3.6.1 Introduction

This form of strip transmission line, shown in cross-section in Fig. 3-7, has come into increasing prominence in recent years. Originally conceived in the early 1950's ^{[32] , [33]}, it enjoyed a brief spell of popularity and intensive investigation, but was eventually rejected for microwave use due to the high loss per unit length occasioned by radiation (cf. the single-wire-above-ground discussed in Chapter 2). This was largely a result of the low dielectric constants (around 2) of the substrate materials then in use. Further developments were prevented by the lack of availability both of high dielectric-constant, low-loss materials, and of suitable methods of processing and production. In more recent years, these difficulties have been completely overcome, and this, coupled with the ever-increasing demands for miniaturized microwave circuitry for use in weapons, aerospace and satellite applications, has led to renewed intensity of interest in microstrip. Further stimulus for the re-investigation of microstrip circuits has come from the somewhat unexpected area of high speed electronic computers: these involve the use of nanosecond pulse techniques, which in turn obviously require the use of circuitry appropriate to the GHz range of frequencies.

Figure 3-7: Cross-section of the microstrip line. The medium of dielectric constant ? ? is assumed to be infinite in extent. Usually, ? > > ? ?, and in most practical cases ? ?=1 (air).

The strip thicknesses used in present-day microstrip circuits are quite small; between 0.0003 and 0.003 in...