versa, through the velocity constant. In free space, the velocity constant is the speed of light (c), or about 300 000 000 m/s. This is the reason why you often see ‘300’ or its submultiples (150 and 75) in equations. When the frequency is speciﬁed in megahertz, then 300 000 000 becomes 300 for one wavelength. The half-wavelength constant is 150, and the quarter-wavelength constant is 75. The relationship is jmeters ¼ 300 FMHz
_________________________________________________ ________________________________________________ INVERSE SQUARE LAW _________________________________________________
When radio waves travel they become weaker by a relationship called the inverse square law. This means that the strength is inversely proportional to the square of the distance traveled (1=D2 ).
Figure 2.6 shows how this works using the analogy of a candle. If the candle projects a distance r, all of the light energy falls onto square ‘A’. At twice the distance ð2rÞ the light spreads out and covers four times the area (square ‘B’).
The total amount of light energy is the same, but the energy per unit of area is reduced to one-fourth of the energy that was measured at ‘A.’ This means that a radio signal gets weaker very rapidly as the distance from the transmitter increases, requiring ever more sensitive receivers and better antennas.
ANTENNA BASICS 23
_____________________________________ _____________________________________ THE ELECTROMAGNETIC WAVE ______________________________________
The electromagnetic (EM) wave propagating in space is what we know as a ‘radio signal.’ The EM wave is launched when an electrical current oscillates in the transmitting antenna (Figure 2.7).
Because moving electrical currents possess both electrical (E) and magnetic (H) ﬁelds, the electromagnetic wave launched into space has alternating E-ﬁeld and H-ﬁeld components. These ﬁelds are transverse (meaning they travel in the same direction) and orthogonal (meaning the E- and H-ﬁelds are at right angles to each other).
When the EM wave intercepts the receiver antenna, it sets up a copy of the original oscillating currents in the antenna, and these currents are what the receiver circuitry senses. The orthogonal E- and H-ﬁelds are important to the antenna designer. If you could look directly at an oncoming EM wave, you would see a plane front advancing from the transmitting antenna. If you had some magical dye that would render the E-ﬁeld and H-ﬁeld line of force vectors visible to the naked eye, then you would see the E-ﬁeld pointing in one direction, and the H-ﬁeld in a direction 908 away (Figure 2.8).
The polarization of the signal is the direction of the E-ﬁeld vector. In Figure 2.8 the polarization is vertical because the electric ﬁeld vector is up and down. If the E-ﬁeld vector were side-to-side, then the polarization would be horizontal. One way to tell which polarization an antenna produces when it transmits, or is most sensitive to when it receives, is to note the direction of the radiator element. If the radiator element is vertical, i.e. perpendicular to the Earth’s surface, then it is vertically polarized. But if
24 ANTENNA TOOLKIT
the radiator element is horizontal with respect to the Earth’s surface, then it is horizontally polarized. Figure 2.9 shows these relationships. In Figure 2.9, two dipole receiver antennas are shown, one is vertically polarized (VD) and the other is horizontally polarized (HD).
In Figure 2.9A, the arriving signal is vertically polarized. Because the E-ﬁeld vectors lines are vertical, they cut across more of the VD antenna than the HD, producing a considerably larger signal level. The opposite is seen in Figure 2.9B. Here the E-ﬁeld is horizontally polarized, so it is the HD antenna that receives the most signal. The signal level difference can be as much as 20 dB, which represents a 10-fold decrease in signal strength if the wrong antenna is used.
______________________________________________________________ ______________________________________________________________ DECIBELS (dB) ______________________________________________________________
In the section above the term decibel (symbol ‘dB’) was introduced. The decibel is a unit of measure of the ratio of two signals: two voltages, two currents, or two powers. The equations for decibels take the logarithm of the ratio, and multiply it by a constant (10 for powers and 20 for voltages or currents).
The use of decibel notation makes it possible to use ratios, such as found in gains and losses in electronic circuits, but use only addition and subtraction arithmetic. It is not necessary to be able to calculate decibels, but you should know that +dB represents a gain, and ÀdB represents a loss. The term ‘0 dB’ means that the ratio of the two signals is 1:1 (neither gain nor loss).
Some common ratios encountered in radio work include those listed in Table 2.1. You can see that doubling a signal strength results in a +3 dB gain, while halving it produces a À3 dB loss. To put these ﬁgures into perspective, most S-meters on receivers use a scaling factor of 6 dB per S-unit (some use 3 dB per S-unit).
Receiver designers tell us that a signal-to-noise ratio of 10 dB is necessary for ‘comfortable listening,’ while a signal-to-noise ratio of 3 dB is
ANTENNA BASICS 25
TABLE 2.1 Ratio 1:1 2:1 1:2 10:1 1:10 Power (dB) 0 þ3 À3 þ10 À10 Voltage (dB) 0 þ6 À6 þ20 À20
26 ANTENNA TOOLKIT
needed for barely perceptible but reliable communication for a listener who tries hard to hear what is being said.
__________________________________________________ __________________________________________________ LAW OF RECIPROCITY ___________________________________________________
Radio antennas obey a kind of law of reciprocity, i.e. they work the same on transmit as they do on receive. If an antenna has a certain gain and directivity on transmit, then that exact same pattern is seen in the receive mode. Similarly, the feedpoint impedance, element lengths, spacings, and other issues are the same for both modes. An implication of this law is that receiver owners are able to translate the theory from discussions of transmitting antennas to their own needs (and vice versa).
Reciprocity does not necessarily mean that the same antennas are the best selection for both transmit and receive. For example, the use of gain on a transmit antenna is to increase the apparent signal power level at a distant point in a particular direction. Alternatively, some licensing authorities use antenna directivity to protect the coverage areas of relatively nearby stations that share the same or adjacent frequencies. To the receiver operator, there may be a reason to want the gain of the antenna to boost the received signal power to a useful level. There is, however, often a powerful argument for aiming an antenna such that the main sensitivity is not in the direction of the desired station. Instead, the receiver antenna owner may wish to position a null, i.e. the least sensitive aspect of the antenna, in the direction of an offending station in order to reduce its effect. The issue is, after all is said and done, the signal-to-noise ratio. Another restriction is that there are several antennas that are ﬁt for receive use, especially those for difﬁcult installation situations, but are either technically unsuited or unsafe for transmitters except at the lowest of power levels. The issue might be impedance, VSWR, voltage arcing, or some other undesirable factor that occurs in transmit situations (even at moderate power levels such as 50–100 W).
_________________________________________________________ _________________________________________________________ ANTENNA TYPES __________________________________________________________
Most antennas fall into either of two broad categories: Marconi or Hertzian. The Marconi antennas (Figure 2.10) are considered unbalanced with respect to ground because one side of the signal source (or receiver input load) is grounded. The Marconi antenna can be either vertical (Figure 2.10A) or at some angle (Figure 2.10B), including horizontal. The name derives from the types of antenna used by Guglielmo Marconi in his early radio experiments. In his famous 1903 transatlantic communication, ol’ ‘Gug’ used a single wire tethered to a kite aloft above the Newfoundland coast.
ANTENNA BASICS 27
The Hertzian antennas (Figure 2.11) are balanced with respect to the ground, i.e. neither side of the oscillator or receiver load is grounded. Dipoles fall into this category. Both Marconi and Hertzian antennas can be either vertically or horizontally polarized. However, most Marconi antennas are designed for vertical polarization and most Hertzian antennas for horizontal polarization. The long wire and vertical dipoles are obvious exceptions to that rule, however.
______________________________ ______________________________ RECEIVER–ANTENNA INTERACTIONS ______________________________
The antenna and the receiver (or the antenna and transmitter on the other end) form a system that must be used together. Either alone is not too useful. Figure 2.12 shows an antenna connected to a receiver through a transmission line. For our practical purpose here, the receiver looks to the
28 ANTENNA TOOLKIT
antenna and transmission line like a load resistor, RIN. On the positive halfcycle (Figure 2.12A), the passing wave creates a current in the antenna– receiver system that ﬂows in one direction (shown here as ‘up’).
When the oscillation reverses and becomes negative, the direction of current ﬂow in the antenna–receiver reverses (Figure 2.12B).
This is the mechanism by which the oscillating electromagnetic wave reproduces a signal in the input of the receiver – the signal that is then ampliﬁed and demodulated to recover whatever passes for ‘intelligence’ riding on the signal.
______________________________________________ ______________________________________________ STANDING WAVE RATIO ______________________________________________
The issue of the standing wave ratio (SWR) is of constant interest to radio enthusiasts. Some of the heat and smoke on this matter is well justiﬁed. In other cases, the perceived problems are not real.
ANTENNA BASICS 29
Figure 2.13 shows how the SWR comes into play in an antenna system. In Figure 2.13A, a single cycle of a signal is launched down a transmission line (it is called the ‘incident’ or ‘forward’ wave).
When it reaches the end of the line, if it is not totally absorbed by a load resistor or antenna, then it (or part of it) will be reﬂected back toward the source. This reﬂected wave is shown in Figure 2.13B. The incident and reﬂected waves are both examples of travelling waves. The reﬂected wave represents power that is lost, and can cause other problems as well. The situation in Figures 2.13A and 2.13B represent a single-cycle pulse launched down a transmission line. In a real radio system, the oscillations of the incident wave are constant (Figure 2.13C).
When this situation occurs, then the reﬂected waves will interfere with following incident waves. At any given point, the amplitude of the wave is the algebraic sum of the interfering incident and reﬂected signals. The resultant caused by the interference of the incident and reﬂected waves is called a standing wave. Figure 2.14 shows what happens when continuous incident and reﬂected waves coexist on the same transmission line. In the case of Figure 2.14A, the two waves coincide, with the resultant as shown. The waves begin to move
30 ANTENNA TOOLKIT
ANTENNA BASICS 31
FIGURE 2.14 (continued)
apart in opposite directions in Figure 2.14B, which causes the overall amplitude of the standing wave to decrease, but the location of the maximum and minimum points remain stable. The condition of 1808 out of phase between the two traveling waves results in a zero-amplitude standing wave, as shown in Figure 2.14C. No current ﬂows in this case. As the waves continue to move apart, the standing wave reappears, as shown in Figure 2.14D. The ﬁnal case, Figure 2.14E, is the case with the traveling waves in-phase with each other (08 difference).
Notice that the standing wave in Figure 2.14E is the same amplitude as that of Figure 2.14A, but is 1808 out of phase with it. Figure 2.14F shows the voltage along a transmission line when a VSWR higher than 1:1 is present. The antenna or load end is on the left, and the lengths along the transmission line are plotted in terms of wavelength. In the
32 ANTENNA TOOLKIT
situation shown, the voltage is a minimum at the antenna end (marked ‘0’).
It rises to a peak or ‘antinode’ at a quarter wavelength, and then drops back to a minimum (‘node’) at the half-wavelength (j/2) point. The voltage then rises again to a peak at 3j/4, falling back to minimum at 1j. The current rises and falls in a similar manner, but the nodes and antinodes are offset from the voltage by 908 (quarter wavelength).
Note that the voltage is at a minimum and the current at a maximum at the same points that are integer multiples of a half wavelength. This situation has the effect of making it advisable to measure the VSWR at the transmitter end only when integer multiples of an electrical half wavelength of transmission line (note: the most valid measurement is made at the interface of the transmission line and antenna feedpoint).
The plot shown in Figure 2.14G is the same line system under the situation where the antenna impedance and transmission line impedance are matched. For obvious reasons, this line is said to be ‘ﬂat.’
Measures of SWR
There are several different methods for measuring the SWR of an antenna transmission line system. For example, one could measure the incident and reﬂected signal voltages along the line, producing a result of VSWR ¼ ðVi þ Vr Þ=ðVi À Vr Þ. The method of Figures 2.14F and 2.14G can be used by comparing the maximum and minimum voltages: VSWR ¼ VMAX =VMIN . We can also measure the forward and reverse power levels to ﬁnd the VSWR. A simple way to predict the VSWR is to compare the antenna feedpoint resistive impedance (ZL) to the transmission line characteristic impedance (Z0).
The value of the VSWR is found from either VSWR ¼ or VSWR ¼ ZL Z0 ðZL > Z0 Þ Z0 ZL ðZ0 > ZL Þ
If, for example, we measure the antenna feedpoint impedance as being 25 ohms, and the antenna transmission line is 52 ohm coaxial cable, then the VSWR ¼ 52=25 ¼ 2:08:1. We can ﬁnd the resonant frequency of an antenna by ﬁnding the frequency at which the VSWR is a minimum. But that point may not be a VSWR of 1:1 unless the resistive component of the antenna feedpoint impedance is the same as the transmission line impedance, and the complex portion of the impedance is zero. To create this situation one might need an impedance matching device (see Chapter 12).
ANTENNA BASICS 33
______________________________________________ ______________________________________________ AN OLD MYTH REVIVED? _______________________________________________
There are a number of myths that are widely held among radio communications hobbyists – and amateur radio is no less infested with some of these myths than others (CB, for example).
Twenty-ﬁve years ago I worked in a CB shop in Virginia, and we kept hearing one old saw over and over again: you can ‘cut your coax to reduce the VSWR to 1’ (actually, they meant ‘1:1’ but routinely called it ‘1’).
Hoards of CBers have ‘cut the coax’ and watched the VSWR reduce to 1:1, so they cannot be talked out of the error. I even know of one shop that kept 30 cm lengths of coaxial cable, with connectors on both ends, so they could insert them into the line at the transmitter in order to ﬁnd the correct length that would reduce the VSWR to 1:1. What actually happens in that case is a measurement artifact that makes it appear to be true. Of course, hams are superior to CBers and so do not believe that error, right? I would like to think so; but having been in both the CB and the amateur worlds, and ‘Elmered’ (mentored) more than a few CBers studying for amateur licenses, I have to admit that at least as many amateurs believe the ‘cut the coax’ error as CBers (sorry, fellows, but that’s my observation).
The only really proper way to reduce the VSWR to 1:1 is to tune the antenna to resonance and then match the impedance. For a center-fed halfwavelength dipole, or a bottom-fed quarter-wavelength vertical, the proper way to resonate the antenna is to adjust its length to the correct point. The formulas in the books and magazines only give approximate lengths – the real length is found from experimentation on the particular antenna after it is installed. Even commercial antennas are adjusted this way. On certain CB mobile antennas, for example, this trick is done by raising (or lowering) the radiator while watching the VSWR meter. On amateur antennas similar tuning procedures are used. Even when the resonant point is found, the feedpoint impedance may not be a good match to the transmission line. A VSWR will result in that case. The impedance matching should be done between the far end of the transmission line (i.e. away from the receiver or rig) at the feedpoint of the antenna. Antenna tuners intended for strictly coaxial cable are little more than line ﬂatteners. They do not really ‘tune’ the antenna, but rather they reduce the VSWR looking into the transmission line so that the transmitter will work properly. If the antenna tuner is not a high-pass ﬁlter (as some are), then it will also provide some harmonic attenuation. An approach used by many amateurs (including myself) is to connect an antenna-matching unit (tuner) at the output of the transmitter. For my Kenwood TS-430, I use either a Heath SA-2060A or an MFJ Differential Tuner to ‘tune-out’ the VSWR presented by my Hustler 4BTV and 23 m of coaxial cable. But I do not even pretend to be tuning the antenna. The
34 ANTENNA TOOLKIT
TS-430 is a solid-state rig, and the ﬁnals are, therefore, not terribly tolerant of VSWR, and will shut down with a high VSWR. The purpose of the antenna tuner is to reduce the VSWR seen by the transmitter – ignoring the actual antenna mismatch on the roof. The tuner also serves to reduce harmonics further, thereby helping to prevent television interference. The best form of antenna tuner is one that both reduces the VSWR (for the beneﬁt of the transmitter), and also resonates to the antenna frequency, preventing harmonics from getting out (a little secret is that many ‘line ﬂattener’ antenna tuning units are actually variable high-pass ﬁlters, and must be used with a low-pass ﬁlter ahead of them if spurious signals are to be kept in abeyance).
