A novel approach for simulation of a coaxial coupler for helix TWTs using HFSS

This paper describes a novel approach to model and analyze a helix to coaxial transition using three-dimensional electromagnetic computer code, the High Frequency Structure Simulator (HFSS) based on the Finite Element Method (FEM). For a K-Ka band helix TWT, RF Coaxial Coupler consisting of a helix to coaxial transition and a RF coaxial window is designed. The novelty of this simulation approach is that the RF matching to helix slow wave structure at the other end was provide by lossy resistive thin films being placed them on either side of three dielectric support rods. For a broadband K-Ka band helix TWT where the optimum VSWR expected for a transition is high, the used approach is promising of giving more realistic estimate of the performance of a transition compared to the method in which at both ends of helix a identical transition is modelled. The simulation approach for the coupler of an X-Ku band production tube compares well with measured results.