Covers the basic principles and fundamental microwave antenna types and techniques.
Inspec keywords: microwave antennas; antenna radiation patterns; dipole antennas; transmission lines
Other keywords: radio science; microwave dipole antennas; antenna radiation patterns; microwave antenna theory and design; microwave transmission lines
Subjects: Antenna theory; Single antennas; Transmission line links and equipment
The following sections are included: the wavelength region; antenna patterns; types of microwave beams; microwave transmission lines; radiating elements; a survey of microwave antenna types; impedance specifications; and program of the present volume.
A waveguide can itself be treated as a system of distributed impedances. Distributed impedances are treated in the same way as lumped impedances, by use of Kirchhoff's current and voltage laws for networks. A system of distributed impedance can, in fact, be replaced by a network of lumped-impedance elements. The latter differ from the conventional radio-circuit elements in that their impedance is a transcendental function of frequency rather than an algebraic function. By means of these equivalent lumped-element networks, the network theorems that are applicable to low-frequency lumped-element networks are carried over to systems with distributed impedance. The first part of this chapter will review several network theorems and the two-wire transmission-line theory that are used in microwave circuit theory. The subjects will be treated briefly, the reader being referred to standard texts for more complete discussions and proofs of the results quoted here.
The fundamental approach to an understanding of microwave antennas is necessarily based on electromagnetic theory. This chapter therefore begins with a discussion of the field equations and the general properties of an electromagnetic field; the treatment is necessarily cursory, being intended as a summary of material that is familiar to the reader. This theory is then applied to the simplest problem of antenna theory, the calculation of the radiation fields due to known current distributions. A discussion of certain idealized current distributions illustrates the principles of superposition and interference and furnishes a theoretical guide to the design of various antenna feeds.
The preceding chapter dealt with radiation fields in their direct relation to the sources. It was found that the field represents a flow of energy outward from the region of the sources; also it was demonstrated separately that the energy flow in a time-varying field is a wave phenomenon. We now turn our attention to the study of wave propagation and the associated energy flow, without direct reference to the sources. Several simple waveforms have already been discussed: plane, cylindrical, and spherical waves. In each case the wave was described by a family of equiphase surfaces or wavefronts, and the propagation of the wave was visualized as a progression of each wavefront into a contiguous one; furthermore, the energy flow at every point was in a direction normal to the wavefront. The main subject of this chapter is the extension of these ideas to general waveforms.
These phenomena -scattering and diffraction-are of fundamental importance in micro wave antennas, for they underlie the formation of antenna patterns by reflectors and lenses. In the present chapter the theory of scattering and diffraction is developed with reference to general techniques; the specific problems associated with antenna patterns will be taken up in Chap. 6.
The discussion of aperture systems will be continued in the present chapter with the object of developing in more detail the relations between the aperture field and the diffraction field. The results will furnish a basis for the design of the reflectors and lenses used in directive microwave antennas. The design considerations for such systems fall into two major groups: (1) transformation of the specifications that the radiation pattern of the antenna as a whole is required to meet into requirements on the aperture-field distribution, and (2) the design of the primary feed and reflector or lens to produce the required aperture field. The radiation pattern of the com posite antenna will be referred to as the secondary -pattern in distinction to the primary pattern of the feed system.
We have dwelt at considerable length on general theoretical considerations underlying the design and operation of microwave antennas as a whole. We now enter upon a program of studying the components of an antenna, starting with an investigation of microwave transmission lines. Usually about a foot, or perhaps two, of the line immediately preceding the radiating system is at the disposal of the engineer for the insertion of matching devices to compensate for the impedance mismatch of his antenna; this section will be referred to as the feed line.
The early trends in microwave antenna design grew out of the practice of using dipole systems at longer wavelengths. Nevertheless, little systematic information has been obtained about microwave dipole systems. This is partly due to the greater difficulty in applying theory to practically useful microwave dipoles and partly to the urgent military needs which prevented systematic research during the early development in this field. More recently, attention has been concentrated on wave guide and horn radiators, which are more amenable to quantitative analysis. Consequently, the design of microwave dipole antennas is still in the empirical stage; quantitative data are available only with reference to particular systems.This chapter includes a discussion of the following: characteristics of antenna feeds; coaxial line termination; asymmetrical line termination; symmetrical energised dipole - slot fed systems; shape and size of the dipole; directive dipole feed; dipole disk feeds; double dipole feeds; and multi-dipole systems.