Should you even worry about VSWR on a system? Or, more correctly stated, given that a 1:1 VSWR often requires a herculean effort to achieve, at what point do you declare the battle won and send the troops home? Some of the issues are:
. Transmitter heating of the transmission line due to power losses. . Reduction of power from solid-state transmitters due to SWR shutdown circuitry. . At kilowatt levels there may be excessive radio frequency voltage at nodes, which could lead to transmission line shorting. . Loss of receiver sensitivity. The feedline loss is added directly to the receiver noise ﬁgure, so may degrade the matched noise ﬁgure of the receiver. This problem is especially severe in the VHF/UHF scanner bands.
Where these problems result in unacceptable performance, work hard to match the antenna to the feedline, and the feedline to the transmitter or receiver. As one of my correspondents said: ‘The question then is to what extent does the problem impair the actually ability to use the receiver or transmitter successfully?’ The same reader provided the guidelines below: (1) The actual SWR on a feedline is 1:1 only if the load (e.g. an antenna) connected to the line is equal to the characteristic impedance of the line. Adjusting an antenna to resonance will improve the SWR only if the impedance of the antenna at resonance is closer to the impedance of the feedline than before it was adjusted. If the impedance of the antenna at resonance is not equal to the impedance of the feedline, you will never get the SWR down to 1:1. (2) The only possible places to measure the SWR with consistent accuracy are: (a) at the load (b) at a distance of 0.5 electrical wavelength (accounting for the velocity factor of the line), or integer multiples thereof, from the load.
ANTENNA BASICS 35
The latter is true because a half-wavelength long piece of feedline ‘repeats’ the impedance of whatever is connected to its far end; it is almost as if you are measuring right up at the feedpoint. As a consequence, many professional antenna installers go to pains to make the transmission line j/2 or an integer multiple of j/2. The VSWR and impedance looking into the line will reﬂect the situation at the feedpoint. Be aware that the velocity factor of the transmission line reduces the physical length required to make an electrical half wavelength. (3) An easy way to check if your SWR measurements are being accurately made is as follows: Add a piece (j/8 to j/4 wavelength) of identical line to your feedline. Repeat the SWR measurement. If it is not the same, your SWR measurement is not accurate, i.e. you are not measuring the actual SWR. (4) Know that a few things can upset SWR measurements, e.g. currents ﬂowing on the outside of a coaxial feedline (a common situation).
This can be caused by not using a BALUN when feeding a balanced antenna (e.g. a symmetric dipole) with an unbalanced (coaxial) feedline. It can also be induced by antenna currents onto a nearby coaxial feedline.
ANTENNA GAIN, DIRECTIVITY, AND RADIATION ___________________________________________________________________ ___________________________________________________________________ PATTERNS ___________________________________________________________________
Antennas do not radiate or receive uniformingly in all directions (and, indeed, you usually do not want them to!).
The interrelated concepts of antenna gain and directivity are of much concern to antenna builders. Gain refers to the fact that certain antennas cause a signal to seem to have more power than it actually can claim. The gain of the antenna can be expressed as either a multiple (e.g. a twofold increase in apparent power) or in decibel notation (e.g. a 3 dB increase).
Both methods of expressing gain reﬂect the fact that a gain antenna seems to produce a signal that is stronger than the signal produced by some comparison antenna (e.g. a dipole).
The apparent power produced by the antenna and transmitter combination is called the effective radiated power (ERP), and is the product of the transmitter power and the antenna gain. For example, if the antenna gain is 7 dB, the power is increased by a factor of ﬁve times. The ERP of a 100 W transmitter is therefore 5 Â 100 W ¼ 500 W. On a receive-only antenna, the signal picked up by a gain antenna is louder than the same signal picked up on a non-gain antenna or the comparison antenna. Because of reciprocity, the gain is the same for both receive and transmit. Directivity refers to the fact that the radiation or reception direction is not constant. Certain directions are favored and others rejected. Indeed, this is how the antenna gets gain. After all, if there is an apparent increase in the
36 ANTENNA TOOLKIT
signal power, there must be some phenomenon to account for it – after all, antennas do not produce power. Figure 2.15A shows how the gain and directivity are formed. Consider an isotropic radiator source located at point ‘A’ in Figure 2.15A. An isotropic radiator is a perfectly spherical
ANTENNA BASICS 37
point source. The signal radiates outward from it in a spherical pattern. At some distance R we can examine the surface of the sphere. The entire output power of the transmitter is uniformly distributed over the entire surface of the sphere. But if the antenna is directive, then all of the power is focussed onto a small area of the sphere with a vertical angle of V and a horizontal area of H . The same amount of energy exists, but now it is focussed onto a smaller area, which makes it appear as if the actual power level was a lot higher in that direction. The antenna pattern is a graphical way of showing the relative radiation in different directions (i.e. directivity).
The most commonly seen pattern is the azimuthal or polar plot. That pattern is measured in the horizontal plane at all angles around the antenna, and is plotted graphically as if seen from above. It is a ‘bird’s eye view’ of the antenna radiation characteristic. But antenna radiation patterns are actually three-dimensional, and the azimuthal plot is merely a slice of that three-dimensional pattern, as seen from but one perspective. Figures 2.15B through 2.15F show how antenna patterns are developed. The antenna in this case is a vertically polarized Hertzian radiator such as a half-wavelength dipole. It radiates poorly off the ends, and very well at angles perpendicular to the wire. The solid-pattern of Figure 2.15B shows the three-dimensional view of what the radiation looks like. To ﬁnd the azimuthal and elevation patterns for this antenna, we take slices out of the solid (Figures 2.15C and 2.15D), and plot the results (Figures 2.15E and 2.15F).
Because the radiator is vertical (and perfect, I might add), it radiates equally well in all horizontal directions (see Figure 2.15E), so the azimuthal or horizontal plot is termed omnidirectional. The elevation or vertical plane view (Figure 2.15F) has to take into account the minima found off the ends of the radiator, so plots as a ﬁgure ‘8.’ If the vertical radiator of Figures 2.15B–2.15F is rotated 908 to form a horizontal dipole, then the radiation patterns remain the same, but are reversed. The omnidirectional pattern of Figure 2.15E becomes the elevation or vertical extent, while the ﬁgure ‘8’ pattern of Figure 2.15F is the horizontal or azimuthal extent. The patterns of Figures 2.15B–2.15F make a rather bold and unwarranted assumption: the antenna is perfect and is installed somewhere in free space. Since most antennas are not in the zone midway between the Earth and Mars, one has to account for the effects of being close to the Earth’s surface. Figure 2.16 shows the approximate patterns for horizontally polarized half-wavelength dipoles close to the ground. In Figure 2.16 we see the azimuthal pattern. It is the ﬁgure ‘8,’ but is pinched a bit. Notice that the maxima are perpendicular to the wire, and the minima (or ‘nulls’) are off the ends. One use for this type of pattern is to position the antenna so that it
38 ANTENNA TOOLKIT
ANTENNA BASICS 39
nulls out interfering signal or noise sources, making the overall signal-tonoise ratio higher – and increasing listenability. Figure 2.16B shows the elevation extent. This view is taken from a site perpendicular to the wire, and is essentially looking into one end of the three-dimensional ﬁgure ‘8’ pattern. In other words, the directivity, or maxima, is in and out of the page. In free space, one would expect this pattern to be omnidirectional. But close to the Earth’s surface, reﬂections from the surface add to the pattern, causing the pinching effect apparent in Figure 2.16B. The shape shown is approximately that for a half wavelength above the ground. Other heights produce variants of this pattern. The patterns shown in Figure 2.17 are for a vertically polarized Marconi radiator, such as a quarter-wavelength, bottom-fed vertical. The azimuthal pattern, as seen from above, is shown in Figure 2.17A. It is the omnidirectional pattern that one would expect. The elevation pattern in Figure 2.17B
40 ANTENNA TOOLKIT
represents a vertical slice taken from the ‘doughnut’ seen earlier. Note that the lobes are elevated, rather than being exactly on the horizon. The angle of these lobes is the angle of radiation, and affects the distance that an antenna will transmit to or receive from. A low angle of radiation is usually prescribed for long-distance ‘DX,’ while a high angle of radiation produces skip to nearer regions. Figure 2.18 shows the conventions for specifying antenna patterns. The antenna is a dipole, with both relative electric ﬁeld strength and relative powers plotted (both types of plot are done).
The 0–2708 line is along the length of the dipole, while the 90–1808 line is perpendicular to the dipole. The beamwidth of the antenna, which is speciﬁed in degrees, is measured as the angle between the half-power points, i.e. the points where the voltage ﬁeld drops off to 70.7% of the maximum (along the 908 line), or to the 0.5 power point. These points are also called the ‘À3 dB’ points. Recall that À3 dB represents a power ratio of 1:2.
_______________________ _______________________ SOME ANTENNA CONSTRUCTION HINTS ________________________
This section will help you think up some good ideas on how to erect the antenna. First, though, let us talk a little bit about antenna safety. It is difﬁcult to say too much about this topic. Indeed, I mention it elsewhere in this book as well as here. Wherever it seems prudent, I warn readers that
ANTENNA BASICS 41
antenna erection can be dangerous if you do not follow certain precautions. Do it right, and the risk is mitigated and the job can be quite safe.
Before dealing with the radio and performance issues, let us ﬁrst deal with safety matters – you do not want to be hurt either during installation, or during the next wind storm. Two problems present themselves: reliable mechanical installation and electrical safety. Electrical safety note. Every year we read sad news in the magazines of a colleague being electrocuted while installing, or working on, an antenna. In all of these tragic cases, the antenna somehow came into contact with the electrical power lines. Keep in mind one dictum and make it an absolute: There is never a time or situation when it is safe to let an antenna contact the electrical power lines! None. Ever. BELIEVE THIS NO MATTER WHAT ANYONE SAYS TO THE CONTRARY. This advice includes dipoles and long wires ‘thrown over’ supposedly insulated power mains lines, as well as antennas built from aluminum tubing. The excuse that the lines are insulated is nonsense. Old insulation crumbles on contact with even a thin wire antenna. Do not do it! The operant word is never! Consider a typical scenario involving a four-band trap vertical antenna made of tubing. It will be 5–8 m tall (judging from adverts in magazines), and will be mounted on a roof, or mast, 4–10 m off the ground. At my home in Virginia, a 7.6 m tall trap vertical is installed atop a 4.6 m telescoping television antenna mast. The total height above ground is the sum of the two heights: 7:6 m þ 4:6 m ¼ 12:2 m. The tip of the vertical is 12.2 m above the ground. I had to select a location, on the side of my house, at which a 12.2 m aluminum pole could fall safely. Although that requirement limited the selection of locations for the antenna, neither my father-in-law (who helped install the thing) nor myself was injured during the work session. Neither will a wind storm cause a shorted or downed power line if that antenna falls over. In some jurisdictions, there might be legal limitations on antenna location. For example, some local governments in the USA have a requirement that the antenna be able to fall over and land entirely on your own property. Before installing the antenna, check local building codes. When installing a vertical, especially one that is not ground mounted, make sure that you have help. It takes at least two people to safely install the standard HF vertical antenna, and more may be needed for especially large models. If you are alone, then go and ﬁnd some friends to lend an arm or two. Wrenched backs, smashed antenna (and house parts), and other
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calamities simply do not happen as often to a well-organized work party that has a sufﬁcient number to do the job safely.
The second issue in installing antennas is old-fashioned mechanical integrity. Two problems are seen. First, you must comply with local building regulations and inspections. Even though the courts in the USA seem to forbid local governments from prohibiting amateur radio activity (on grounds that it is Federal prerogative), local governments in the USA and eslewhere have a justiﬁable interest, and absolute right, to impose reasonable engineering standards on the mechanical installation of radio antennas. The second issue is that it is in your own best interest to make the installation as good as possible. View local regulations as the minimum acceptable standard, not the maximum; go one better. In other words, build the antenna installation like a brick outhouse. Both of these mechanical integrity issues become extremely important if a problem develops. For example, suppose a wind or snow storm wrecks the antenna, plus a part of your house. The insurance company will not pay out (in most cases) if your local government requires inspections and you failed to get them done. Make sure the mechanical and/or electrical inspector (as required by US law) leaves a certiﬁcate or receipt proving that the ﬁnal inspection was done. It could come in handy when disputing with the insurance company over damage. A quality installation starts with the selection of good hardware for the installation. Any radio/television parts distributor who sells television antenna hardware will have what you need. I used Radio Shack stand-off brackets, ground pin, and a 5.8 m telescoping mast. Wherever you buy, select the best-quality, strongest material that you can ﬁnd. Opt for steel masts and brackets over aluminum, no matter what the salesperson behind the counter tells you. Keep in mind that, although salespeople can be knowledgeable and helpful, you, not they, are responsible for the integrity of the installation. In my own case, I found that the 5.8 m mast was considerably sturdier at 4.6 m than when fully up, so I opted to use less than the full length because the installation is unguyed. Because I have never trusted the little cotter pin method of securing the mast at the slip-up height, I drilled a single hole through both bottom and slip-up segments (which telescope together), and secured the antenna mast with a 8 mm stainless steel bolt. The bolt was ‘double nutted’ in order to ensure that it did not come loose over time. The television mast is set on a ground mounting pin/plate that is set into a 76 cm deep (local frost line regulations required only 71 cm) fencepost hole ﬁlled with concrete. The top end of the mast was secured to the roof overANTENNA BASICS 43
hang of the house (see Figure 2.23).
That overhang was beefed up with 5 Â 20 cm kiln-dried lumber that was bolted between two, 60 cm center, roof rafters. I felt it necessary to do that because the roof is only plywood, and the gutter guard is only 2:5 Â 20 cm lumber (and is old), and the sofﬁts are aluminum. There was not enough strength to support a 12 m lever arm, whipping around in a 35 knot wind. Wind can be a terrible force, especially when acting on the ‘sail area’ of the antenna through a 7.5–12 m lever arm. A shabby installation will tear apart in wind, causing the antenna to be damaged, damage to the house, and destruction of the installation. That is why I recommend ‘brick outhouse’ construction methods. Over the 33 years I have been in amateur radio, I have seen a lot of verticals toppled over. Except for a few shabby models that were so poorly built that they should not have been on the market in the ﬁrst place, all of these failed installations were caused by either poor installation design or poor-quality materials.
