Introduction to Antennas

Introduction to Antennas

Macmillan New Electronics Series Series Editor: Paul A. Lynn Paul A. Lynn, Radar Systems A. F. Murray and H. M. Reekie, Integrated Circuit Design Martin S. Smith Introduction to Antennas

Introduction to Antennas Martin S. Smith MA, PhD, CEng, MIEE Senior Principal Research Engineer, S.T.L., Harlow formerly Lecturer in Electronic Engineering, University College London Macmillan New Electronics Introductions to Advanced Topics M MACMILLAN EDUCATION

Martin S. Smith 1988 All rights reserved. No reproduction, copy or transmission of this publication may be made without written permission. No paragraph of this publication may be reproduced, copied or transmitted save with written permission or in accordance with the provisions of the Copyright Act 1956 (as amended), or under the terms of any licence permitting limited copying issued by the Copyright Licensing Agency, 33-4 Alfred Place, London WC1E 7DP. Any person who does any unauthorised act in relation to this publication may be liable to criminal prosecution and civil claims for damages. First published 1988 Published by MACMILLAN EDUCATION LTD Houndmills, Basingstoke, Hampshire RG21 2XS and London Companies and representatives throughout the world British Library Cataloguing in Publication Data Smith, Martin S. Introduction to antennas.-(macmillan new electronics). 1. Antennas (Electronics) I. Title 621.38'028'3 TK7871.6 ISBN 978-0-333-44559-4 ISBN 978-1-349-19384-4 (ebook) DOI 10.1007/978-1-349-19384-4

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Contents Series Editor's Foreword Preface List of Symbols ix X xi 1 Basic Antenna Concepts 1 1.1 Introduction 1 1.2 From ELF to mm waves 1 1.3 Maxwell's equations and plane waves 5 1.4 Basic antenna properties 8 1.5 Reciprocity and receiving antennas 10 1.6 Choice of frequency 13 1. 7 Antenna analysis and design 15 Reference 16 2 Wire Antennas 17 2.1 Introduction 17 2.2 The Hertzian dipole 18 2.3 Dipoles and monopoles 21 2.4 Parasitic elements 23 2.5 Loops and slots 25 2.6 Printed antennas 27 2. 7 An introduction to moment methods 29 2.8 Case study: antennas on aircraft 31 References 32 3 Aperture Theory 33 3.1 Diffraction by apertures 33 3.2 Aperture theory in two dimensions 35 3.3 Applications of aperture theory 44 3.4 Aperture theory in three dimensions 48 3.5 Gain 55 3.6 Case study: airport radar antennas 56 4 Linear Arrays 59 4.1 Introduction 59 4.2 The radiation pattern of a linear array 59 4.3 Nulls 63 vii

viii Contents 4.4 Pattern multiplication 4.5 Beam steering 4.6 Woodward synthesis of antenna patterns 4. 7 Case study: the ILS array system References 5 Antenna Measurements 5.1 Introduction 5.2 Input impedance 5.3 Radiation patterns 5.4 Gain measurements 5.5 Polarisation measurements 5.6 Case study: a vertical antenna range References 6 Further Array Topics 6.1 Introduction 6.2 Scanning antennas 6.3 Orthogonal beams and multiple beam formers 6.4 Planar arrays 6.5 Mutual coupling in phased arrays 6.6 Circular arrays 6. 7 Case study: a Rotman lens multiple beam former References Appendix A: Vector Differential Operators Appendix B: The Fields of a Hertzian Dipole Appendix C: Derivation of Aperture Theory in Three Dimensions Exercises Bibliography Answers to Exercises Index 67 68 70 73 76 77 77 77 78 82 84 85 88 89 89 89 92 100 105 106 109 112 114 116 118 121 123 124 125

Series Editor's Foreword The rapid development of electronics and its engineering applications ensures that new topics are always competing for a place in university and polytechnic courses. But it is often difficult for lecturers to find suitable books for recommendation to students, particularly when a topic is covered by a short lecture module, or as an 'option'. Macmillan New Electronics offers introductions to advanced topics. The level is generally that of second and subsequent years of undergraduate courses in electronic and electrical engineering, computer science and physics. Some of the authors will paint with a broad brush; others will concentrate on a narrower topic, and cover it in greater detail. But in all cases the titles in the Series will provide a sound basis for further reading of the specialist literature, and an up-to-date appreciation of practical applications and likely trends. The level, scope and approach of the Series should also appeal to practising engineers and scientists encountering an area of electronics for the first time, or needing a rapid and authoritative update. ix

Preface Antennas are a vital part of many communication and radar systems, as they form and control the link between cable and radiative transmission of signals. As such they are usually an important topic in final year undergraduate courses and M.Sc. courses in Electronic Engineering and related disciplines. This book is intended to cover the theory and practice of antennas at a level suitable for such courses; it should also be a useful introduction to the subject for recently qualified engineers working in industry. The first five (of six) chapters treat the basic theoretical analysis and experimental techniques. The main topics are wire antennas, aperture antennas, linear arrays and antenna measurements. There is an extensive literature covering more advanced and specialised antenna topics. The final chapter of this book includes a small selection of these topics, mainly associated with phased arrays. These have been chosen on the grounds that they contain some important principles of which a future antenna, or system, designer should be aware. Each chapter contains a case study of a practical application to illustrate the work of the chapter. Thanks are due to many colleagues who have passed on their own experience to me. I am particularly indebted to the.following people. Dr Kenneth Budden supervised my research in radio wave propagation. Mr Norman Pavey first introduced me to antennas, particularly for aircraft use. Professor 'Den' Davies led me into various microwave antenna research topics, while Professor Alec Cullen introduced me to lecturing on antennas. I would also like to thank Mrs Margaret Ringsell for her expert typing of the manuscript. MartinS. Smith X

List of Symbols c d d db f ga(sr, Sz) h h(sj, S 2 ) u x,y,z p, e, z r, e, <f> A A Ae Ael B c C,C' D D E G(S,<f>) G H I I( c:f>) J lm(x) M P,PT,PR P(S), P(S 1, S 2 ) R R Rin Rrad speed of light in vacuo array element spacing antenna linear dimension decibel, 10 log 10 (power ratio) frequency active element gain pattern height array factor for a planar array unit vector Cartesian coordinates cylindrical polar coordinates spherical polar coordinates magnetic vector potential geometrical area effective area area per element in a planar array magnetic flux intensity cos e contours of integration electric flux intensity directivity electric field gain function gain magnetic field current circular array excitation current density Bessel function of order m magnetic current density power, transmitted power, received power angular spectrum in two dimensions, three dimensions measurement range length circular array radius real part of input impedance radiation resistance xi

xu Rohmic s sl, s2 ds Xin Zin Zo Zo Eo Er TJ 6 A. 1-Lo 1-Lr 1-Leff CT CT T w r List of Symbols ohmic loss resistance power flux density sin 6 sin 6 cos <!>, sin 6 sin <!> element of surface area imaginary part of input impedance input impedance characteristic impedance of free space, 3770 transmission line characteristic impedance permittivity of free space relative permittivity (dielectric constant) efficiency polar coordinate wavelength permeability of free space relative permeability effective 1-Lr conductivity radar cross-section tilt angle of polarisation ellipse spherical polar coordinate circular array element angular coordinate incremental phase shift any field component angular frequency reflection coefficient