Analysis of it-mode Stopband in an Asymmetric Millimeter-Wave Helical Slow-Wave Structure

摘要

A simple closed form formula for the estimation of 7r-mode stopband in anazimuthally asymmetric helical slow-wave structure (SWS) was developed followingcoupled-mode analysis of multiple reflections of the degenerate space-harmonic modesfrom the support rod discontinuities. The method incorporates the effects of circuit loss, andaccrues the accuracy of 3D electromagnetic analysis by allowing the use of dispersioncharacteristics obtainable from any standard electromagnetic modeling. The formula issimple and amenable to easy computation, even using a scientific calculator, and withoutresorting to exhaustive and time-intensive numerical computation, and at the same time,without sacrificing the accuracy in results. The analysis was benchmarked against publishedresults and excellent agreement observed. The analysis was further used for demonstratingthe stopband phenomenon for a typical millimeter-wave helical slow-wave structure.Compared to low frequency structures, the stopband phenomenon for a millimeter-wavestructure was found to be more pronounced, and an interesting inference was drawn as tohow asymmetry induced stopband might be made to advantage in combating 7-modeinstabilities in a millimeter-wave traveling-wave tube.