Antenna erection methods
In this section we will look at some methods for erecting wire antennas. It is recommended that you also consult Chapter 3 to ﬁnd out how to make connections to wire antennas and the usual ﬁttings (end insulators, center insulators, BALUN transformers, etc.).
Chapter 3 also deals with the type of wire used for antenna construction. The ideas in this chapter are not the beall-end-all discussion, but rather points of departure of representative methods. Your own innate intelligence can ﬁgure out other methods suitable for your own situation (keeping safety foremost in your plans).
Figure 2.19 shows one of the most basic methods for installing a wire antenna. A mast is installed on the roof of the house, or on the side of the building close to the roof. Alternatively, the mast could be replaced by some
44 ANTENNA TOOLKIT
other support on the house. The other support is a convenient nearby tree. It could also be another building or a mast erected on the ground. The antenna is supported between two end insulators, that are in turn supported by ropes connected to the mast and the tree. For a simple random length Marconi antenna, as shown here, the direction of the wire can be up, down, or horizontal, depending on the convenience of the situation (horizontal is preferred but not always easily attained).
The down-lead to the receiver or antenna tuner is run through a convenient window. There are special straps and ﬁttings for making the connection under the window (see your radio dealer).
The installation shown in Figure 2.20 is a half-wavelength dipole, although it could represent a variety of doublet antennas. As with the previous case, the supports can be masts, a building, or a convenient tree. The installation is very much like that in Figure 2.19. The goal in making this installation is to make the antenna horizontal, with the feedline coming away from the antenna center at a right angle for as far as is practical. The installation in Figure 2.21 is for a dipole, but it must be a small dipole (either inductively loaded or for a high frequency), or built over a very wide roof. Both end supports are on the roof, or on the walls at either end of the roof. The coaxial cable comes away at a right angle, and then passes through a window. A vertically polarized dipole is shown in Figure 2.22.
ANTENNA BASICS 45
46 ANTENNA TOOLKIT
Figures 2.23A and 2.23B show methods for installing quad loops and delta loops, respectively. As above, the end supports can be a tree, a mast, or a building. It is necessary to support both the top and bottom of loops. The weight of the BALUN transformer and coaxial cable is not sufﬁcient to keep the antenna from whipping around in the wind, and possibly doing some damage to either itself or other structures (not to mention the odd passerby).
The bottom support can be either separate ropes to the end supports (Figure
ANTENNA BASICS 47
2.23A), or to a stake in the ground (Figure 2.23B).
Make sure that the stake is secure against pulling out under the stress of wind blowing against the sail area of the antenna. Now, let us go to Chapter 3 to talk about wire and connections.
Wire, connection, grounds, and all that
In this chapter we will take a look at the types of wire useful for antenna elements, the different types of transmission lines, and grounding. The wire used in an antenna project can make or break the results, especially the reliability of the project over time. Antennas, especially outdoor antennas, are exposed to a lot of stress and environmental factors that tend to deteriorate both the performance and life expectancy.
____________________________________________________________ ___________________________________________________________ TYPES OF WIRE ____________________________________________________________
There are basically two types of wire used for the radiating element and other parts of antennas: solid (Figure 3.1A) and stranded (Figure 3.1B).
The solid type of wire is made of a single thick strand of copper or copper alloy. Pure copper is rarely used because of both cost and strength problems, while aluminum is almost never used because it does not take solder without special, high-cost, industrial equipment. Solid wire is extruded from a single piece of copper. This is the type of wire used most often in household electrical wiring. In its smaller sizes, it is also the type of wire most commonly used as hook-up wire in electronic construction projects. The stranded form of wire is made of two or more (sometimes many more) lengths of solid wire twisted together (Figure 3.1B).
The number of strands used to make the wire sets the ﬂexibility of the wire. Coarse stranded wire has fewer strands, and is less ﬂexible than wire of the same overall diameter that is made up of many more strands of ﬁner wire.
Antennas should normally be built with stranded wire. The reason is that the stranded wire stands up better to the constant ﬂexing an antenna experiences, especially in windy areas. All wire will begin to stretch because of both gravity and wind. Solid wire will stretch rather rapidly, and eventually come to a point where it is too thin at the strain point to hold its own weight – and will come tumbling down. Stranded wire, on the other hand, withstands the normal stresses and strains of antenna installations much better, and will stay up a lot longer. Stranded wire antennas may come tumbling down as well, but in general it takes a lot more time and effort for the stranded wire antenna to break, compared to the solid wire type.
_________________________________________________________________ _________________________________________________________________ WIRE SIZES __________________________________________________________________
Both stranded and solid wire comes in certain standard sizes. The speciﬁcations for each size wire were set basically for their ability to carry certain standard loads of direct current or the 50/60 Hz alternating current (AC) used in power systems. Table 3.1 gives some of the standard wire sizes in the range normally used by antenna constructors. In most cases, it is a good idea to stick to sizes around AWG 14 (SWG 16) for antenna work. The larger sizes are expensive, and difﬁcult to work (most antenna hardware assumes AWG 14 or SWG 16, unless otherwise marked).
The smaller sizes, on the other hand, are nice to use but often provide poor reliability – they break easily. The trade off is to use wires, of whatever standard size, that are in the 1–2.2 mm diameter region.
50 ANTENNA TOOLKIT
TABLE 3.1 Diameter AWG (USA) 8 10 12 14 16 18 20 22
mm 3.264 2.588 2.053 1.628 1.291 1.024 0.812 0.644
Nearest SWG (UK) 10 12 14 16 18 19 21 24
128.5 101.9 80.8 64.1 50.8 40.3 32.0 25.3
1 1 mil ¼ 1000 inch.
_____________________________________________________ _____________________________________________________ COPPER-CLAD WIRE ______________________________________________________
Pure copper wire is not terribly good for radio antenna work because it is too soft to withstand the rigors of being outdoors, or supporting its own weight plus the weight of the ﬁxtures and transmission lines used. As a result, one needs to ﬁnd a better wire. Some people will use copper wires of harder alloys, but even that type of wire is not all that good. Others buy ‘hard drawn’ copper wire, and while it is better than straight electrical wiring copper wire it is not the best suited to antenna construction. The best wire for radio antennas is copper-clad steel wire, perhaps the most famous brand of which is Copperweld. This type of wire is made of a solid steel core coated with a copper layer. It looks like ordinary copper wire on the outside. The steel provides strength and thereby increases the reliability of the antenna installation. But steel is not a terribly good electrical conductor, so the manufacturer places a layer of copper over the outside of the wire. The standard wire sold with the better antenna kits and by the better sources is copper-clad steel, stranded wire of AWG 14 (SWG 16) size. Each strand is copper-clad steel, and the strands are wound together to form the antenna wire. The copper-clad wire is better for the purposes of strength, but there is a workability problem with it: copper-clad steel-core wire knots easily. If you get a knot or kink (even a minor bend) in it, that feature is always present from then on. Even if you attempt to straighten it out, it will not come anywhere near the original state. Although ordinary copper wire also possesses this attribute, it is not nearly as severe as with copper-clad steel-core wire, and can often be worked into a negligible size when it does occur. The trick is to make sure that the wire does not knot or kink during the installation process (and that can be a monumental task!).
WIRE, CONNECTION, GROUNDS, AND ALL THAT 51
One fair question regarding copper-clad steel-core wire concerns its resistivity. After all, any basic electrical text tells us that the cross-sectional area of the wire sets the current-carrying capacity. That is true for direct current (DC), and power line frequency AC, but only partially true for higher-frequency AC. Radio signals cause AC currents to be set up in the antenna wire, and because their frequency is so much higher than power line AC the currents ﬂow only near the surface. The operating principle here is called the skin effect.
_______________________________________________________________ _______________________________________________________________ SKIN EFFECT ________________________________________________________________
The skin effect refers to the fact that AC tends to ﬂow on the surface of a conductor. While DC ﬂows in the entire cross-section of the conductor, AC ﬂows in a narrow band near the surface (Figure 3.2).
Current density falls off exponentially from the surface of the conductor toward the center (inset to Figure 3.2).
At the critical depth (), also called the depth of penetration, the current density is 1/”, or 1/2.718 ¼ 0.368, of the surface current density. The value of is a function of operating frequency, the permeability () of the conductor, and the conductivity (s).
_______________________________ _______________________________ WIRE SIZE AND ANTENNA LENGTH ________________________________
The size of the wire used to make an antenna affects its performance. For amateur radio operators, one concern is the current-carrying capacity. For normal amateur radio RF power levels, the AWG 14 wire size works well,
52 ANTENNA TOOLKIT
although those tempted to use smaller diameters with the higher legally permitted powers are forewarned: it is not such a good idea. Both short-wave listeners and amateur radio operators need to be aware of another problem with the wire: the length to diameter ratio, or as it is sometimes speciﬁed, the signal-wavelength-to-wire-diameter ratio, determines the velocity factor (VF) of the conductor. The VF is deﬁned as the velocity of the wave in the medium (i.e. wire) compared with the velocity in free space (which is c, the speed of light).
Figure 3.3 shows the velocity factor as a function of the L=D ratio. Wire antennas tend to have very high L=D ratios, especially at high frequencies. For example, a half wavelength at 10 MHz is 15 m (or 15 000 mm), while AWG 14 wire is about 1.63 mm diameter, resulting in L=D ¼ 15 000= 1:63 ¼ 9200. The VF of this L=D ratio is around 0.97–0.98. At very high/ ultrahigh frequencies (VHF/UHF), on the other hand, the L=D ratio declines. At 150 MHz, the AWG 14 wire half-wavelength antenna has a physical length of 1500 m, making L=D ¼ 1500=1:63 ¼ 920. The VF drops to about 0.95–0.96. At 450 MHz, the L=D ratio drops to 200, and the VF runs to 0.94 or so. So what? Why is the VF important in antenna design? Most of the antennas in this book are designed to be resonant, which means that the element lengths are related to wavelength: quarter wavelength (j/4), half
WIRE, CONNECTION, GROUNDS, AND ALL THAT 53
wavelength (j/2), full wavelength (j), and so forth. These lengths are required electrical lengths, and are only found in free space with perfect connectors. In antennas and transmission lines, the VF shortens the physical length required to achieve a speciﬁed electrical length. To ﬁnd the physical length, multiply the required electrical length by the velocity factor. For example, a half wavelength in free space is deﬁned as 150/FMHz, while in a wire it is ð150 Â VFÞ=FMHz . The result is that antenna elements must be shortened a small amount compared to the free space length. If you cut some antennas to a physical length that is comparable to the wavelength of the desired resonant frequency, then you will ﬁnd that the actual resonant frequency is a tad low – meaning that the antenna is a little bit too long. This effect is almost negligible on the 80 m band, but becomes more substantial at 10 m and downright annoying at 2 m.
___________________________________________________ __________________________________________________ TRANSMISSION LINES ___________________________________________________
The wire between the antenna and either a transmitter or receiver is called the transmission line. Although it is tempting to think of the transmission line as a mere wire, it is actually equivalent to a complex inductor–capacitor network. The details of the transmission line network are a bit beyond the scope of this book, but can be found in antenna engineering textbooks for those who are interested. The main aspect of the transmission line network that you need to understand is that the line possesses a property called the characteristic impedance or surge impedance, which is symbolized by Z0 . The rigorous deﬁnition of Z0 can get a bit involved, but it breaks down to one thing for most practical situations: Z0 is the value of load impedance that will result in the maximum power transfer between the antenna and the transmission line, or between the receiver/rig and transmission line. The act of making these impedances equal to each other (a very desirable thing, as you will ﬁnd out) is called impedance matching, and is covered in Chapter 11. Transmission lines come in a bewildering variety of species, but there are only a few basic types: parallel open lead, twin-lead, and coaxial cable are the main types that we need to consider. Figure 3.4 shows all of these varieties.
Parallel open-wire line
Parallel line is shown in Figure 3.4A. It consists of a pair of wires, run parallel to each other, and separated by a constant space. Insulating spacers (made of ceramic, nylon, or some other material) are used in practical parallel open line to keep the distance between the conductors constant. The characteristic impedance of open line is determined by the diameter (d) of the conductors and the spacing (s) between them. Typical values run from 300 to 1000 ohms. Although you can calculate the spacings needed
54 ANTENNA TOOLKIT
WIRE, CONNECTION, GROUNDS, AND ALL THAT 55
(and the wire diameters are given in some of my other books), there are some general guidelines in Table 3.2. The conductor type is along the vertical axis on the left, and the characteristic impedances are along the horizontal axis at the top. The entries in each cell are the spacings (s) in centimeters. As you can see from the table, only some of the calculated values are useful (who wants to make a 129 cm wide parallel line?).
Parallel open-wire transmission line can be either built or bought. For some applications, such as tuned feeders, it might be the best selection. But for many balanced antennas that use parallel line one might want to consider using twin lead instead.
The twin-lead form of transmission line is shown in Figures 3.4B and 3.4C. This type of line is like parallel line, except that the conductors are buried in an insulating material that also spans the space between the conductors. Spacings are shortened a little bit compared to the open-wire variety because the dielectric constant of the insulating material reduces the VF. The most common form is 300 ohm television antenna twin-lead, of the sort shown in Figure 3.4B. This line is about 1 cm wide, and has a characteristic impedance of 300 ohms. Television twin-lead is easily available from a wide variety of sources, including shops catering for television and video customers. It can be used for transmitting at powers up to about 150 W, but even that level is questionable if the line is of a cheaper variety. Receive-only installations have no such limitation. Television 300 ohm twin-lead may be solid as shown, or may have oblong or rectangular holes cut into the insulation separating the conductors. This tactic is used to reduce the losses inherent in the transmission line. In other cases, the center insulating material will be rounded and hollow. This
TABLE 3.2 Characteristic impedance, Z0 (ohms) Conductor 8 AWG 10 AWG 12 AWG 14 AWG 16 AWG 18 AWG 20 AWG 22 AWG 300 2.0 1.6 1.25 1.0 0.80 0.63 0.50 0.40 400 4.6 3.6 2.9 2.3 1.8 1.4 1.1 0.90 500 10.6 8.4 6.7 5.3 4.2 3.3 2.6 2.1 600 24.4 19.3 15.3 12.2 9.6 7.6 6.1 4.8 800 129 102 81 64 51 40 32 25
approach to design also reduces the losses, and is especially effective at UHF. The type of twin-lead shown in Figure 3.4C is 450 ohm line. This type of line is specialist, and must be sought at shops catering for amateur and commercial radio operators. It is about twice as wide as the 300 ohm variety, and almost always comes with rectangular or oblong loss-reducing holes cut into the insulation. The 450 ohm twin-lead is considerably heavier than 300 ohm line, and uses larger conductors. As a result, it can handle higher power levels. Twin-lead is wonderful stuff, but can cause problems in some installations. The problems are especially severe at VHF/UHF, but nonetheless exist at HF as well. For example, care must be given to where the line is run: it should not be in close proximity to metal structures such as aluminum house siding, rain gutter downspouts, and so forth. It is also susceptible to picking up local electrical ﬁelds, which means a possibility of interference problems. One solution is to use coaxial cable transmission line.
