3.8.1 Introduction

From the cross-sectional diagram shown in Fig. 3-20 it is apparent that one may regard this line either as a microstrip derivative (if one locates a "mirror-reflection" plane midway between the two strips), or, perhaps, as a triplate derivative (regarding it as a "split-strip" version of triplate). Neither viewpoint, unfortunately, leads to any significant simplification of the analysis problem, but as in so many of the previous instances, the pseudo-TEM mode assumption is reasonably valid, and on this basis the structure can, in principle, be analyzed either by conformal transformation or by numerical techniques.

Figure 3-20: Cross-section of the High-Q triplate stripline.

This form of stripline developed virtually in parallel with triplate and microstrip; although not quite enjoying the widespread popularity of the former (nor suffering the temporary rejection of the latter!) it has nevertheless come into very extensive use, probably because it is so easy and cheap to produce from double-clad printed circuit boards of standard type by standard methods.

It was originally thought that this configuration of conductors and dielectric would yield much lower loss than the "standard" dielectric-filled triplate, because of the low proportion of dielectric included in the propagation area (in the most common version, where ? ? = 1). However, this expectation was not entirely fulfilled, because although it is true that only a small proportion of electric field line lengths are "immersed" in dielectric, it is unfortunately the case that the dielectric-filled region adjacent to the...