Microstrip is the name given to a type of open waveguiding structure that is now commonly used in present-day electronics, not only as a transmission line but for circuit components such as filters, couplers, resonators etc. Readers not completely familiar with the microstrip concept are referred to Chapter 2 for some explanatory details and references. The idea of using microstrip to construct antennas is a much more recent development and an example of a microstrip antenna is given in Fig. 1.1. The antenna assembly is physically very simple and flat which are two of the reasons for the great interest in this new antenna topic. The upper surface of the dielectric substrate supports the printed conducting strip which is suitably con toured while the entire lower surface of the substrate is backed by the conducting ground plate. Sometimes the antennas are referred to as printed antennas because of the manufacturing process.

It was concluded in the previous chapter that little had been established in the literature, either by measurement or analysis, about the precise radiation mechanism of discontinuities in microstrip. Radiation is, of course, a highly undesirable effect in microstrip circuits and the priority in this respect has clearly been to avoid radiation rather than investigate its origin and behaviour. Lewin (1960) was perhaps the first to consider the analysis of radiation from microstrip discontinuities, although microstrip antenna arrays comprised of step discontinuities had already been reported several years previous by Gutton and Baissinot (1955). Very recently there has been an upsurge of interest in the analysis of various forms of microstrip antennas and these will be described in later chapters. In this present chapter, attention will be confined to the behaviour of an open-circuit microstrip termination. This element is of particular interest because it is amenable to analysis and enables a clearer understanding of the physical aspects of other more complicated discontinuities to be obtained.

In this chapter, after reviewing the forms of feeding arrangements for linear arrays, the practical forms of microstrip linear arrays that have been reported are surveyed. For this, it is convenient to split the field into arrays of resonators, arrays formed by discontinuities in a continuous line and higher-order mode arrays. In order to discuss analysis and design techniques for linear arrays, two examples are taken, namely the disc array fed through the substrate by a feed network mounted behind the ground plane, and the combline array, an example of an integral feed array structure. These examples represent the classes of array that have the feed network, respectively, behind and coplanar with the array and represent considerably different challenges to the designer. In addition, the feed types are generally corporate and series type, respectively. In the first, the feed network will be influenced by the radiating elements in terms of element impedance and inter element coupling. Thus a scattering parameter approach is used in the array analysis. In the second class, the feed system is further constrained by the geometry and has to fit in the space between the radiating elements. Furthermore as microstrip is an open structure, the feed system will also radiate and perturb the radiation pattern. Here the first order analysis incorporates the transmission-line matrix approach.

This chapter is concerned with the various techniques which may be used to generate circularly polarised radiation from microstrip antennas. Circular polarisation is particularly useful for a number of radar, communication and navigation system requirements because of the behaviour it exhibits upon reflection from regular objects of reversing the hand of circularity to produce predominantly orthogonal polarisation. The system will then tend to discriminate against reception of such reflected signals in favour of direct paths or reflections from irregular shapes. A radar system using circular polarisation will therefore receive reflected signals from irregularly shaped aircraft rather than nearly spherical raindrops, or a communication system will be able to reject multipath reflections from ground or large buildings. A further attribute is that in a communication system using circular polarised radiation the rotational orientation of the transmitter and the receiver are unimportant in relation to the received signal strength, whereas with linearly polar ised signals there will be only very weak reception if the transmitter and receiver orientations are nearly orthogonal. An example of this is a telemetry link to a small aircraft or missile which is capable of performing complex manouvers. The available techniques of generating circular polarised radiation from antennas is of great interest then for many systems applications.

The analyses of microstrip antenna element configurations discussed in earlier chapters were based on some degree of approximation concerning the current and field distributions. The results obtained show reasonable agreement with practical results in some cases, but various adjustments have to be made to the parameters used in the calculations to improve agreement. Examples of this are the use of effective widths of open circuit transmission lines rather than the physical width to calculate the radiation conductance and the allowances for fringe fields in calculating the resonant frequency of patch antennas. In this chapter, recent advances in the mathematical analysis of microstrip antennas are outlined and the extent to which they model the precise physical action is particularly discussed together with likely sources of error.