Analysis of Insular Guide Launcher Radiation Loss and Comparison with Microstrip Counterparts

Abstract

An analysis of a launching configuration enables the radiation loss associated with the feed region of a millimetre wave insular guide to be assessed. Series expansions of the guide fields are used in conjunction with a variational formulation of the launcher loss which is computed for various guide parameters and dimensions . An alternative assessment of loss based on a chopped surface wave distribution is also presented and reveals other effects present. Finally a comparison with typical launching losses for microstrip lines is made and some experimental data is also given. It is concluded that for some circuit and antenna applications the use of micros trip rather than insular guide could lead to an overall lower system loss. INTRODUCTIONThe past decade has witnessed an upsurge of interest in designing radar and communication systems at millimetre wavelengths. Much attention has been focussed on the use of dielectric structures, such as image and insular guide [1] in filters, waveguides and antennas [2], due to their low transmission loss at these wavelengths. In the case of antennas, the planar construction of the new dielectric arrays is particularly useful for conformal applications [31 and has the potential for full integration with similarly constructed feed systems. At present, however, conventional waveguides are predominantly being used for millimetre systems, in preference to microstrip lines, due to their lower transmission losses. Whilst this achieves a reduction in system loss, it does introduce a degree of incompatibility with the new dielectric planar array structures which can result in a significant radiation loss at the feed point. This places a different emphasis on the merits of these dielectric-type arrays in comparison to their microstrip counterparts and the purpose of the paper is to obtain by analysis an assessment of * typical launcher radiation loss associated with use of insular guide. * comparative data for microstrip structures.* system implications for millimetre wave circuits and antemnas using these guides. ANALYSISThe launcher used in practise, shown in Figs 1 (c) and 3(a), consists of a conventional waveguide horn, with a tapered insular guide insert to obtain a good VSWR. For the purposes of obtaining some estimation of the radiation loss, we have analysed a more simplified model of the launcher consisting of a parallel plate waveguide, fitted with an insular guide insert and the top metal plate abruptly terminated. Before we consider the launching mechanism, we have to obtain an adequate representation of the basic insular guide fields, firstly bounded by the top metal plate of 534