SHIELDED SURFACE WAVEGUIDES

Abstract

Recently a large number of papers have been published on the topic of Shielded Surface Waveguides. H.M. Barlow has been one of the earliest workers in this field. In his first paper on the 'topic(3)he potn-ted out the possibility of a screened surface waveguide and followed it up with another paper (4), where he -presented an analysis of both the parallel plate and coaxial waveguide structures with dielectric coated guiding surfaces. Especially, he talks about the hybrid TEM dual surface waves which he considers to be the natural mode in two conductor transmission lines, the usual TEM mode being present ai only when the guiding surfaces are perfect conductors. Barlow maintains that this hybrid model under suitable circumstances, suffers an attenuation which is roughly one half of that exper-ienced by the usual TEM mode • To support this modes one is to enhance the surface reactance by coating the surface with a practically loss-free dielectric • Through his experiments, whose results appeared in the literatures, $ Barlow has been able to substantiate his observations • He has found that to reduce the attenuation one has to have an asymmetry in the field distribution, that is= the surfaces should be of un-equal reactances and further, only one of the surfaces should be coated with dielectric,, the thickness of coating being optimum. Barlow's work thus revealed the possibility of propagating• in the usual transmission line structures, a surface wave mode, which is for less attenL ted 'rhls compared to the usual TEM mode, naturally aroused the curiosity of other workers and Wait came up with his theory of Shielded Surface Wave guides(5)0 His con- clusions are rather interesting and they stimulated the present work. It is in order to mention that this work is an extension of Wait t s work. We have ventured to make a thorough investigation of the various modes which can exist in a parallel plate guide;, with reactive guiding surfaces• Wait has merely indicated the approach and what we have done is s to pursue this course to its logical end. We have analysed the symmetric case (i•e. s where the two surfaces are of equal reactances) and the unsymmetric case (i.e. surface reactances unequal). Also we have considered the case where one surface is inductive and the other is capacitive* In the course of these investigations we have come up with some new and interesting results. Mostly* the results have supported [Fait's conclusions• But on some counts we differ with him• A significant pahx~ of departure is in the f acts emerging out of this works that the modes are not orthogonal in the sense of Waits for the general un symmet ric case. Furthers Wait has not mentioned TE type modes existing in a guide with inductive guiding surfaces. It is shown that 'o would have TE type waveguide modes in such a structure though no TB type surface wave can exist. Another significant result is that there are two surface waves in general s of which* one vanishes at a particular value of surface reactance called the threshold value and turns into a zero order waveguide mode; Wait had said that this zero order mode would split into two surface waves at a particular value of reactance. Finally it is shown that one of the surface waves which stays on till zero frequency coincides with the type Barlow has been considering • Thus this work links the works of Barlow and Wait. Also a few new prob-lems are brought to light as a result of this work. They are mentioned at the end. While writing this dissertatiai s we have divided it into f our Chapters. The first Chapter gives a brief but comprehensive introduction to surface waves This Chapter draws its material chiefly from references (1) and (2) . The second is devoted to a review of the past work in the field of Shielded Surface IVaveguides. The third constitutes the author's contribution • Here all the results of calculations performed on the IW 1620 computer, along with fairly detailed analysis and discussions are presented. The last Chapter s after presenting in outline, the new results which have come to light after the present work, goes on to unf old areas for further work. Before we closes it is to be pointed out that much work is in progress on this topic. Recently papers have appeared which are critical of Barlowts approach and his conclusions. Especially n the plausi-bility of reduced attenuation is being debated. Millington., Brown and Cullen have conducted theoretical investigations wherein they have examined the possibi- lity of reduced attenuation. Their results seem to div-erge from Barlowts conclusions. As yet the last wore. has not been written on this topic. Already much absorbing work is going on and scientific world is awaiting the results. a