A novel contra-wound slow-wave structure for travelling-wave tube amplifier

Abstract

Backward-wave oscillation is frequently a problem in TWT amplifiers that use the conventional circular helix slow-wave structure (SWS). Circular contra-wound helix SWS, proposed in the past to solve this problem, is hard to fabricate at high frequencies. A planar version of the contra-wound structure is easily realizable but has not been studied so far.

This project involves design, fabrication and testing of a novel a planar contra-wound planar helix SWS that can be fabricated using printed circuit techniques. A simple PH-SEC structure has been modified into a contra-wound PH-SEC structure and benchmarked against previously simulated results to understand the dynamics of the CST software, requisite settings for running reliable simulations and the intricacies of CAD designing. The parameters studied include phase velocity, coupling impedance and S-parameters. It is shown with the help of simulated dispersion characteristics that the proposed structure decreases the risk of backward-wave oscillation in a TWT. The performance of the discussed structure has also been measured against that of a RRB-SEC structure designed earlier [14] and the new design proves to be an improvement over the latter in terms of having comparable S-parameters over a broader frequency range.

The fabrication has been done using RO4003 substrate and annealed copper. The testing results fair reasonably well against the simulated results and have also been compared to the testing results for the RRB-SEC structure. The research work done towards this project has been selected for presentation at the European Microwave Conference 2014 to be held in Cecilia, Italy in October 2014.