IEE ELECTROMAGNETIC WAVES SERIES 12 MICROSTRIP ANTENNA THEORY AND DESIGN J. R. JAMES, P. S. HALL and C. WOOD Peter Peregrinus Ltd on behalf of the Institution of Electrical Engineers IEE ELECTROMAGNETIC WAVES SERIES 12 Series Editors: Professors P. J. B. Clarricoats E. D. R. Shearman and J. R. Wait MICROSTRIP ANTENNA THEORY AND DESIGN Other volumes In this series Volume 1 Geometrical theory of diffraction for electromagnetic waves Graeme L. James Volume 2 Electromagnetic waves and curved structures Leonard Lewin, David C. Chang and Edward F. Kuester Volume 3 Microwave homodyne systems Ray J. King Volume 4 Radio direction-finding P. J. D. Gething Volume 5 ELF communications antennas Michael L. Burrows Volume 6 Waveguide tapers, transitions and couplers F. Sporleder and H. G. linger Volume 7 Reflector antenna analysis and design P. J. Wood Volume 8 Effects of the troposphere on radio communication Martin P. M. Hall Volume 9 Schumann resonances in the earth-Ionosphere cavity R V. Bliokh, A. P. Nikolaenko and Y. F. Filippov Volume 10 Aperture antennas and diffraction theory E. V. Jull Volume 11 Adaptive array principles J. E. Hudson Volume 12 Microstrip antenna theory and design J. R. James, P. S. Hall and C. Wood Volume 13 Energy In electromagnetism H. G. Booker Volume 14 Leaky feeders and subsurface radio communications Volume 15 The handbook of antenna design, volume 1 Editors: A. W. Rudge, K. Milne, A. D. Olver, P. Knight Volume 16 The handbook of antenna design, volume 2 Editors: A. W. Rudge, K. Milne, A. D. Olver, P. Knight Volume 17 Surveillance radar performance prediction P. Rohan Volume 18 Corrugated horns for microwave antennas P. J. B. Clarricoats and A. D. Olver Volume 19 Microwave antenna theory and design Editor: S. Silver Volume 20 Advances In radar techniques Editor. J. Clarke Volume 21 Waveguide handbook N. Marcuvitz Volume 22 Target adaptive matched Illumination radar D. T. Gjessing MICROSTRIP ANTENNA THEORY AND DESIGN J. R. JAMES, P. S. HALL and C W O OD Peter Peregrinus Ltd on behalf of the Institution of Electrical Engineers Published by: Peter Peregrinus Ltd, London, United Kingdom ©1981: Peter Peregrinus Ltd Paperback reprint 1986 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means- electronic, mechanical, photocopying, recording or otherwise—without the prior written permission of the publisher. While the author and the publishers believe that the information and guidance given in this work is correct, all parties must rely upon their own skill and judgment when making use of it. Neither the author nor the publishers assume any liability to anyone for any loss or damage caused by any error or omission in the work, whether such error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed. ISBN 086341 088 X Contents Page Preface ix Acknowledgments xi Principal symbols xiii 1 Flat-plate antenna techniques and constraints on performance 1 1.1 Waveguide antennas 3 1.1.1 Slot element design and deployment 3 1.1.2 Waveguide arrays 5 1.1.3 Dielectric waveguide arrays 6 1.2 Triplate antennas 7 1.2.1 Design of the slot element 7 1.2.2 Triplate slotted arrays 7 1.3 Cavity-backed printed antennas 9 1.3.1 Printed dipole elements 10 1.3.2 Cavity-backed printed arrays 12 1.4 Microstrip antenna concept 12 1.4.1 Fundamental limitations of microstrip antennas 13 1.4.2 Performance trends 16 1.5 Summary comment 17 1.6 References 1 g 2 Microstrip design equations and data 21 2.1 Equations based on TEM properties 22 2.2 Estimations of losses in microstrip 25 2.2.1 Dielectric loss 25 2.2.2 Conductor loss 26 2.3 Higher-order analytical methods 29 2.3.1 Practical design implications 29 2.3.2 Methods of analysis 30 2.4 Discontinuities in microstrip lines 34 2.4.1 Quasistatic analysis of discontinuities 35 2.4.2 Equivalent waveguide analysis of discontinuities 35 2.4.3 First-order closed-form equations for antenna design 36 2.5 Manufacturing and operational aspects of microstrip substrates 36 vi Contents 2.6 Summary comments 38 2.7 References 39 3 Radiation mechanism of an open-circuit microstrip termination — fundamental design implications 42 3.1 Comparison of estimates of radiation loss 42 3.2 Direct measurement of the end admittance 48 3.3 Substrate surface waves and their significance 51 3.4 Improved analysis of microstrip open-circuit terminations 53 3.5 Fundamental design implications based on first-order patch design 59 3.6 Considerations for other antenna forms and discontinuities 54 3.7 Summary comments 64 3.8 References 65 4 Basic methods of calculation and design of patch antennas 67 4.1 Patch analysis 71 4.1.1 Field structure of patch 71 4.1.2 Calculation of radiation fields 73 4.1.3 Antenna bandwidth and impedance 74 4.2 Analysis of rectangular patch using modal method 77 4.2.1 Radiation fields 77 4.2.2 Radiation Q-factor 81 4.2.3 Square antenna 82 4.3 Analysis of circular patch using modal method 82 4.3.1 Radiation fields 82 4.3.2 Radiation Q4actor 84 4.3.3 Circular polarisation 86 4.4 Input impedance analysis 87 4.4.1 Network model for (m = 0, n = 1) mode rectangular patch 87 4.4.2 General patch shapes 91 4.5 Refinements to theoretical models 93 4.5.1 Complete modal field description 95 4.5.2 Corrections for fringe-field leakage 97 4.6 Short-circuit patches 102 4.6.1 Radiation patterns 103 4.6.2 Resonant frequency modification using shorting pins 104 4.6.3 Bandwidth and resonant impedance 106 4.6.4 Bandwidth increase using parasitic elements 106 4.7 Summary comments 108 4.8 References 109 5 Linear array techniques HI 5.1 Review of feed methods for linear