Substrate materials play an essential role in microstrip antenna design, production and finished-product performance. Several aspects of materials must be considered in the design stage when substrates are selected. What may seem ideal from a design viewpoint must be balanced against production and final product requirements. Ability to measure and control critical properties, especially relative permittivity and dissipation factor, cannot be ignored. The possible adverse effects of necessary processing steps or environment in the final application must be taken into account. Successful antenna production will depend on the use of appropriate processing techniques. New substrate types and special substrate features are becoming increasingly available and often can offer significant advantages for designers and producers.

The basic purpose of this chapter is to provide general background information about circuits in microstrip and balanced stripline (triplate) technologies, that are currently used to interconnect elements and realise antenna feed networks. It will describe the general appearance of the circuits, the techniques utilised to fabricate them and interconnect them, the materials most currently used and, finally, the very powerful computer programs presently available to analyse, design and actually draw the pattern and cut the masks required for the manufacturing process. The chapter will be completed with worked examples and an extended Bibliography.

Mobile communications often require antennas having small size, light weight, low profile and low cost. Microstrip antennas (MSA) are a type of antenna which can meet these requirements, and various MSAs have so far been developed and used for mobile communication systems. The practical applications for mobile systems are in portable or pocket-size equipment and in vehicles. UHF pagers, manpack radars, and car telephones are typical of those. Base stations for mobile communications need antennas with sector radiation patterns. Small, simple antennas are also favoured, since the antenna tower built for the base station can then be smaller and need less support for the weight. In satellite communications, circularly polarised radiation patterns are required and MSAs of either square or circular patches with one or two feeding points can be used for generating the circular polarisation. In this chapter, various types of MSAs which have been developed and applied in mobile and satellite systems are described.

In this chapter we are concerned with the relatively undeveloped subject of surface-field metrology for 'open' microstrip and other related 'open' planar transmission structures. Early measurements on microstrip lines were aimed at determining the dispersive properties of the medium using a number of different techniques, some of which are listed below: (a) Measurements on open-circuited and short-circuited resonant microstrip lines (b) Ring-resonator techniques (c) Variation of phase shift of a line with frequency (d) Nodal-shift techniques. None of these methods, however, provide much insight into the detailed field distributions that actually reside on the conductors. Liquid crystals have been employed to render mode patterns and regions of high electric field 'visible' to the unaided eye, but the spatial resolution and general applicability of the technique is very limited. The growing need for reduction in the size, weight and cost of millimetric guidance and radar equipments has stimulated the design of circuits with a much higher degree of component integration than is possible with the modular approach to circuit construction. With the growing complexity of circuit-integration techniques, both fault location and test procedures become increasingly difficult. The accessible RF input and output ports provide little information about the detailed internal operation of the system, and the conventional network analyser, much valued for the assessment of isolated modular components, is of limited use as a diagnostic for large-scale circuits. The use of a greater degree of component integration not only makes circuit evaluation difficult, but can also create severe problems owing to unwanted electrical coupling between components.

The extensions and variations to the microstrip-antenna concept described are vastly different in application, and emphasise both the ingenuity that has already been experienced and the fund of innovative ideas that will continue to be created in future years. It was stated in Chapter 1 that the inspiration for innovation lies in the demands made by new systems, and this is well illustrated by the efforts on one hand to exert more control of radiation patterns, and on the other to increase bandwidth by an order. In each case one parameter is optimised at the expense of the others in a way which is compatible with the system constraints prevailing, but above all it is the microstrip technology that is diluted by other construction techniques to effect the desired optimisation. As already mentioned, it is a sign of maturity in a technology when a designer has learnt just how to exploit selectively its best features with confidence.