This type of cable is shown in Figure 3.4D. The name ‘coaxial’ comes from the fact that the two conductors are cylindrical (the inner one being a wire) and share the same axis. The inner conductor and outer conductor are separated by the inner insulator. The outer conductor is usually a braided wire or foil shield, and is covered with an outer insulating material. The characteristic impedances of commercial coaxial cables are of the order of 36–120 ohms, with 52 and 75 ohms being the most common. The speciﬁc impedance value is a function of the conductor sizes and the spacing between them. There are several standard types of coaxial cable, and these are available in many subtypes (Table 3.3) that use similar numbering. The terms ‘thick’ and ‘thin’ would have been replaced with actual dimensions if this book were being written only a few years ago, but because of the wide array of different types now available it was decided to use these designations instead. One reason is that there are some RG-8/U specialist cables used by amateur radio and commercial antenna installers that are smaller than regular RG-8/U but larger than RG-58/U or RG-59/U.
TABLE 3.3 Type RG-8/U RG-11/U RG-58/U RG-59/U Characteristic impedance, Z0 (ohms) 52 75 52 75 Size Thick Thick Thin Thin
WIRE, CONNECTION, GROUNDS, AND ALL THAT
One of the differences between the different forms of coaxial cable is the insulating material used for the center insulator. Several types are used: polyethylene, polyfoam and Teﬂon. A principal effect of these materials is to change the VF of the line.
The VF (denoted by V in some textbooks) of transmission line is calculated in exactly the same way as the VF of other conductors: it is the ratio of the velocity of the signal in the transmission line over the velocity of the signal in free space. The free space velocity is the speed of light (c), or about 300 000 000 m/s. The VFs of several popular transmission lines are shown in Table 3.4. The effect of the VF is to reduce the physical length of the coaxial cable that is cut for a particular frequency. And this foreshortening can be quite drastic. For example, if polyethylene cable is cut for an electrical half wavelength at, say, 5 MHz, the actual length L ¼ ð150 Â 0:66Þ=5 ¼ 19:8 m, while the wavelength of the signal in free space is 1.52 times longer, or 30 m.
CONNECTING THE TRANSMISSION LINE TO THE RIG OR ____________________________________________________________________ ___________________________________________________________________ RECEIVER ____________________________________________________________________
The method used to connect the transmission line to a receiver or transmitter depends on the type of output/input connector that is provided for the antenna, and the type of transmission line. In the discussion to follow, the illustrations show a receiver, but apply equally well to transmitters. The only difference is that parallel output on transmitters tends to be large ceramic feedthrough insulators, while on receivers, parallel line inputs tend to be small screw terminals on a plastic, Bakelite or other insulating carrier. Figure 3.5 shows the basic coaxial connector on a receiver. Although an SO-239 ‘UHF’ connector is shown, and may well be the standard, you may also ﬁnd BNC, Type-N, or RCA ‘phono’ plug connectors used on some rigs (I’ve even seen phono plug connectors used on 100 W transmitters, but that
TABLE 3.4 Type of line VF
0.95–0.99 Parallel open-wire 0.82 300 ohm twin lead 300 ohm twin lead (with holes in insulation) 0.87 0.87 450 ohm twin lead 0.66–0.80 Coaxial cable 0.66 Polyethylene 0.80 Polyfoam 0.72 Teﬂon
is not the recommended practice!).
In this type of installation, the coaxial cable from the antenna (or any antenna tuning unit that might be present) is ﬁtted with a connector that matches the chassis connector on the receiver. The threads of the male PL-259 connector are tightened onto the female SO239 connector. Or, in the case of BNC connectors, they are pressed on and then rotated a quarter of a turn (it is a bayonet ﬁtting), and phono plugs are just pushed into the socket. If the receiver or transmitter is ﬁtted with a ground connection, then it should be used. In short order we will discuss what a ‘good ground’ means in this context, but for the time being rest assured that you will need a heavy wire to the ‘good ground’ (whatever it is).
If your receiver or transmitter lacks a ground terminal, then wonder why and consider that next time you select a new model. Although most modern receivers are ﬁtted with coaxial connectors for the antennas, there are a few models (including many that are old but still useful) which are ﬁtted with a balanced input scheme such as that shown in Figure 3.6. In the case of Figure 3.6A, there are three screw terminals: A1, A2, and G (or the variants ‘ANT1’, ‘ANT2’, and ‘GND’).
The A1 and A2 terminals form a balanced input for a parallel transmission line, while G is the chassis ground. When an unbalanced antenna transmission line, such as a single-wire downlead (Figure 3.6B) is used, then the line goes to either A1 or A2, while the other is connected to G (in the case shown, A1 receives the antenna downlead, while A2 and G are connected together and then connected to the earth ground).
Some models are equipped with only two terminals (Figure 3.6C), and in that case the selection is easy: connect the downlead to A1 (or just ‘A’ or ‘ANT’), and the ground lead to ‘G’ or ‘GND.’ When parallel line, twin lead, or any other form of balanced line is used (Figure 3.6D), then connect ‘G’ or ‘GND’ to ground, and the two conductors of the transmission line to A1 (or ANT1) and A2 (or ANT2).
WIRE, CONNECTION, GROUNDS, AND ALL THAT
60 ANTENNA TOOLKIT
FIGURE 3.6 (continued )
___________________________________ ___________________________________ CONNECTING WIRES TOGETHER ___________________________________
Wire antennas often require that two or more wires be spliced together (for example, two antenna elements or an antenna element and a downlead).
There are right ways and wrong ways to accomplish this task. Let us look at some of the right ways. All of the correct methods (only a few of which are shown here) have two things in common: they are (a) electrically sound, and (b) mechanically sound. Electrical soundness is created by making a good electrical connection, with the wires bound tightly together, and protected from the elements. The electrical soundness of the joint is made better and longer lasting by soldering the connection. Figure 3.7 shows how to make the connection mechanically sound. Overlap the two wires being spliced by a few centimeters (Figure 3.7A), and then wrap each one onto the other (Figure 3.7B), forming seven or more tight turns (Figure 3.7C).
Once the splice is made, then solder both knots to form a better electrical connection. One mistake made by novice antenna builders is to assume that the purpose of soldering is to provide mechanical strength. This is false. Solder only improves and keeps good the electrical connection. Solder for radio work is a 50/50 or 60/40 lead/tin mixture with a resin core (NEVER acid core!).
It is soft and has no inherent mechanical strength. Always depend on the splice to provide mechanical strength, and NEVER use solder for strength.
WIRE, CONNECTION, GROUNDS, AND ALL THAT 61
______ ______ CONNECTING WIRES TO ANTENNAS AND SUPPORTS ______
The methods for connecting wires and antenna elements are basically the same as above, but with differences to account for the types of hardware being used in the antenna construction. Figure 3.8 shows how the ends of the wire antenna are supported. An end insulator is used. These insulators may be of glass, ceramic, or a synthetic material (nylon is common), but all will have a body and two holes. The wire from the antenna element is passed through one hole, and then looped back onto itself to form seven or more turns. As in the previous case, the splice may be plated with solder to prevent corrosion from interfering with the electrical integrity of the connection. The other end of the insulator is ﬁtted to a rope that is run to a tree, edge of the building, or a mast of some sort. The rope should be treated similarly to the wire in that it should be wrapped over on itself several times before being knotted. If you use an antenna that has a single-wire downlead, then you should connect it to the antenna in the manner of Figure 3.9. The downlead is almost always insulated wire, even when the antenna is not insulated. The insulation of the downlead prevents it being shorted out somewhere along its run. The scheme in Figure 3.9A shows how most people connect the downlead (and, indeed, how I have connected them quite often).
This is not the best practice, although it will work most of the time. The problem is that the connection is stressed by the downlead, and may break. More
62 ANTENNA TOOLKIT
than once I have been off-the-air due to a spliced downlead breaking right at the junction with the antenna wire. A better solution is shown in Figure 3.9B. In this installation, the downlead wire is passed through the hole in the insulator, and then wrapped back on itself three or four times before being spliced to the antenna wire in the usual manner. This arrangement provides some strain relief to the downlead, and that (usually) translates into a longer life expectancy. Some antennas, such as the center-fed half-wavelength dipole (a very popular wire antenna), use a center insulator to separate the two halves of the antenna. Coaxial cable is connected such that the inner conductor is spliced to one antenna element, and the shield (outer conductor) is connected to the other antenna element. Good practice is to splice the shield to the element closest to the house, if this is a factor. The method shown in Figure 3.10A will work, but it is also a bit foolish (I know, I know, I have used it and even recommended it in the past).
The problem is that the center conductor is quite weak, and may be even smaller than single-wire downleads. As a result, it has no strength. To make matters worse, coaxial cable weighs more length-for-length than single-conductor downleads (or even some twin lead).
The connection of Figure 3.10A will work, but only for
WIRE, CONNECTION, GROUNDS, AND ALL THAT 63
a while – it will surely come down prematurely. A better scheme is shown in Figure 3.10B. In this case, the coaxial cable is wrapped once around the center insulator, and secured with a length of twine or string. The electrical connections are made in exactly the same manner as shown in Figure 3.10A. Perhaps the best solution is to use a manufactured center insulator (Figure 3.11).
These devices come in a wide variety of sizes and shapes, so this illustration represents a lot of different devices. What they all share in common, however, are means for strain relieving the antenna element wires and coaxial cable. Some devices may be straight insulators, while others may
64 ANTENNA TOOLKIT
contain BALUN transformers inside (of which, more shortly).
Some models include special strain relief features for the coaxial cable, or they may have a screw-eye on top to secure the antenna center to a mast or other support. The electrical connections between the center insulator and the wire antenna elements are done in the usual manner (see above), but the ends of the wires are usually attached to eyelet connectors, which are crimped and then soldered. It has been my experience that these connectors are often corroded (as seen by the dull appearance) when they arrive, so brighten them with sandpaper or steel-wool before attempting to make the connections permanent.
________________________________________________ ________________________________________________ LIGHTNING ARRESTORS ________________________________________________
Lightning is messy stuff, and can create havoc with your radio – and might set your house on ﬁre. But there are some things you can do to protect yourself. But ﬁrst, let us take on one myth right away: contrary to popular belief, antennas do NOT inherently attract lightning. Lightning will usually strike the highest object around, and if that happens to be an antenna, then the antenna will take a hit. And that hit could set the house on ﬁre, and the radio will be damaged or even destroyed. The lightning would have come anyway, and if it takes out the antenna rather than your house, then it acted like a lightning rod and protected your property. If lightning actually strikes your antenna, then it will probably wipe out your transmitter or receiver no matter what you do. But a direct hit is not needed for fatal damage to occur: even lightning overhead or striking nearby will induce a large enough voltage into your antenna to wreak havoc inside the equipment. And it may be silent damage because it is not so spectacular as the problems seen with direct hits.
WIRE, CONNECTION, GROUNDS, AND ALL THAT 65
The key to protecting your equipment is to use a lightning arrestor (Figure 3.12).
These devices come in several types, each suitable for coaxial line, parallel line, or single-wire downleads. Each will have at least one connection for the antenna wire, and a ground connection. It is placed in the line between the antenna lead and the receiver or transmitter, and should be mounted outside the house. There is very little that can be done for a direct hit, but I have heard tales of near hits only causing a small amount of rig or receiver damage because an effective lightning arrestor was in place. Also, keep in mind that some local laws or insurance regulations make the lightning arrestor mandatory. If you fail to use one, you may be liable for a ﬁne from the local government, or ﬁnd your homeowner’s insurance not valid after a lightning episode. Lightning arrestors are cheap, and so make very reasonably priced insurance.
________________________________________________ _______________________________________________ BALUN TRANSFORMERS ________________________________________________
Both impedance transformation and conversion between unbalanced and balanced lines can be accomplished with a BALUN transformer (the name comes from BALanced–UNbalanced).
Two sorts of transformers are used (Figure 3.13).
The 1:1 BALUN transformer converts the balanced load to an unbalanced form (so it can be fed with coaxial cable).
The impedance of the load must be the same as the source. The version shown in Figure 3.13B is a 4:1 BALUN. This transformer is used to provide not only a balanced-to66 ANTENNA TOOLKIT
unbalanced conversion, but also a 4:1 impedance transformation. For example, a half-wavelength folded dipole antenna is balanced with respect to ground, and has a feedpoint impedance near 300 ohms. A 4:1 BALUN transformer converts the 300 ohm load to 75 ohms, and makes it appear an unbalanced rather than balanced load. BALUN transformers can be easily built using toroidal coil forms or ferrite rods. The 1:1 BALUN is wound in the triﬁlar style, i.e. the three wires are wound side by side so that all three coils interleave with one another. The 4:1 BALUN is wound in the biﬁlar manner. If you do not wish to make your own BALUN transformers, then you will ﬁnd that there are many commercially available BALUN transformers on the market. VHF/UHF scanner users will ﬁnd few BALUNs specially for their needs, but there are plenty of devices available. The television antenna transformers are not usually called ‘BALUNs,’ but when you ﬁnd a transformer that converts 75 ohm coaxial cable to 300 ohm twin-lead, then it is (rest assured) a BALUN, and will work over the entire VHF/UHF range unless marked otherwise. BALUN transformers are bidirectional devices, so can be used either way around.
_______________________________________________________________________ ______________________________________________________________________ SAFETY _______________________________________________________________________
Antenna safety is an absolute must. First, let me make one thing absolutely, positively, clear in your mind: DO NOT EVER TOSS AN ANTENNA WIRE OVER THE AC POWER LINES!
WIRE, CONNECTION, GROUNDS, AND ALL THAT
If you plan to do this trick, then have your family prepare for some solemn praying, sad music, and slow marching, for your funeral is imminent. This action kills radio hobbyists every year. There is no safe way to do it, so do not. Even if both the antenna wire and the power line are insulated, the forces of erecting the antenna will slice through both insulations! When that happens, the wires will come into contact with each other and with you – with disastrous consequences: human bodies make lousy fuses. The next matter is like unto the ﬁrst: do not erect your antenna where it can fall onto the power lines, or other people’s property, either during erection or as a result of a structural failure. Falling onto the electrical lines can make the antenna deadly, and falling onto your neighbor’s property is just plain rude (and might get you sued).
Finally, do not install an antenna without assistance. I know they are lightweight, and all that, but they have a remarkable ‘sail area,’ and even very weak winds will increase their apparent weight. I’ve suffered injury myself (wrenched back) by working alone, and a friend of mine broke a pelvis and a leg falling off a roof while trying to install a television antenna.