arrays 112 5.1.1 Series feeding 112 5.1.2 Parallel feeding 114 5.2 Practical forms of linear microstrip array 117 5.2.1 Arrays of resonators 117 5.2.2 Arrays formed from line discontinuities 123 5.2.3 Higher-order mode arrays 129 5.2.4 Overall assessment of available linear arrays 130 5.3 Analysis and design of rear-fed resonator arrays 131 5.3.1 Radiating element choice and first-order design 131 Contents vii 5.3.2 Mutual coupling effects 134 5.4 Analysis and design of arrays with integral feed networks 138 5.4.1 Analysis and first-order design of series arrays \ 39 5.4.2 Theoretical performance limitations of travelling-wave arrays 141 5.4.3 Design parameter characterisation effects 145 5.4.4 Mutual coupling, unwanted radiation and cross-polarisation effects 152 5.5 Summary comments 156 5.6 References 157 6 Techniques and design limitations in two-dimensional arrays 160 6.1 Review of feed methods 160 6.2 Practical forms of two-dimensional arrays 162 6.2.1 Corporately fed arrays 162 6.2.2 Arrays of series-fed radiators 164 6.3 Mutual coupling 172 6.4 Techniques and problems in feed design 175 6.4.1 Feed design and component characterisation 175 6.4.2 Transitions between triplate and coaxial lines and microstrip 183 6.5 Array gain limitations due to losses 186 6.6 Summary comments 190 6.7 References 191 7 Circular polarisation techniques 194 7.1 Orthogonal resonators with separate feeds 195 7.1.1 Polar diagram control 195 7.1.2 Effect of radiator mismatch 196 7.1.3 Cross-fed patch arrays 199 7.2 Resonators with single feed points 201 7.2.1 Equivalent circuit 202 7.2.2 Frequency response 204 7.2.3 Pentagon patch 205 7.3 Curved line antennas 206 7.3.1 Radiation mechanism of curved line 207 7.3.2 Characteristics of antennas 211 7.4 Travelling-wave arrays 213 7.4.1 Rampart line antenna 215 7.4.2 Chain antenna 218 7.5 Summary comments 222 7.6 References 223 8 Some manufacturing and operational problems of microstrip antennas 225 8.1 Material and manufacturing tolerance problems 225 8.1.1 Resonant antenna design 226 8.1.2 Array design 228 8.2 Substrate surface-wave generation 230 8.3 Summary comments 232 8.4 References 232 9 Recent advances in microstrip antenna analysis 233 9.1 Wire antenna analysis 236 viii Contents 9.2 Narrow rectangular strips 239 9.3 Circular patch 242 9.4 Rectangular patch 249 9.5 Discussion of analytical methods 251 9.6 Summary comments 253 9.7 References 254 10 Other trends and possible future developments 256 10.1 Variations on the basic resonant element 256 10.1.1 Dual-frequency elements 256 10.1.2 Coplanar stripline 258 10.2 Multilayer techniques 259 10.2.1 Directly coupled forms 260 10.2.2 Electro magnetically coupled forms 261 10.2.3 Low-profile dipole antennas 265 10.3 Antenna feed structures in microstrip 269 10.3.1 Feeds for surface-wave structures 269 10.3.2 Feeds for reflector configurations 270 10.3.3 Slot antenna feeds 273 10.4 Conformal microstrip antennas 274 10.5 The future 280 10.6 References 280 Appendixes 283 Appendix A 283 Appendix B 283 Appendix C 287 Index 288 Preface In the past few years, the concept of creating microwave antennas using microstrip has attracted increasing attention and viable practical designs are now emerging. The purpose of this monograph is to present the reader with an appreciation of the underlying physical action, up-to-date theoretical treatments, useful antenna design approaches and the overall state-of-the-art situation. The emphasis is on antenna engineering design but to achieve this goal it has been necessary to delve into the behaviour of microstrip in a much wider sense and also include aspects of electro- magnetic analysis. As a consequence, the monograph will also be of interest to microstrip circuit designers and to some extent those seeking electromagnetic problems of a challenging nature. It is worthwhile saying a little about the trends that led up to these new types of antennas. To begin with, the idea of utilising a printed conductor on a substrate as a radiating element is probably as old as the idea of printed transmission lines them- selves. Microstrip transmission lines were introduced in the 1950s and it was evident then that the lines, together with their compatible microstrip components, were prone to radiation losses and parasitic coupling effects which became more trouble- some as the operating frequency increased. It is interesting to reflect that the antenna designer actually makes good use of, and optimises, one of these unwanted properties, the radiation. One may well wonder why the research into microstrip antennas did not intensify at an earlier date and there are perhaps several reasons that can be pointed out. The astronomical progress in miniaturising and integrating electronic circuits in the past decade has recently created a positive demand for a new generation of antenna systems. In principle, microstrip antennas are thin planar configurations that are lightweight, low cost, easy to manufacture and can be made conformal with the surfaces of vehicles, missiles etc. The compatibility of micro- strip antennas with integrated electronics is thus very evident and is a great impetus to antenna designers particularly so, now that many new types of substrate materials are commercially available. However, the microstrip wave-trapping effects, so vital for circuit operation, inhibit the radiation mechanism and must be taken into account in antenna design. This seems an obvious statement when one considers that the original intended purpose of the microstrip structure was to act as an