Microwave Antennas

674 Pages·1961·15.37 MB·English

by  A. Z. Fradin (Auth.)

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MICROWAVE
ANTENNAS
by
A. Z. FRADIN
Translated from the Russian by
MORTON NADLER
Translation edited by
R. C. GLASS
Lecturer in Applied Physics
Northampton College of Advanced Technology
London
PERGAMON PRESS
OXFORD · LONDON · NEW YORK · PARlS
1961
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Copyright
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1961
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FOREWOHD
Microwave antennas form, at present, a very wide independent
branch of radio engineering having its theoretical basis and
its methods of calculation and design. However, there is no
book on the subject available at present which deals with all
questions of microwave antenna theory and design in a suffi­
ciently complete and satisfactory manner.
The existing books on microwave antennas are either mono­
graphs on particulau: questions or works presenting the tech­
niques of the forties and not reflecting the achievements of
recent years.
This book is an attempt to fill this gap. It is a new,
substantially expanded and revised edition of the author's
book which appeared in 1950.
In the first chapter the classification of microwave
antennas is introduced with descriptions of their structure
and operating principles.
The second chapter presents the fundamental principles of
microwave antenna theory and both rigorous and approximate
methods for their analysis are described.
The third chapter is devoted to a solution of a number of
mathematical problems of diffraction at openings in plane
screens by approximate methods, to which the determination of
the radiation characteristics for the majority of microwave
xi
xii MICROWAVE AlνTENNAS
antennas is reduced. The presentation of this material in a
separate chapter enables us to achieve, on the one hgmd, a
simplification of the discussion in the section devoted to
individual types of antennas while, on the other hand, it
enables general conclusions to be drawn on the relationship
between the field amplitude and phase distribution in aper­
ture and radiation characteristics.
The remaining chapters examine in detail individual types
of antennas. Por each of the types the various forms are
analysed, the question of structures is discussed and the
contemporary state of the theory is explained. In addition,
design formulae and graphs are presented, relating the
design parameters to the electrical characteristics of the
antennas and an idea of the procedure in performing the
computations is given.
Together with data on antennas which have been examined and
tested in practice, the book also includes data taken from
reports appearing in the technical journals of new develop­
ments and ideas which may well be worth further investiga­
tion.
Naturally this book cannot pretend to give a full presenta­
tion of both theoretical and experimental material on micro­
wave antennas known at present. The bibliography should,
however, help the reader to find additional material on the
questions dealt with here, where required.
It is my pleasant duty to express my deep appreciation to
the editor, V,A, Khatskelevich, for generous assistance
given in preparing the manuscript for press and also to
B,V. Braude and A.I. Potekhin for valuable suggestions and
advice which the author took into consideration in revising
the manuscript.
A.Z.P.
CHAPTER I
MICRCWAVE AUTEMAS. THEIR CLASSIFICATION.
STRUCTURE ΑΙΠ) OPERATING PRINCIPLES
1, Scope of the Investigation
By microwaves or super-high frequencies (abbreviated SHP)
we understand that part of the radio spectrum which includes
decimetre and centimetre waves. Wire antennas which are
also used at lower frequencies are encountered in microwave
engineering (for example antennas of the "wave duct" type,
broadside dipole antennas, helical antennas, etc.). However,
at microwave frequencies the most widely-used antennas are
the so-called diffraction antennas, many of which are analo­
gous to optical systems (reflectors, lenses, etc.).
The term "microwave antennas" in this book is used to mean
only diffraction antennas. An explanation of the concept of
diffraction antennas will be given below (section 9) after a
description of the structure and principles of operation.
Together with diffraction antennas, to which we give most
attention, we present also a brief description of wire heli­
cal antennas since they are mainly employed at microwave
frequencies and are not described in the usual textbooks.
There are no specific receiving antennas at microwave
frequencies as is the case on long and short waves. The same
antennas are used both for transmission and reception. In
certain practical cases, for exeimple, in radar, one and the
same antenna is frequently used for both forms of operation.
Since the basic characteristics of the antenna, in accordance
with the reciprocity theorem, do not depend on the form of
operation, the antennas may be studied either as transmitting
or as receiving.
2 MICROfAYE AITFENNAS
From the point of view of presentation it is more conveni­
ent to consider antennas as transmitting. Therefore, in
what follows microwave antennas will everywhere he considered
as radiating systems.
The scope of the study of microwave antennas includes the
following questions:
1. Radiation theory.
2. Theory of coupling antennas and feeder lines.
5. Design featvires for various fields of application.
In this book chief attention will be given to the first
question, i.e., the theory of radiation of microwave antennas
including:
a) the initial physical principles
b) rigorous mathematical methods of solving radiation
problems
c) formulae and procedures of approximate calculation for
radiation characteristics: directional patterns,
gain factors and others.
The remaining questions mentioned above are described to
the extent to which they serve to illustrate the theory and
contribute to the understanding of the fundamental questions.
2. Classification of Microwave Antennas
At present, many different forms of microwave antennas are
known. However, as will be shown below, they have the same
operating principles and differ substantially from antennas
at lower frequencies. We shall therefore consider them as a
particular class of antennas. The differences between
individual microwave antennas appear in the design features,
feeding methods, etc.
Microwave antennas may be divided into six types:
1. horn; 2. lens; 3. slot; 4- dielectric; 5. reflector;
6. surface-wave.
CUSSIFICATION, STRUCTURE AND OPERATING PRINCIPLES 3
Each of these types is divided further into several sub­
types , many of which, in turn, are encountered in a large
number of variations.
This classification of microwave antennas is illustrated
in the diagram (Pig. l.l). In this figure only the more
important variants found in practice and in theoretical
investigations are represented. A number of other variants
of interest will be mentioned later. In addition, it should
be borne in mind that, in practice, antennas are encountered
which are a combination of several of the types shown here
(for example, horn and reflector, horn and lens, reflector
and slot antenna, etc.). To avoid excessive complication of
the classification diagram these antennas are not included.
They will be considered in the appropriate places in the
text.
Below we give a description of the construction and operat­
ing principles of the individual types.
5. Horn Antennas
In Fig. 1.2 the basic types of horn antennas are shown:
a) sectoral; b) acute pyramidal; c) wedge-shaped pyra­
midal; d) conical; e) bi-conical double-ended and f) bi-
conical single-ended horns.
Of these the first four [Figs. 1, 2 (a - b - c and d)] are
systems with an ordinary waveguide and a horn connected
directly to it. In effect the latter is also a waveguide,
but of variable cross-section. For these antennas the
ordinary waveguides of constant cross-section serve as feeder
lines, transmitting energy from the source (S) to the horn.
The source S or other exciter of the waveguide is, as is
familiar, a symmetrical or unsymmetrical dipole. In Fig. 1.2
the form of the open end of the inner conductor of a co­
axial cable is shown.
The bi-conical horns [Figs. 1.2 (e and f)] consist of two
truncated cones, with common axis AA' and common apex 0 and
located on one side (Fig. 1.2f)* or on opposite sides
• The bi-conical symmetrical dipole may also be considered in
essence as a bi-conical horn.
MICROWAVE AHTEMAS
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CLASSIFICATION, STRUCTUBE AND OPERATING PRINCIPLES 5
Fig, 1,2. Horn antennas,
a) sectoral horn
b) acute pyramidal horn
c) wedge-shaped pyramidal horn.