Marconi and other unbalanced antennas
Marconi antennas are those that are fed unbalanced with respect to ground (Figure 4.1).
In the case of a transmitter, one side of the output network is grounded, while the other side is connected to the radiator element of the antenna. The length of the radiator element varies with type, and determines many of the operating properties of the antenna. The direction of the radia-
tor element also varies. It may be vertical as shown (indeed, vertical antennas are a species of Marconi antenna), or at a slant. In some cases, the Marconi antenna is connected directly to the rig or receiver, but in other cases an antenna tuning network is needed.
_________________________________ _________________________________ BASIC RECEIVING ‘TEE’ ANTENNA __________________________________
Although most of the antennas in this chapter can be used on either receive or transmit, this one (Figure 4.2) is recommended for receive use only unless an antenna tuner is provided. This antenna is not particularly well thoughtof, but is the antenna of choice for many ﬁrst-time receiver owners, or those who live in a difﬁcult antenna erection venue. The radiator element is a 10–40 m long (more or less) piece of wire, preferably in the 12 AWG (%14 SWG), 14 AWG (%16 SWG), or 16 AWG (%19 SWG) size. Smaller sizes can be used, but they are not recommended for structural reasons (they break and fall down too easily).
________________________________ ________________________________ THE HALF-LAMBDA-‘TEE’ ANTENNA ________________________________
The antenna in Figure 4.3 is called the half-lambda-‘tee’ because it is onehalf wavelength long (i.e. j/2) and is a ‘tee’ antenna. The ‘tee’ is seen by the fact that a downlead section is provided, and it comes away from the radiator element at a 908 angle. The length of this downlead section is quarter wavelength. This length differs from the top-hat tee antenna used by many
70 ANTENNA TOOLKIT
short-wave listeners where the radiator is half wavelength or longer and the downlead is a random length of wire. The lengths of these elements are 2L ¼ and L¼ 71:5 meters FMHz 143 meters FMHz
These lengths make the antenna resonant. Examples of the proper lengths for various bands are:
Band 60 m (5 MHz) 31 m (9.75 MHz) 20 m (14.2 MHz) L 14.30 m 7.33 m 5.03 m 2L 28.60 m 14.66 m 10.06 m
An antenna tuner is required for the half-lambda-‘tee’ antenna. Both the inductor and the capacitor should have reactances of 450 ohm, i.e.
MARCONI AND OTHER UNBALANCED ANTENNAS 71
XL ¼ 450 ohms and XC ¼ 450 ohms. Typical values for these components are:
Band 60 m (5 MHz) 31 m (9.75 MHz) 20 m (14.2 MHz) C 65 pF 33 pF 23 pF L 16.0 mH 8.2 mH 5.6 mH
For operation over a wider band, try using variable capacitor and/or inductor elements.
THE ‘QUICK ’N’ DIRTY’ TWIN-LEAD MARCONI __________________________________________________________________ __________________________________________________________________ ANTENNA __________________________________________________________________
The antenna shown in Figure 4.4 is one of the ﬁrst that I ever used (when a mentor, MacIvor Parker, W4II, gave me a roll of twin-lead wire).
I ran this antenna out of a basement window, and attached the top end to an old pine tree, and the middle supported by a 5 Â 10 cm piece of construction lumber nailed rather unceremoniously to a tool shed on the back of the house. Basically a quarter-wavelength ‘round robin’ Marconi, it consists of a 300 or 450 ohm twin-lead transmission line with a length (in meters) of 71.5/ FMHz. Thus, for the 60 m (5 MHz) band the length would be about 14.3 m long overall. The length of the coaxial cable feedline is not critical. The 52 ohm coaxial cable feedline is connected such that the center conductor goes to one conductor of the twin-lead, and the shield goes to the other. This antenna is basically a species of Marconi antenna, so requires a good ground in order to work properly. At the minimum, one of the longer copper or copper-clad steel ground rods should be driven into the earth at
72 ANTENNA TOOLKIT
or very near the feedpoint. The best method is to use both a ground rod and a system of two to eight quarter-wavelength radials as a counterpoise ground. The ‘quick ’n’ dirty’ (or ‘round robin’ as my mentor called it) can be installed at an angle (as shown), although I suspect best performance occurs when the wire is straight. In the version that I had, there was only a small voltage standing wave ratio (VSWR) with one bend that put the vertical and horizontal segments at about a 1208 angle. Note the far end of the twin-lead wire (i.e. the end away from the coaxial cable).
The two conductors of the twin lead are shorted together. I have encountered some confusion over this matter, after all a short is a short, isn’t it? No, because this antenna is a quarter wavelength long, so the impedance reﬂected to the feedpoint by a short at the end is very high. This antenna works better than single-conductor Marconi antennas because regular Marconi antennas suffer from ground losses. By supplying a return line, the radiation resistance is raised from 10 or 15 ohms to something of the order of 40–50 ohms – which makes it a good match to 52 ohm coaxial cable.
_____________________________________ _____________________________________ THE SWALLOW TAIL ANTENNA ______________________________________
Figure 4.5 shows two variants of the swallow tail multiband antenna. The radiator elements in both Figures 4.5A and 4.5B are cut to speciﬁc frequency bands, and are quarter wavelength (Lmeters ¼ 75=FMHz ).
As many as needed to cover the bands of interest may be used, provided that they do not bear a 3:1 frequency ratio. The reason for the constraint is that for all antennas but the resonant one the impedances are so high that connecting them in parallel with each other does not affect the overall feedpoint impedance. However, at the third harmonic, the impedance again drops low and will load down the impedance seen by the transmission line. This is another situation where the results are more profound for amateur radio transmitters than for shortwave receivers, but it is a good idea to avoid it if possible. Besides, the antenna is actually resonant on its third harmonic, so you lose nothing. The version in Figure 4.5A uses a pair of insulating masts, or support structures (roof of a house, tree, etc.) with a rope stretched between them. The quarter-wavelength resonant wires are spread out along the length of the rope, spaced approximately evenly. The version shown in Figure 4.5B uses a large wooden cross-like structure. The antenna wires are connected to the cross-piece at the top end, and to the coaxial cable at the bottom end. This antenna apparently worked well for a fellow who wrote to me recently. He lives in a townhouse community that has a ‘homeowners’ association’ of nit-picking little dictators who like
MARCONI AND OTHER UNBALANCED ANTENNAS 73
74 ANTENNA TOOLKIT
to tell people what they may do with their houses. One of the rules is ‘no outdoor antennas’ of any sort. He erected a mast in his rear garden shaped like a ship’s mast and yardarm, and then convinced the busy-bodies snooping for ‘the committee’ (dread!) that it was somehow nautical, not radio.
________________________________________ ________________________________________ RANDOM LENGTH MARCONI ________________________________________
There are several antennas that qualify as ‘all-time favorites’ with both ham operators and short-wave listeners. In the top three or four is the randomlength wire antenna. This form of antenna consists of whatever length of wire that you happen to have (although there are reasonable limits!) run from the rig or receiver to a convenient elevated support (e.g. a tree, mast, or roof of a building).
Figure 4.6 shows this most basic form of Marconi antenna. Although a receiver owner can connect the random-length radiator wire to the antenna input of the receiver, the transmitter operator most certainly requires an antenna tuning unit (ATU).
Even receiver operators will ﬁnd this antenna a better performer when an ATU is used between the coaxial cable to the receiver and the radiator wire.
MARCONI AND OTHER UNBALANCED ANTENNAS 75
One requirement of this antenna is that it requires a good ground. It is essential that either an earth ground or a counterpoise ground (i.e. a system of radials) be provided. Methods of providing a good grounding system are described in Chapter 13. There are several conditions for this form of antenna: an antenna that is shorter than a quarter wavelength, an antenna that is a quarter wavelength, and longer than a quarter wavelength. The inset in Figure 4.6 shows several options for the antenna tuning unit. At position 1 is a variable inductor. This is used when the antenna is quite short compared to a quarter wavelength. At position 2 is a variable capacitor. It is used when the antenna is longer than a quarter wavelength. When the antenna is exactly a quarter wavelength, then it may be possible to eliminate the antenna tuner. It is also possible to use the L-section couplers at positions 3 and 4. These ATUs combine the inductor and capacitor. These are used to match the feedpoint impedance to the 52 ohm coaxial cable. Many commercial L-section couplers are designed with either switching or shorting straps that allow any of the combinations in the inset to Figure 4.6 to be realized by making some simple changes. Receiver operators who use this antenna are well advised to use an ATU, just like transmitter operators. But there is at least one difference: you should be able to switch the ATU out of the system when you are receiving a frequency outside the limits of the tuner.
_____________________________________________ _____________________________________________ THE WINDOM ANTENNA ______________________________________________
One of the ﬁrst ham radio transmitting antennas that I ever used was not my own, but rather the antenna at my high school station in Arlington, Virginia (K4BGA, Washington-Lee High School).
Although I had owned receive antennas, the K4BGA club station was one of the ﬁrst I had operated. It used an HF Windom antenna running across the roof of the industrial arts building. The Windom antenna (Figure 4.7) has been around since the 1920s. Although Mr Loren Windom is credited with the design, there were actually a number of contributors. Co-workers with Windom at the University of Illinois were John Byrne, E. F. Brooke, and W. L. Everett, and they are properly co-credited. The designation of Windom as the inventor was probably due to the publication of the idea (credited to Windom) in the July 1926 issue of QST magazine. Additional (later) contributions were rendered by G2BI and GM3IAA (Jim MacIntosh).
We will continue the tradition of crediting Loren Windom, with the understanding that others also contributed to this antenna design. The Windom is a roughly half-wavelength antenna that will also work on even harmonics of the fundamental frequency. The basic premise is that the antenna radiation resistance varies from about 50 ohms to about 5000
76 ANTENNA TOOLKIT
ohms, depending upon the selected feedpoint. When fed in the exact center, a current node, the feedpoint impedance will be 50 ohms; similarly, endfeeding the antenna ﬁnds a feedpoint impedance of about 5000 ohms. In Figure 4.7 the feedpoint is tapped away from the center at a point that is about one-third (0.36L) the way from one end, at a point where the impedance is about 600 ohms. The feedline for the basic Windom of Figure 4.7 is an insulated length of wire. Of course, the size of the wire depends on the power level, but I suspect that No. 14 insulated stranded wire will do for most people who run less than 200 W of power (indeed, I would not like to use a Windom at highpower levels because of the ‘radio frequency (RF) in the shack’ problem).
The Windom antenna works well – but with some serious caveats. For example, the antenna has a tendency to put ‘RF in the shack’ because of the fact that it is voltage fed. This is why Windom antenna users (along with those using the random-length Marconi) get little ‘nips’ of RF shock when touching the transmitter chassis, or putting a lip on the microphone. Second, there is some signiﬁcant radiation loss from the feedline. Finally, the antenna works poorly on odd harmonics of the fundamental frequency. The antenna tuning unit can be either a parallel resonant, link-coupled, LC tank circuit or a -network (i.e. high-impedance end toward the antenna).
Note that a good ground should be used with this antenna (note the ground connection at the output of the antenna tuning unit).
This basically means (for most people) a 2–2.5 m ground rod, or a system of radials (see earlier discussion for random-length wire antennas).
MARCONI AND OTHER UNBALANCED ANTENNAS 77
A reasonable compromise Windom, which reduces feedline radiation losses, is shown in Figure 4.8. In this antenna a 4:1 BALUN transformer is placed at the feedpoint, and this in turn is connected to 75 ohm coaxial cable. It is necessary to ensure that the feedpoint is at a current node, so it is placed a quarter wavelength from one end. Also, the antenna on the other side of the BALUN transformer should have a length that is an odd multiple of a quarter wavelength. In the case shown it is three-quarter wavelength long.
_________________________________ ________________________________ QUARTER WAVELENGTH VERTICAL _________________________________
Perhaps the most classic form of Marconi antenna is the quarter wavelength vertical (Figure 4.9).
It consists of a vertical radiator element that is quarter wavelength long, and a system of quarter wavelength radials for the ground system. The length of the radiator element is found from: Lmeters ¼ 75 FMHz
where Lmeters is the length in meters and FMHz is the frequency in megahertz. The radiator element can be made of pipe or wire. The former case, the pipe can be copper or aluminum, although I suspect that aluminum will be the choice of most readers. In the case of a wire radiator, it will be necessary to support the wire up top, so some form of mast or support is necessary. Make the mast or support out of an insulating material such as wood. The radials are somewhat optional if you use it with a ‘good ground’. But in the absence of a ‘good ground’, I would use at least four radials and as many as sixteen. Above sixteen the beneﬁt of adding further radials drops off substantially. Radials should be buried if the vertical is ground mounted (that is to prevent lawsuits over pedestrians tripping over them).
In the case where the
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52-OHM COAX TO RIG OR RECEIVER
vertical is mounted on a mast, off ground, the radials are not optional – they are mandatory. In that case, use four to sixteen radials arranged in a circular pattern around the base of the radiator element. Connect the system so that the radiator element is connected to the coaxial cable center conductor, and the radials are connected to the shield of the coaxial cable. Because the feedpoint impedance of the vertical antenna is 37 ohms, use of 52-ohm coax will result in a VSWR of only 50/37 = 1.35:1. In the case where the vertical has a substantially lower impedance (as low as 2 ohms!), use a broadband transformer between the coaxial cable and the antenna.
________________________________ _______________________________ FOLDED MARCONI ‘TEE’ ANTENNA ________________________________
The antenna shown in Figure 4.10 is popular for receiving and transmitting on the lower bands (5 or 6 cm in diameter) are metal fasteners usable, and even then there is some bad effect. The scheme in Figure 11.12B is used for transmitter ATUs and similar applications where the power is higher. Two or more 5 cm or larger toroids are stacked one on top of the other. Each toroid core is ﬁrst wound with ﬁberglass tape to insulate it from the other core. After the cores are wrapped with tape and stacked on top of each other, a ﬁnal layer of tape can be added to keep the whole assembly stable. The biﬁlar or triﬁlar windings are then laid down on the stacked cores. A number of manufacturers offer BALUNs, BAL–BALs, and UN–UNs, in both voltage and current conﬁgurations. Some are designed to replace the center insulator of an antenna such as the dipole. Others are intended for mounting elsewhere. The transformer in Figure 11.13 is intended for use on vertical antennas. The feedpoint impedance of the quarter-wavelength vertical will vary from something less than 5 ohms to about 37 ohms, depending on the installation. The transformer of Figure 11.13 is an UN–UN that has a single attachment point for 52 ohm coaxial cable to the rig or receiver.
IMPEDANCE MATCHING 217
218 ANTENNA TOOLKIT
The other four coaxial connectors are various load impedances that will be transformed to 52 ohms if that port is used.
__________________________________ __________________________________ COMMERCIAL ANTENNA TUNERS __________________________________
An ‘antenna tuner’ located at the transmitter end of the transmission line will not effect a good impedance match to the antenna, but it can mitigate the effects of a high VSWR on the transmitter. Even a single-band resonant antenna will exhibit a high enough VSWR at the ends of the band to upset the operation of the transmitter that is equipped with automatic load control (that is, all modern solid-state rigs).
