RADIO ANTENNAS AND PROPAGATION This Page Intentionally Left Blank RADIO ANTENNAS AND PROPAGATION WILLIAM GOSLING Newnes OXFORD BOSTON JOHANNESBURG MELBOURN~ NF_.WDELHI SINGAPORE 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 1998 Transferred to digital printing 2004 0 William Gosling 1998 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 WlP 9HE. 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 3741 2 Library of Congress Cataloging in Publication Data A catalogue record for this book is available from the Library of Congress Typeset by David Gregson Associates, Bales, Suffolk CONTENTS Preface vii 1 introduction 1 Part One: Antennas 17 2 Antennas: getting started 19 3 The inescapable dipole 26 4 Antenna arrays 52 5 Parasitic arrays 72 6 Antennas using conducting surfaces 81 7 Wide-band antennas 103 8 Odds and ends 116 9 Microwave antennas 133 Part Two: Propagation 151 10 Elements of propagation 153 11 The atmosphere 165 12 At ground level 177 13 The long haul 202 Appendix: Feeders 244 Further reading 254 Index 255 This Page Intentionally Left Blank PREFACE Textbooks on radio antennas and propagation have changed little over the last 50 years. Invariably they base themselves on the famous electromagnetic equations described by James Clerk Max- well, a great nineteenth-century genius of theoretical physics (Torrance, 1982). Maxwell's equations brilliantly encompassed all the electromagnetic phenomena known by his time (except photo- electric long-wave cut-off, which remained a mystery). To this day, the classic textbooks on antennas and propagation treat the subject as a series of solutions of Maxwell's equations fitted to practical situations. Doing this turns out to be far from easy in all but a very few cases. Even so, by ingenuity and approximation, solutions are revealed which correspond quite well to what may be observed and measured in real life. Maxwell's equations work; they did when he announced them and they still do. As applicable mathematics they remain a valid and valuable tool. Nevertheless, the physics he used to derive them is entirely discredited. Maxwell based his electromagnetics on the notion of forces and waves acting in a universal elastic medium called the ether. Invisible and impalpable, it nevertheless permeated the whole universe. Yet only six years after his death the famous Michelson-Morley experiments began to cast doubt on the exist- ence of the ether. Now the idea is dead, thanks to the universal adoption of relativistic physics and quantum theory. In our present- day interpretation, radio energy consists of photons, electromag- netic quanta which are incredibly small and strange, particles that also have wave properties. Quantum mechanics, because of the very oddness of some of its predictions, has been subjected to the most rigorous processes of experimental testing conceivable, more so than any other branch of physics. One day things might change, but for now and the foreseeable future, quantum theory is the most firmly established of all physical ideas. Yet for half a century we have gone on RadioAntennas and Propagation viii ,, ,, H,,J , , i , teaching electromagnetics to generations of engineering students as if the quantum revolution had never happened. Why so? It is true that the classical Maxwell approach does provide a good mathematical model of electromagnetic phenomena. Nowhere in radio engineering does it blatantly fail, as it does in optics and spectroscopy. The radio frequency quantum is much less energetic than its optical counterpart, so any detectable energy involves very many of them. As a result, effects attributable to individuals are not seen, and everything averages out to the classical picture. So if radio engineers ignore quantum mechanics nothing actually goes wrong for them, and this was long thought reason enough for leaving it out of books and courses. It seemed an unnecessary complication. Times change, however. Modern electrical engineering students must pick their way through some quantum mechanics to under- stand semiconductor devices; it is no longer an optional extra. But to use quantum explanations about transistors and microcircuits yet ignore them when it comes to radio destroys the natural unity of our subject, fails to make important connections and seems arbitrary. Besides, 'difficult' ideas grow easier with use and a quantum orientation to radio no longer makes the subject less accessible to modern students. On the contrary, sometimes quan- tum notions give an easier insight than the old classical approach. The 'feel' is so much less abstract, so much more real-world oriented. Anyway, I cannot help believing that we ought to teach our students the best we know, particularly since we have no idea what will be important to them in the future. So, start to finish, this book takes an approachable but persistently quantum-oriented stance, and in my mind that is what justified writing it. My hope is that it will encourage those who have long wanted to teach the subject in a more modern way. As to acknowledgements, first my undying gratitude to generations of final year students at the University of Bath, from whom I discovered how best to teach this subject. Heartfelt thanks also to Duncan Enright at Newnes, for encouraging me to turn the course into a book. William Gosling CHAPTER 1 1 INTRODUCTION This book is about how radio energy is released (transmission), how it moves from one place to another (propagation) and how it is captured again (reception). Understanding all this is indispensable for communications engineers because during the twentieth century radio has become a supremely important means of carrying information. First used by ships at sea, soon after 1900, radio systems were quickly developed for broadcasting (sound from around 1920, television after 1936). At much the same time came air traffic control, emergency services (police, fire, ambulance) and later private mobile radio, with users ranging from taxi drivers in the city streets to civil engineers on major construction projects. The military were enthusiastic users of radio from the start, notably for battlefield communication (especially in tanks), for warships, both surface and submarine, and the command and control of military aircraft. In the second quarter of the twentieth century radio navigation systems, which enabled ships and aircraft to obtain accurate 'fixes' on their position, spread to give worldwide cover- age. A modification of standard radio techniques, permitting reception of reflected energy, led (from about 1938) to the extensive use of radar for the detection, and later even imaging, of distant objects such as ships, aircraft or vehicles. The worldwide annual turnover of the radio industry (in all its many forms) still exceeds that of the computer industry, and it is growing just as fast. In recent times, optical fibres have replaced 2 Radio Antennas and Propagation radio, to some extent, for communication between fixed locations, but for all situations in which one or both ends of a communication link may be mobile or subject to movement, radio remains the only information-bearer technology. Early radio engineers struggled to get the maximum possible range from their systems, but today, as well as continued interest in long ranges, there is also an explosive growth in the use of short-range radio systems. Cellular radio telephones are the most obvious example. Short-range radio has the important advantage that it enables more users to be accom- modated in the same radio bands without interfering with each other. All of this explosive technological development depends on the transmission, propagation and reception of radio energy. So what is radio energy? 1.1 What radio energy really is Radio energy is similar to light. It propagates freely in space as a stream of very small, light particles called electromagnetic quanta or photons. The difference between the quanta of light and of radio energy is solely that each quantum of light carries far more energy than those of radio, but in other ways they are identical. The term 'quantum' (plural 'quanta') is a general one for any particle of energy. We can, for example, have quanta of gravita- tional energy (which are called gravitons) or of acoustic energy (phonons). When the energy is electromagnetic, that is involving electrical and magnetic forces, the quantum is called a photon. In what follows the terms 'quanta' and 'photons' will be used interchangeably, since this book is concerned with electromag- netics. However, because these particles are very small indeed they do not obey the laws of classical mechanics (Newton's laws), as do snooker balls, for example. Instead they behave in accordance with the laws of quantum mechanics, as do all very small things. This gives them some strange properties, quite unfamiliar to us from everyday life, which may even seem contrary to common sense. Two properties are important. Introduction ,,, ,, The first is that radio quanta can exist only when they are in motion, travelling at their one and only natural speed, which is the velocity of light.