A Novel Method to Estimate the Conductivity and Surface Roughness of Folded Waveguide From S-Parameters for Submillimeter Traveling Wave Tube

The effective submillimeter-wave folded waveguide traveling wave tube (FW-TWT) design requires accurate metal surface morphology information, and predictive methods are currently urgent in this regime. In this work, a novel method based on the S-parameters of folded waveguide high-frequency components (FW-HFCs) is proposed to estimate the conductivity and surface roughness of oxygen-free copper. First, the attenuation by the loss of folded waveguide slow wave structure (FW-SWS) and rectangular waveguide was studied, and new formulas for estimating the conductivity and surface roughness were derived. Then, the surface roughness of the inner wall of the folded waveguide plates was measured, and the $S11$ and $S21$ values of FW-HFC were measured. The estimated surface roughness is $0.16~\mu $ m, which is slightly higher than the measured value of $0.1313~\mu $ m due to the fact that multiple forces and high-temperature welding for fabricating the FW-HFC indeed lead to the deterioration of the surface roughness inevitably. Finally, an FW-SWS with the estimated effective conductivity and surface roughness was modeled in HFSS and the $S21$ -parameter and attenuation were calculated to compared with the measured values. Comparison shows that the simulated curves of $S21$ and attenuation are basically consistent with the measured curves, which proves that the new method is effective and accurate. This novel method can easily and directly obtain the important information of the surface morphology of oxygen-free copper, which is of great significance for the accurate and rapid design of submillimeter traveling wave tube (TWT).