Although antenna tuner construction is well within the abilities of most do-it-yourself ‘homebrewers,’ it is also likely that most will opt instead for a ready-built ATU. A number of reasonably priced models are available on the market. Figure 11.14 shows the MFJ-956 receiver S/M/L wave preselector/tuner. This device places a series resonant circuit in series with the antenna downlead before it goes to the receiver. A single variable capacitor (365 pF maximum) is in series with an inductance. Four different switch-selected inductance values are provided (2.5 mH, 330 mH, 22 mH, and 1 mH) to cover the range 150 kHz to 30 MHz. When the inductor switch is in the BYP (‘bypass’) position, the preselector/tuner is effectively out of the circuit, and the antenna–receiver combination works as normal. In the GND (‘ground’) position, the antenna input of the receiver (but not the antenna) is grounded. Tuners for transmitting use are shown in Figures 11.15 and 11.16. The tuner in Figure 11.15 is the MFJ-949E Deluxe Versa Tuner II, while that in Figure 11.16 is the MFJ-986 Differential Tee tuner. These tuners are a bit larger than the standard receiver-only tuner, but are nonetheless just as
IMPEDANCE MATCHING 219
useful to the receiver owner as well. The only thing that will not work for receiver owners is the VSWR metering system. These are based on the same technology as RF wattmeters, so require the output of a radio transmitter for excitation. Both models allow more than one antenna (although only one at a time!) to be switch selected.
Simple antenna instrumentation and measurements
It is very difﬁcult to get an antenna working properly without making some simple measurements. Although it would be nice to make pattern measurements, those are beyond the reach of almost all of us. On the other hand, the ‘I worked [or heard] a guy on the other side of the world’ type of measurement tells us little or nothing. During the peak of the sunspot cycle a breath of hot air on the antenna can be picked up on all continents. There are some things that can be measured, however. For example, the voltage standing wave ratio (VSWR) and the resonant frequency of the antenna are readily accessible. It is also possible to measure the impedance of the antenna feedpoint. Hams can measure the VSWR either with a special VSWR meter (often built into transmitters or antenna tuning units), or by using a radio frequency (RF) wattmeter. By stepping through the band and testing the VSWR at various frequencies, one can draw a VSWR curve (Figure 12.1) that shows how the antenna performs across the band. The resonant frequency is the point where the VSWR dips to a minimum. You can use the resonant frequency to ﬁgure out whether the antenna is too long (resonant frequency lower than the hopedfor design frequency), or too short (resonant frequency above the design frequency).
But resonant frequency and VSWR curves are not the entire story because they do not tell us anything about the impedance presented by the antenna. One cannot get the VSWR to be 1:1 unless the antenna impedance and transmission line impedance are the same. For example, a dipole has a nominal textbook impedance of 73 ohms, so makes a very good match
to 75 ohm coaxial cable. But the actual impedance of a real dipole may vary from a few ohms to more than 100 ohms. For example, if your antenna exhibits a feedpoint impedance of (say) 25 ohms, using 75 ohm coaxial cable to feed it produces a VSWR ¼ 75/25 ¼ 3:1. Not too great. Measuring the feedpoint impedance is therefore quite important to making the antenna work properly. There are any number of instruments on the market that will aid in making antenna measurements. Some of them are quite reasonably priced (or can be built), while others are beyond the reach of all but the most ardent and well-endowed enthusiasts. In this chapter we will look at RF power meters, VSWR meters, dip meters, noise bridges, impedance bridges, and a newer breed of more universal instrument called the SWR analyzer. We will also look at a few accessories that are useful in the measurement or adjustment of radio antennas (e.g. coaxial switches, attenuators, and dummy loads).
__________________________ _________________________ RF POWER METERS AND VSWR METERS __________________________
The RF power meter is designed to indicate the actual value of RF power (in watts).
VSWR meters are similar, but are either calibrated in VSWR units or have a selectable scale that is so calibrated. Some RF wattmeters measure the voltage on the transmission line, while others measure the current ﬂowing in the line. If things are done right, then these approaches work nicely. But sometimes those methods can be fooled. A better system is to measure both the current and the voltage and to display their product. Although this task sounds like a formidible instrumentation job, it is actually the way one of the most popular RF power meters works. The Bird Electronics Model 43 (Figure 12.2) is one of the oldest and most popular professional-grade instruments. It uses a specially designed internal transmission line segment
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with sampling loops inserted in the form of plug-in ‘elements’ (the round object with an arrow in Figure 12.2).
The elements are customized for the RF power level and frequency range. The arrow on the element indicates the direction of the power ﬂow. When the arrow points to the antenna feedline, the instrument is measuring the forward RF power, and if pointed toward the transmitter it is measuring the reﬂected RF power. Although the Model 43 type of meter will provide the RF power measurements more accurately under a wider range of conditions, a ham radio style VSWR meter/RF power meter will work well (and measure accurately) under most ham conditions. In addition, these instruments will provide a measure of the VSWR (with an RF wattmeter you need to calculate the VSWR).
Figure 12.3 shows two typical ham-style VSWR/RF power meters. The instrument in Figure 12.3A is small and simple, but quite effective. It will measure forward and reﬂected power over the entire high-frequency (HF) band, as well as the VSWR. To measure the VSWR, set the fullscale/SWR switch (marked ‘FS’ and ‘SWR’) to the FS position, key the transmitter, and adjust the knob to achieve a full-scale deﬂection of the meter pointer. When the FS/SWR switch is set to the SWR position, it will read the VSWR. These instructions, by the way, ﬁt a large number of different instruments, by many manufacturers, in this same class. The instrument in Figure 12.3B is for the very high/ultrahigh frequency (VHF/UHF) bands. It does not have to be set as does the meter above. The
SIMPLE ANTENNA INSTRUMENTATION AND MEASUREMENTS 223
reason is that it uses the ‘crossed-needles’ method of displaying forward and reverse power. Lines of intersection with the two scales form a species of nomograph that reads out the VSWR from observation of the point where the needles cross. This instrument also includes a peak reading function. Most passive RF power meters inherently measure the average RF power because of the inertia of the meter movement. But with active circuitry, i.e. circuits that require direct-current power to operate, it is possible to sample and hold the peaks of the RF waveform. This meter then becomes more useful for measuring continuous-wave and amplitude-modulated waveforms (including SSB).
Some authorities claim that the peak reading meter is preferred for SSB operation – and I concur.
224 ANTENNA TOOLKIT
_________________________________________________________________ _________________________________________________________________ DIP METERS _________________________________________________________________
The dip meter works because a resonant circuit will draw power from any nearby ﬁeld oscillating at its resonant frequency. An antenna that is cut to a speciﬁc frequency is resonant, so acts like an LC resonant circuit. At one time, these instruments were called grid dip meters because they were vacuum tube (‘valve’) operated. The metering monitored the grid current to detect the sudden decrease when the instrument was coupled to an external resonant circuit. Today, some people still use the archaic term ‘grid dipper’ even though dip meters have not had grids for nearly 30 years. Figure 12.4A shows a typical dip meter. It consists of an LC oscillator in which the inductor is protruding outside the box so that it can be coupled to the circuit under test. A large dial is calibrated for frequency, and a meter monitors the signal level. Coils for additional bands are stored in the carrying case. The frequency dial of the dip meter bears some additional comment. It typically covers a very wide range of frequencies, yet the dip we need is very sharp. The calibration is none too accurate. As a result, it is wise to ﬁnd the dip, and then use a receiver to measure the operating frequency while the dip meter coil is still coupled to the circuit being tested. This operation is a neat trick unless you are possessed with good manual dexterity. It is necessary to measure the frequency while the instrument is coupled because this type of LC oscillator changes frequency markedly when the loading of the coil changes. The proper method for coupling the dip meter to an antenna transmission line is shown in Figure 12.4B. Construct a ‘gimmick’, i.e. a small coil of one to three turns, of sufﬁcient diameter to slip over the coil protruding from the dip meter. Either slip the dipper coil inside the gimmick, or bring it into close proximity. Energy is transferred from the dipper to the gimmick and thus to the antenna. When the dipper is tuned to the resonant frequency of the antenna, a sudden drain is put on the dipper, and the signal level drops sharply. The frequency of this dip tells you the resonant frequency of the antenna. The dipper must be tuned very slowly – very, very slowly – or you will miss the dipping action of the meter. Tuning too fast is the principal reason why many newcomers fail to make the dipper work. To make matters more confusing, the meter needle will drift all over the scale as the instrument is tuned. This occurs because it is natural for LC-tuned oscillators to produce different signal level at different frequencies (generally, but not always, decreasing as frequency increases).
The effect to watch for is a very sharp drop in the meter reading.
SIMPLE ANTENNA INSTRUMENTATION AND MEASUREMENTS 225
_________ ________ ANTENNA IMPEDANCE (OR ‘RESISTANCE’) BRIDGES _________
While it is necessary to measure the VSWR of an antenna to see how well it is working, the VSWR alone is insufﬁcient to optimize performance. If you do not know the feedpoint impedance at resonance (minimum VSWR), then you cannot do much to correct a problem. Several instruments are available that will measure the feedpoint resistance of the antenna (it is called ‘impedance’ in some user manuals, but it is really only the resistive component of
226 ANTENNA TOOLKIT
The instruments in Figure 12.5 are examples of antenna resistance meters. The instrument in Figure 12.5A is based on the Wheatstone bridge circuit. It uses a variable resistor (connected to the thumbwheel resistance dial) as one leg of the bridge, and the antenna
SIMPLE ANTENNA INSTRUMENTATION AND MEASUREMENTS 227
resistance another leg (ﬁxed resistors form the remaining two legs of the classic Wheatstone bridge).
This model is passive, and requires an external signal source. There is an internal ampliﬁer that allows you to use a signal generator, or the ampliﬁer can be bypassed if you wish to drive it with a transmitter signal. The instrument in Figure 12.5B is a little different. It is basically a sensitive VSWR meter with a built-in signal generator. Knowing the system impedance it can thereby calculate the resistance from knowledge of the VSWR.
_______________________________________________________ _______________________________________________________ RF NOISE BRIDGES _______________________________________________________
The noise bridge (Figure 12.6A) is an interesting antenna instrument that is used in conjunction with a radio receiver to measure the antenna feedpoint resistance, approximate the reactance component of the impedance, and ﬁnd the resonant frequency. These instruments use white noise (‘hiss’ is what it sounds like on the receiver speaker) to generate a wide spectrum of radio frequencies. When the receiver is used to monitor the noise, a null is noted at the resonant frequency. Figure 12.6B shows how the noise bridge is connected between the antenna transmission line and the antenna input connector of the receiver. The line between the bridge and the receiver can be any length, but it is wise
228 ANTENNA TOOLKIT
to keep it short. One reader of my columns wrote in to take exception to the ‘short as possible’ requirement because it is not strictly necessary in a perfect world. My own experience says differently, especially at higher frequencies where the line is a signiﬁcant fraction of a wavelength. A meter or so should not affect any HF measurement, however. The line to the antenna should either be as short as possible (preferably zero length), but in the real world, where one has to be more practical than theoretically pure, it is sufﬁcient to make the transmission line an integer multiple of an electrical half wavelength long (the physical length is shorter than the free space half wavelength because of the velocity factor).
The impedance hanging on the far end of the half wavelength Â N line is repeated every half wavelength. A consequence of this effect is that the impedance measured at the transmitter or receiver end is the same as at the antenna end. To use the noise bridge, set the resistance (R) and reactance (X) controls to mid-scale ðX ¼ 0Þ. Adjust the receiver frequency to the expected resonant frequency. At this point you can do either of two different measurement schemes.
First, you can set the R-control of the noise bridge to the desired antenna impedance, and then tune the receiver until a dip in the noise occurs (as heard in the output or noted on the S-meter).
For example, suppose you have a dipole cut for 11.75 MHz, and installed such that the expected feedpoint impedance should be around 70 ohms. It is fed with a half wavelength of polyfoam dielectric 75 ohm coaxial cable (V ¼ 0:80).
The cable length is ð150Þð0:8Þ=11:75 ¼ 10:2 m. The noise bridge X-control is set to 0, and the resistance control is set to the letter that represents 70 ohms (given in the calibration manual).
This resistance setting can also be determined experimentally by connecting a 70 ohm resistor across the antenna terminal and adjusting the R-control for the minimum noise. When the noise bridge is set, then adjust the receiver for minimum noise. The frequency at which the dip occurs is the actual resonant frequency. I used this method to investigate a vertical cut for 21.25 MHz, only to ﬁnd the actual resonance was 19.2 MHz, which explained the high VSWR.
The other method is to set the receiver to the design resonant frequency of the antenna. You then adjust the R-control for a dip in the noise level. The value of the resistance is the feedpoint impedance of the antenna. You can either match that impedance, or adjust the antenna to bring the actual resonant frequency closer to the design frequency.
SIMPLE ANTENNA INSTRUMENTATION AND MEASUREMENTS 229
_______________________________________________________ _______________________________________________________ VSWR ANALYZERS _______________________________________________________
The basic premise in this section is that the instruments used must be accessible to people who do not have a ham radio or professional radio operator’s license. Some of the instruments discussed above meet that requirement, but a relatively new breed of instrument called the SWR analyzer provides a lot of capability to the short-wave listener, scanner operator, and ham radio operator alike. It uses a low-power RF signal generator and some clever circuitry to measure the VSWR of the antenna. One model also measures the feedpoint resistance. The simple version shown in Figure 12.7 is for the low VHF band (up to and including the 6 m ham band).
It is a hand-held, battery-powered instrument. The meter reads the VSWR of the antenna at the frequency set by the TUNE dial. A somewhat more sophisticated instrument, the MFJ Enterprises Model MFJ-259, is shown in Figure 12.8. The front panel has two meters, SWR and RESISTANCE. The SWR meter is calibrated up to 3:1, with a little uncalibrated scale to indicate higher SWRs. The RESISTANCE meter is calibrated from 0 to 500 ohms, which is consistent with the SWR range. Two controls on the front panel are TUNE and FREQUENCY (MHz) (a
230 ANTENNA TOOLKIT
The MFJ-259 has a digital frequency meter to measure the operating frequency of the internal oscillator. This frequency counter can also be used to measure the frequency of external signal sources (DO NOT connect the counter to the output of a transmitter – the instrument will be destroyed).
The top end of the MFJ-259 has a number of controls and connectors. An SO-239 ‘UHF’ style coaxial connector is provided for the antenna connection. A BNC coaxial connector is provided to apply an external signal to the frequency counter, while a pushbutton INPUT switch is available to switch the counter from internal to external signal sources. Another pushbutton switch is used to set the gate timing of the counter (a red LED on the front panel blinks every time the gate is triggered).
