Minimization of return loss using minimum steps coaxial coupler for ka-band helix TWT

In this paper the impedance matching between the characteristic impedance of coaxial coupler and helix and the return loss improvement has been done using step by step modeling of the input and output coaxial coupler with minimum number of steps in the centre conductor for ka-band helix travelling-wave tube. Applying single port excitation and proper boundary conditions in CST Microwave Studio, for both input and output section in a helix travelling-wave tube, the return loss has been investigated for single step as well as multiple steps uniform, quarter-wavelength long sections, coaxial coupler along with the sever and the attenuator coating in the T-shaped dielectric support rods. Using this technique more than -15 dB return loss, throughout the frequency band, and -46 dB return loss, at the design frequency, has been achieved.