The problems and techniques of linear-array design have been divided in this chapter into three general parts. The first concerns itself with general pattern theory, that is, the relation between the far-zone pattern of an array and the amplitude and phase distribution among the elements and their spacing; in this section no attention is paid to the problem of realizing a given amplitude and phase distribution. The second part is a survey of the radiating elements that have been developed for micro wave arrays. The final division treats the problems associated with combination of the elements into linear arrays and the techniques avail able to produce the desired amplitude and phase distributions.
The problem of radiation from the open end of a waveguide could be discussed in principle from several points of view. Rigorously, the radiation can be considered to arise from the current distribution on the inside walls of the guide, which is just the current distribution associated with the fields propagated in the interior of the guide, together with the currents flowing from the open end out upon the exterior guide surface. Were it not for difficulties in the analysis, this current distribution and the radiation field at an external point could be calculated. This has, however, not yet been accomplished.
The utilization of optical methods is an outstanding feature of microwave antenna design. It is natural, therefore, to consider a much-used optical device, the lens. It is the aim of this chapter to point out the methods of design, types of structure, and general problems involved in the use of lenses. Correcting lenses and other lenses designed for special purposes will not be considered. Discussion will be confined to those lenses whose function is to convert the spherical (or cylindrical) phase front from a point (or line) source at the focus of the lens into a plane phase front across the aperture. This is the most frequently recurrent problem in microwave antenna design, because, by diffraction theory, a plane phase front results in the most directive pattern for an aperture of a given size with a given amplitude distribution across it.
The term “pencil beam” is applied to a highly directive antenna pattern consisting of a single major lobe contained within a cone of small solid angle and almost circularly symmetrical about the direction of peak intensity. As used here, it will apply to beams with half-power width less than 15°. These beams are analogous to searchlight beams, and, as with an optical search light, the elevation and azimuth coordinates of a target in space can be simply correlated with the similar coordinates that define the orientations of the antenna. In connection with the technique of using radar echoes for obtaining range information, the pencil-beam antenna serves to define the position of a target completely.
The highly directive beams attainable with microwave antennas have been utilized to achieve large antenna gain, precision direction finding, and a high degree of resolution of complex targets. The exploration of a wide angular region with such sharp beams requires an involved scanning operation in which the scanning time becomes a limiting factor. This problem is much simplified if the required scanning can be reduced to only one direction, the coverage of the angular region being completed by fanning the beam broadly. The purpose of this chapter is to describe several applications for shaped beams, to discuss requirements imposed on the beam by these applications, and to present a number of design techniques for producing shaped-beam antennas.
The customary procedure in microwave radar antenna development has been to design the antenna and to carry out the early experimental tests on the assumption of free-space conditions surrounding the antenna. Whereas this represents a good approximation in general, it is necessary eventually to consider the effect upon the antenna performance of the supporting structure on which it must be mounted. Also, it is generally necessary to place the antenna in a dielectric housing-the radome- which likewise affects its performance. It is sometimes possible so to choose the antenna location on the structure and to design the radome that the original performance of the antenna is unimpaired. The final result can be predicted with greater certainty, however, if the electrical design of the antenna is considered from the beginning in conjunction with that of the radome and with a view to the structure and location that the antenna-radome system must occupy. The purpose of this chapter is to present the problems imposed by installation requirements and the practices that have been adopted for dealing with them. These considerations are intended to serve merely as background for one engaged in antenna design.
The principles and techniques of antenna design were developed in the preceding chapters without consideration of the methods for obtaining design data and for testing the performance of the completed antenna. This and the following chapter will be devoted to a discussion of measurement techniques and a survey of the equipment required for such measurements. The antenna characteristics to be measured fall into four groups: impedance, primary feed patterns, secondary patterns, and gain. The impedance measurement techniques differ little in detail from those for other r-f components of microwave systems; the problem is complicated to a small degree by the fact that the antenna is a radiating load. The importance of the primary feed pattern increases with the progress that is made in reducing antenna design from an empirical to a calculable procedure. The study of pencil beams and fanned beams has shown the need for a detailed knowledge of both the phase and intensity distribution in the primary feed pattern. The over-all characteristics of the antenna are particularly sensitive to the phase characteristics of the feed.
Measurements on microwave antennas differ in character from those carried out on most other radar components. A high order of amplitude stability is required of measuring equipment for the study of antennas and associated components, whereas high accuracy in timing and frequency control are the main requisites in measurements on other radar components. These requirements make the design of special equipment for antenna measurements most desirable.