The tuning is from 1.8 to 174 MHz, while the counter will measure up to 200 MHz. The MFJ-259 will work from an external 12 V DC source, or from an internal battery pack consisting of eight size-AA standard cells. MFJ recommends that either alkaline or rechargeable batteries, rather than ordinary zinc–carbon cells, be used in order to reduce the possibility of leakage that could damage the instrument (this is good practice in all battery-powered instruments).
I have a homebrewed battery pack that uses eight size-D nickel cadmium batteries (4 A-h rating) that can be recharged from a 12 V DC power supply, and it works well with the MFJ-259.
SIMPLE ANTENNA INSTRUMENTATION AND MEASUREMENTS 231
Unlike many lesser SWR meters, this instrument is not fooled by antennas that have impedances consisting of both resistance and reactance elements. An example in the manual demonstrates an impedance of 25 + j25 ohms (i.e. R is 25 ohms and reactance, X, is also 25 ohms).
When connected to a 50 ohm load one might be tempted to think the VSWR is 1:1, and some cheaper meters will so indicate. But the actual SWR is 2.6:1, which is what the MFJ-259 will read. The resistance measurement assumes a resistive load (i.e. the measurement is made at the resonant frequency of the antenna), and is referenced to 50 ohms. The VSWR and resistance measurements should be consistent with each other. If the VSWR is 2:1, then the resistance should be either 100 ohms (100/50 ¼ 2:1) or 25 ohms (50/25 ¼ 2:1).
If the resistance is not consistent with the VSWR reading, then you should assume that the impedance has a signiﬁcant reactive component, and take steps to tune it out. In addition to antenna measurements, the MFJ-259 is equipped to measure a wide variety of other things as well. It will measure the velocity factor of transmission line, help in tuning or adjusting matching stubs or matching networks, measure capacitance or inductance, and measure the resonant frequency of LC networks. Figure 12.9 shows an MFJ-249 meter (similar to the MFJ-259, but without the resistance measurement) equipped with the MFJ-66 dip meter adapter. It can be used to make the MFJ-249 or MFJ-259 work in the same manner as a dip meter. Using this adapter allows you to measure the resonant frequency of tank circuits using the dipper approach, as well as to measure things such as the coefﬁcient of coupling between two LC circuits, transformers, and other radio circuits.
_______________________________ _______________________________ DUMMY LOADS (ARTIFICIAL AERIAL) ________________________________
The dummy load, or artiﬁcial aerial as it is also called, is a resistor used to simulate an antenna when adjusting a transmitter or other radio apparatus. These devices consist of a non-inductive, 50 ohm resistor inside a shielded enclosure. The dummy load offers several beneﬁts over regular antennas. First, it is a constant resistance over the entire frequency range, and, second, it does not present any appreciable reactance. Third, and perhaps most important, the dummy load provides the ability to conduct tests off the air where you will not cause television interference, broadcast interference, or interference to other users of the test frequency that you select. Besides, it is illegal and just plain rude to radiate when you do not have to! One use for dummy loads is to adjust antenna tuners. You can connect the dummy load to the antenna output of the tuner, and then adjust the
232 ANTENNA TOOLKIT
controls for best VSWR at a number of frequencies. By recording the knob settings, you can ‘rough in’ the antenna tuner when changing frequency. Several types of dummy load are shown in Figures 12.10 and 12.11. The version in Figure 12.10 is a small 5 W HF model intended for measuring the output of CB transmitters. I have used this particular dummy load for adjusting a wide range of RF projects and instruments over the years. To make accurate power measurements, the dummy load is placed at the ANT connector of the RF power meter, and the transmitter connected to the other end. The power reading is not obscured by transmission line effects or the reactance that a real antenna might present. The dummy loads shown in Figure 12.11 are intended for ham radio power levels. The model in Figure 12.11A is air cooled, and operates at RF power levels up to 1500 W, over a range of 1–650 MHz. The version in Figure 12.11B is oil cooled, and will handle up to 1000 W of RF power.
_____________________________________________________ _____________________________________________________ COAXIAL SWITCHES _____________________________________________________
A coaxial switch (Figure 12.12) is used to allow a receiver or ham radio set to use any of several antennas (models with up to 16 ports are available, but this one is a two-port model).
The common connector is for the receiver or transmitter, while the two antennas are connector to the ‘A’ and ‘B’ ports,
SIMPLE ANTENNA INSTRUMENTATION AND MEASUREMENTS 233
234 ANTENNA TOOLKIT
respectively. Alternatively, one can turn the switch around backwards (it is bidirectional, after all), and use the same antenna on two different receivers or transmitters. The use of the coaxial switch in antenna measurement is in comparing the antenna being tested with either another antenna or a dummy load. The kind of off-the-air checks that amateurs and short-wave listeners can make are notoriously inaccurate, but can be made a lot more useful by making comparisons with known antennas. For example, a friend of mine, the late Johnnie H. Thorne (K4NFU/5) had an antenna farm in Texas (and it did seem that he grew antennas, judging from the number he had).
He kept a standard dipole, optimally installed and cut for 20 m, and made all of his test designs for the same frequency. He would compare new designs to the dipole by switching back and forth while monitoring the signal strength on the receiver S-meter. He could also compare two different antennas by comparing them against each other or against the dipole.
_____________________________________________________ _____________________________________________________ STEP ATTENUATORS _____________________________________________________
The step attenuator (Figure 12.13) is a precision instrument that provides highly accurate levels of attenuation in steps of 1 dB, 2 dB, 3 dB, 5 dB, 10 dBV, and one or more 20 dB settings. You can use these to calibrate instruments, or measure signal levels. One use is to make comparison measurements of two signal sources, two antennas, etc. For example, suppose you want to measure the gain of a new antenna relative to a dipole (assuming that both are cut for the same frequency).
You would pick or provide a distant signal on the resonant frequency of the antenna. The attenuator is inserted into the feedline path of the test antenna (Figure 12.14).
The signal strength is then measured with the dipole in the
SIMPLE ANTENNA INSTRUMENTATION AND MEASUREMENTS 235
circuit (see the coaxial switch discussion above), and the reading of the S-meter noted. If you have an RF sensitivity control, then adjust it to a convenient point on the meter dial, like S-9 or some other marked spot. Next, cut to the new antenna and note the signal strength. If it increased, then select the various levels of attenuation until the new signal level is the same as the initial signal level. The set level of attenuation is equivalent to the gain difference between the antennas. If the new signal level is lower than the initial signal level, then the new antenna has less gain than the old antenna. Put the attenuator in line with the comparison antenna, rather than the test antenna.
Getting a ‘good ground’
Because this book is about radio antennas it is necessary to also spend some time on the topic of antenna grounds. The effectiveness and safety of a radio antenna often hangs on whether or not it has a good radio frequency (RF) ground. Poor grounds cause most forms of antenna to operate at less than best efﬁciency (sometimes a whole lot less).
In fact, when transmitting it is possible to burn up between 50 and 90% of your RF power heating the ground losses under the antenna, instead of propagating into the air. Ground resistances can vary from very low values of 5 ohms, up to more than 100 ohms (5–30 ohms is a frequently quoted range).
RF power is dissipated in the ground resistance, so is not available for radiation. The factors that affect the ground resistance include the conductivity of the ground, its chemical composition, and its water content. The ideal ground depth is rarely right on the surface, and, depending on the local water table level, might be a couple of meters or so below the surface. It is common practice among some amateur radio operators to use the building electrical ground wiring for the RF antenna ground of their station. Neglecting to install an outdoor ground that will properly do the job, they opt instead for a single connection to the grounded ‘third wire’ in a nearby electrical outlet. Besides being dangerous to work with, unless you know what you are doing, it is also a very poor RF ground. It is too long for even the low RF bands, and radiates RF around the house in large quantity. Stations that use the household electrical wiring as the radio ground tend to cause television interference, and other electromagnetic interference both in their own building and in nearby buildings.
Other people use the plumbing piping in the house, but this is also a problem. While the cold water pipe entering older houses is metal, and is buried for quite a distance underground, it is also a single conductor, and is often made of a metal that is not optimal for radio work. Also, in modern buildings, the main water service and the distribution pipes in the plumbing system of the building may well not be metal at all, but rather PVC or some other insulating synthetic material. Even if you get a metal water pipe, if it is the hot water pipe then no effective ground exists. There is also a very severe danger of hooking your ground to the gas service line, which could cause a ﬁre or explosion. In the USA, gas pipes are usually painted black, or are unpainted, but enough water pipes also ﬁt this description that it is good practice to avoid the pipes in a building when searching for a radio grounding solution.
_______________ _______________ TRADITIONAL RADIO GROUNDING SOLUTIONS ________________
The goal in making a radio ground is to provide a low resistance path to the earth for radio signals. An additional goal is to make a path to the earth for lightning that strikes the antenna, or even nearby (see Chapter 2 for the use of lightning arrestors in antenna lines).
We can reduce the ground resistance by either altering the composition of the earth surrounding the ground point, or by using a large surface area conductor as the ground point. Figure 13.1 shows the traditional ground rod used by small radio stations, including amateur and receive-only stations. Use either a copper (or copper-clad steel) rod at least 2–3 m long. Electrical supply houses, as well as amateur radio and communications equipment suppliers, also sell these ground rods. Do not use the non-clad steel types sold by some electrical supply houses. They are usable by electricians when making a service entrance ground in your home or workplace, but RF applications require the low skin resistance of the copper-clad variety. The rod need not be all copper, because of the skin effect forcing the RF current to ﬂow only on the outer surface of the rod. Try to use a 2.5 m or longer rod if at all possible, because it will work better than the shorter kind. Do not bother with the small television antenna 100–150 cm ground rods; they are next to useless for high-frequency (HF) radio stations. Drive the ground rod into the earth until only a few centimeters remain above the surface. Connect a heavy gage ground wire from your station to the ground rod (some people use heavy braid or the sort once used for the return connection from the battery to the body of an automobile; an additional source is salvaged braid from the heavier gages of coaxial cable).
The ground wire should be as short as possible. Furthermore, it should be a low-inductance conductor. Use either heavy braid (or the outer conductor stripped from RG-8 or RG-11 coaxial cable), or sheet copper. You can buy
238 ANTENNA TOOLKIT
rolls of sheet copper from metal distributors in widths from 10 cm up to about 50 cm. Some amateurs prefer to use 17.5 cm-wide rooﬁng foil that is rated at a weight of 1.5 kg/m. Sweat solder the ground wire to the rod. You can get away with using mechanical connections like the electricians use, but will eventually have to service the installation when corrosion takes its toll. I prefer to use soldered connections, and then cover the joint with either petroleum jelly or acrylic spray lacquer. Another alternative is to use a copper plumbing pipe as the ground rod. The pipe can be purchased in 2–5 m lengths from plumbing supply shops or hardware stores. The pipe selected should be 19 mm in diameter or larger. Some people report using up to 50 mm diameter pipe for this application. The surface area of the hollow pipe is greater than that of a solid rod of the same diameter. Because of certain current ﬂow geometries in the system, however, the ground resistance is not half the resistance of a rod the same diameter, but is nonetheless lower. Unlike the copper-clad steel rod, the copper pipe has no compression strength and will deform when hit with a hammer or other driving tool.
GETTING A ‘GOOD GROUND’
To overcome this problem (see Figure 13.2) you can use a garden hose as a ‘water drill’ to sink the ground rod. Sweat solder a ‘tee’-joint on the top end of the pipe, and then sweat solder a faucet or spigot ﬁtting that matches the garden hose end on one end of the ‘tee’-joint. Cap off the other port of the ‘tee’-joint. Use the ‘tee’-joint as a handle to drive the pipe into the ground. When water pressure is applied, the pipe will sink slowly into the ground as you apply a downward pressure on the tee-handle. In some cases, the pipe will slip into the ground easily, requiring only a few minutes. In other cases, where the soil is hard or has a heavy clay content, it will take considerably greater effort and more time. When you ﬁnish the task, turn off the water and remove the garden hose. Some people also recommend unsoldering and removing the ‘tee’-joint.
240 ANTENNA TOOLKIT
______________________________________ ______________________________________ ALTERING SOIL CONDUCTIVITY ______________________________________
The conductivity of the soil determines how well, or how poorly, it conducts electrical current (Table 13.1).
Moist soil over a brackish water dome conducts best. In the USA, the southern swamps make better radio station locations than most of the rest of the country; and the sand of the western deserts make the worst. In Arizona, for example, one ﬁnds that few buildings have cellers or basements. The reason is that there is a thick calcium-based layer (called ‘caleche’) a meter or two down that resists even mechanized diggers. To build homes in that sand they have to drill holes in the caleche and then sink pylons down to bedrock. The same caleche also prevents water from sinking into the sand, so it ﬂows off the land in natural channels in the desert. The area is rather dangerous for out-of-state campers who do not understand that those nice ﬂat, areas at the bottom of gulleys have a habit of ﬁlling in seconds if there is a distant thunder storm in the mountains. They look up, and to their horror see a 3–5 m high wall of water rushing toward them at 50 km/h! During the late summer months, Arizona has a ‘monsoon’ season when it rains hard and ﬂoods every day (90% of the state’s annual rainfall comes in a 6-week period from sometime in July to mid-September).
Evaporation is so rapid that an hour after the rain stops, there is little evidence of the rainfall except in the gulleys. If you have never felt hot rain, then visit southern Arizona in August – but be prepared for temperatures around 438C in the shade and 808C on the front seat of an automobile if you are so unwise as to leave all the windows rolled up. Figure 13.3 shows a method for reducing the soil electrical resistance by treating with one of three chemicals: copper sulfate, magnesium sulfate or common rock salt. Rock salt is one of several salt materials used for snow and ice melting in snow-prone areas. If you cannot locate rock salt in a hardware store, then look for a store that sells ice cream making supplies. Rock salt is a principal ingredient in the process (but not the product).
TABLE 13.1 Type of soil Salt water Fresh water Pastoral hills Marshy, wooded Rocky hills Sandy Cities Dielectric constant 81 80 14–20 12 12–14 10 3–5 Conductivity (siemans/m) 5 0.001 0.03–0.01 0.0075 10 0.002 0.001 Relative quality Best Very poor Very good Average/poor Poor Poor Very poor
GETTING A ‘GOOD GROUND’
Figure 13.3A shows a side sectioned view of a circular slit trench method of applying the chemical treatment to reduce ground resistance. WARNING: before using this method, you need to determine whether or not you really want to do it. The method involves putting conductive chemicals into the ground. Make sure that local, state or national environmental regulations do not prohibit the practice. Also, if you ever want to use the spot to grow vegetables or ﬂowers – or even grass – it will prove difﬁcult because the chemicals essentially poison the soil for several years. Fortunately, they are water-soluble, so the effect will not last forever. Dig a 15–30 cm deep circular trench, about 30–60 cm from the ground rod. Fill that trench with a 10–12 cm layer of rock salt, magnesium sulfate, or copper sulfate (use only one).
Water the trench well for about 15 minutes. Cover the remaining depth with soil removed when you have dug the trench. If you use about 100 kg of material, the treatment will need to be repeated every 24–36 months, depending upon the local rainfall and soil composition. Figure 13.3B shows an alternative method: the saltpipe. Use either copper or PVC plumbing pipe, up to 10 cm in diameter (although 4–8 cm is easier to work).
The overall length of the pipe should be at least 50–150 cm. Although longer pipes are useful, they are also difﬁcult to install. Drill a large number of small holes in the pipe (no hole over 10 mm), sparing only the end that will be above the surface. Cap off both ends.
242 ANTENNA TOOLKIT
FIGURE 13.3 (continued )
Install several saltpipes in a pattern around the ground rod. Installation is best accomplished using a fencepost hole digger. Drop the saltpipe into the hole and backﬁll with water. Remove the top endcap and hose down for about 15 minutes, or until all the material is gone. Reﬁll the pipe occasionally to account for the salt leaching out. When the chemical is completely leeched out of the pipe, reﬁll the pipe and recap the top end. Leave the pipe in place. Be certain of local environmental laws before using the above methods.
_____________________________________________ _____________________________________________ MULTIPLE GROUND RODS ______________________________________________
The key to a low-resistance ground is the surface area in contact with the soil. One means for gaining surface area, and thereby reducing ground
GETTING A ‘GOOD GROUND’ 243
resistance, is to use multiple ground rods. Figure 13.4 shows the use of four ground rods in the same system. The rods are placed 30–100 cm apart for low- and medium-power levels, and perhaps more than 100 cm for higher amateur radio power levels. An electrical connection is made between the rods on the surface using either copper stripping or copper braid. The connections are sweat soldered in the manner described above, with the feedpoint at the center rod.
_______________________ ______________________ RADIALS AND COUNTERPOISE GROUNDS _______________________
The effectiveness of the ground system is enhanced substantially by the use of radials either above ground, on the surface, or buried under the surface. Figure 13.5 shows a vertical antenna with a set of ground radials. It is not unreasonable to use both radials and a ground rod. Vertical antennas are relatively ineffective unless provided with a good ground system, and for most installations that requirement is best met through a system of ground radials. An effective system of radials requires a large number of radials. Although as few as two quarter-wavelength resonant radials will provide
244 ANTENNA TOOLKIT
a signiﬁcant improvement, the best performance is to use more. Broadcasters in the AM band (550–1640 kHz) are advised to use 120 halfwavelength radials. Installing more than 120 radials is both expensive and time-consuming, but does not provide any substantial improvement. For amateur and small commercial stations, a minimum of 16 quarter-wavelength radials should be used. Above ground, the use of insulated wire is recommended to reduce the possibility of anyone receiving RF burn if it is accidentally touched while transmitting. Although some sources claim that any size wire from AWG 26 up to AWG 10 (SWG 0–12) can be used, it is best to use larger sizes in that range (i.e. AWG 14 through AWG 10 (SWG 16–12)).
Either solid or stranded wire can be used. When viewed from above, the radials should be laid out in a symmetrical pattern around the antenna. This coverage provides both the lowest resistance and the best radiation pattern for the antenna. Solder all radials together at a common point, which might be the ground or mounting rod used to support the vertical antenna. If your antenna is ground mounted, or if the radials are used with the station ground, then use a system such as that in Figure 13.6 to make the connections. A clamp on the ground rod is used to bind together the rod, the radials and the ground wire from the receiver or transmitter. The radials
GETTING A ‘GOOD GROUND’ 245
should be buried a few centimeters below the surface. Some people report using the tip of a spade or shovel to dig a very thin trench into which the wire can be pushed. Be careful to get the radial wire deep enough. Ordinary soil dynamics can make the wire rise to the surface within a year or so.
_________________________________________ ________________________________________ TUNING THE GROUND WIRE _________________________________________
An alternative that some operators use is the ground wire tuner. These instruments insert a series-tuned inductor–capacitor (LC network) in series with the ground line. These devices are usually called artiﬁcial ground tuners, and are inserted into the ground line of the station (Figure 13.7).
Ground line tuners are especially recommended for cases where the wire to the ground is too long, in the case of single-wire antennas (e.g. Windom antennas or random-length wires), or anytime the tuning seems peculiar, but the antenna tuning unit works well into a dummy load. The purpose of the unit is to tune out any inductive or capacitive reactance on the line. Transmitter operators adjust the ground line tuner for maximum ground current at the operating frequency, although receiver operators will have to experiment a bit to ﬁnd the correct settings. MFJ Enterprizes, Inc. (PO Box 494, Mississippi State, MS 39762, USA) makes several such devices. The version shown in Figure 13.8 is a stand-alone unit that contains only the ground current meter, inductor, and capacitor. The instrument in Figure 13.9 is a combination unit that includes both the ground line tuner and antenna tuner in one box.
246 ANTENNA TOOLKIT
GETTING A ‘GOOD GROUND’
__________________________________________________________________ _________________________________________________________________ WARNING __________________________________________________________________
Grounds for radio installations are intended for two purposes: making the radio antenna work better and lightning protection. Be absolutely sure to check with local authorities over the minimum requirements for grounding antenna structures – and then doing more than is minimally required. Not only is it possible to get into trouble with the authorities with a poor ground, you may also ﬁnd that your homeowners and liability insurance is not valid if an uninspected installation is made. My own homeowners policy states that the insurance coverage is not in effect if an antenna is erected that meets the minimum requirements for a mechanical and electrical inspection by the county government but does not have a valid inspection certiﬁcate. Check with your local government and insurer before building any antenna or ground system.
_____________________________________________________________ _____________________________________________________________ CONCLUSION _____________________________________________________________
Now that you have learned how to build safe and efﬁcient antennas and grounds, in the words of Captain Piccard ‘make it so.’
Air core frame loops, 179–81 Angle of radiation, 16, 41 Antenna patterns, 38–41 Antenna tuner/tuning unit (ATU), 75–6, 81, 114, 205, 208 Array antennas, 153 Artiﬁcial aerials see Dummy loads Artiﬁcial ground tuners, 247 Aussie doublet antennas, 109–10 Automatic load control (ALC), 204 BALUN transformers, 65, 67, 78, 91–2, 153, 213–18 erection, 47 use on receiving antennas, 108 Beam antennas, 133–4 impedance matching, 201–202 VHF/UHF quad, 152 Beamwidth, antenna, 41 Beverage antennas, 82 Bi-square large loop antennas, 151 Biﬁlar winding, transformer, 216–17
Bobtail curtain array, 168–69 Thorne, 169–70 Box loops see Air core frame loops Broad-band transmission line transformers, 213 Broadside arrays, 153, 160–1 Bruce array, 165–6 Characteristic impedance, 54 Coaxial cable, 57–8 Coaxial switches, 233, 235 Collinear array, 153, 159 stacked, 174 two-element, 157–60 Connections: balanced input schemes, 59 transmission line to receiver/ transmitter, 58–60 wire joints, 61 wires to antennas/supports, 62–5 Construction hints, 41 Copper-clad steel wires, 51–2 Critical depth, 52
Curtain arrays, 164–6, 168–70
D-layer (ionosphere), 8 Decibel (unit), 25 Dellinger fade, 13–14 Delta loop antennas, 135–6 double, 143–4 multiband switchable, 139–43 Depth of penetration, 52 Depth winding, wire loops, 179 Dip meters, 225 Dipole antennas, 87 Aussie doublet, 109-10 bow-tie, 101–102 capacitor-tuned wide-band, 110 delta-fed, 100–101 folded, 95–7 half-wavelength, 88–91, 100–101 inductor-loaded shortened, 111–12 inverted vee, 98 multiband, 112–13 multiband half-wavelength, 92–4 multiband tuned, 113 radiation patterns, 87–8 resonant, 113 sloping, 99 vertical, 99–100 wide-band, 108–9 wide band half-wavelength, 94, 101 wider bandwidth folded, 102 zig-zag, 122–3 Directivity, antenna, 36–7 Diurnal variation, ionosphere, 10 Double-delta loop antenna, 143–4 Doublet antennas, 87 Dummy loads, 232–3
Effective radiated power (ERP), 36 Eight-element broadside array, 161 Electrical (E) ﬁelds, 24–5 Electrical safety, 42 Electromagnetic radiation, 11–12 waves, 24–5 End-ﬁre arrays, 153 Erection methods, 44–7, 44–8 EWE antennas: basic, 81–2 bidirectional, 85 Koontz, 82–4 reversible, 85 F-layer (ionosphere), 9 Feedpoint impedance, measurement, 222, 226–28 Feedpoint resistance, 226 Flat-top beam see W8JK array Folded dipole antennas, 95–7 X-array, 171 Four-element broadside array, 160–1 Franklin array, 159 G5RV antennas, 106 physical structure, 107 Gain, antenna, 36–7 Gamma match, 201, 202 Great circle paths, 15 Grid dip meters, 225 Ground: alternative symbols, 19–20 importance of good, 237–8 radial system, 245 traditional solutions, 238–41 Ground rods, multiple, 244 Ground waves, 4 propagation, 5–6 Ground wire tuner, 246–7 Half-lambda tee antennas, 70–2
E-layer (ionosphere), 8 Earth see Ground Earth’s atmosphere, 2–3 Effective height, loop arrays, 180
Hertzian antennas, 27, 87 shortened radiator, 124–7 HF band lengths, 158 Impedance bridges, 226–28 Impedance matching, 54, 203–205 beam antennas, 201–202 LC networks, 205–8 Impedance matching stubs, 153, 210–13 see also Q-section Inductor-loaded antennas, 119 Insulators, center, 64–5 Inverse square law, 23 IONCAP programs, 1 Ionization, atmosphere, 7 Ionosphere, 2, 7 11 year cycle, 9, 11 27 day cycle, 9, 10 disturbances, 12–14 diurnal cycle, 9, 10 propagation, 4–5, 7–9 seasonal cycle, 9, 10 storms, 14 use for radio communication, 15–16 variations, 9–12 Isothermal region, 2 Isotropic radiators, 37–8 K-factor, wave, 17–18 Large loop antennas, 129, 176 Large-scale reﬂector array, 175 Law of reciprocity, 27 Lazy-H array, 161–4 LC matching networks, 205–08 Lightning arrestors, 66–7 Limited space antennas, 119–22 radial layout, 128 Line of sight, 6 Linearly loaded tee antennas, 123 Long-wire antennas, 87
Loop antennas, 129 bi-square, 151 delta, 135–6 diamond, 148 half-delta, 148–9 half-wavelength, 144–5 inductor-loaded, 146–8 preampliﬁers, 193 quad, 129–34 sharpening, 193 shielded, 186–91 testing, 191 tuning schemes, 181–4 two-band compact, 150 using, 191 Low-Noise RX antennas, 83, 84 Magnetic (H) ﬁelds, 24–5 Marconi antennas, 27, 69 basic tee, 70 folded tee, 80–1 quarter wavelength vertical, 78–79 random length, 75–6 right-angle, 124 twin-lead, 73–4 Matching stubs see Impedance matching stubs Mechanical integrity, installation, 43–4 MFJ-249 analyzer, 232 MFJ-259 analyzer, 230–42 MFJ-949E tuner, 219 MFJ-956 tuner, 219 MFJ-986 tuner, 219 Multiband tuned doublet antennas, 103, 105 Multipath phenomena, 6 Noise bridges, 228–29 Parasitic elements, 133 Peak reading meter, 224 Phase reversal stubs, 159
Phased vertical arrays, 171–2 Planar winding, wire loops, 179 Polarization, signal, 24–5, 26 Positive ions, 2 Propagation paths, 4–5 anomalies, 1–2, 3 Q-multiplier, 193–4 Q-section (matching stub), 133, 135–6, 208–10 Quad loop antennas, 129–33 beam version, 133–4 ﬁxed, 131 rotatable, 131, 132 Quarter-wavelength matching sections see Q-section Quarter-wavelength vertical antennas, 78–9 Radial layout, for limited space, 128 Radio signal, 24 Radio waves, 20–23 frequencey, 22–3 wavelength, 22–3 Receiver-antenna interactions, 28–9 Reciprocity law, 27 Resistance bridges, 226–8 Resonant frequency (RF): measurement, 221 power meters, 222–4 Roof-mounted antennas, 120–2 Room loop antennas, 136–9 Safety precautions, 68 altering ground resistance, 242–4 electrical 42, 68 ground systems, 247–8 Seasonal cycle, 10 Shielded loop antennas, 186–91 SI units, viii–ix Signal-to-noise ratio, 25 Six-shooter array, 166–8
Skin effect, 52 Skip communications, 7 Skip zone, 6 Sky wave, 7 Small loop antennas, 176–8 Soil conductivity, altering, 238–44 Solar disturbances, 10–12 charged particles, 11–12 Solar energy, 11–12 Solar ﬂux index (SFI), 12 Solar noise, 12 Solid wires, 49 Space waves, 4, 5–6 Split frequencies, 178 Spoiler loop, 192 Sporadic-E layer, propagation, 8, 13 Sports fan’s loop antenna, 186 Square hobby board loop antennas, 184–6 Stacked collinear arrays, 174 Standing wave ration (SWR), 29–33, 203 measuring, 33, 35–6 Standing waves, 203 Step attenuators, 235–6 Sterba curtain array, 164–5 Stranded wires, 49–50 Stratosphere, 2 Subrefraction, 18 Sudden ionospheric disturbances (SIDs), 13–14 Sunspots, 12 Super-refraction, 17–18 Surface waves, 4, 5–6 Surge impedance, 54 Swallow tail antennas, 73–5 Symbols, 19–20 Thorne array bobtail curtain (TABC), 169–70 Tilted, center-fed terminated, folded dipole (TCFTFD), 114–17
Transformer loop antennas, 180–1 Transmission lines, 54 coaxial cable, 57–8 parallel open-wire, 54, 56 twin-lead, 56–7 Trap dipole antenna, 112–14 Triﬁlar winding, transformer, 216–17 Troposphere, 2 propagation, 4 Tuners: commercial, 219–20 see also Antenna tuner/tuning unit (ATU) Tuning schemes, loop antennas, 181–4 Twin-lead lines, 56–7 Two-element collinear array, 157–60 Varacators, 183 Velocity factor (VF), conductor, 53–4, 58 Vertical antennas, 119 VHF/UHF band lengths, 159 VOACAP program, 1 Voltage standing wave ratio (VSWR), 32–3, 34–5, 203, 204–5 analyzers, 230–2
(Voltage standing wave ratio (VSWR) continued) measurement, 221, 222–4 performance curves, 221, 222 W8JK array, 172–4 Waves, see Radio waves Windom antennas, 76–8 Wire array antennas, 153–6 Wire joints, 61 Wires: basic types, 49–50 connections to antennas/ supports, 62–5 copper-clad, 51–2 size to length problems, 52–4 sizes, 50 Wolf number, 12 Yagi beam antennas, 134, 195–6 four-element array, 199–200 six-element array, 200–201 three-element array, 198 two-element array, 196–7 Zepp antennas, 81 double extended, 103, 104, 154 Zig-zag dipoles, 122–3 Zurich smoothed sunspot number, 12