Wideband Circular TE21 and TE01 Mode Converters With Same Exciting Topologies

To accurately measure the performance of dielectric loaded circuits for gyrotron traveling wave tubes, a novel topological structure to generate the TE₂₁^O or TE₀₁^O (superscript represents circular waveguide) mode is proposed and verified in this paper. The topology is constituted by an overmoded TE₁₀^□ to TE₂₀^□ (superscript represents rectangular waveguide) mode converter with multiple matching steps and a taper from TE20^□ to TE₂₁^O or TE₀₁^O mode. This topology allows the TE₂₁^O or TE₀₁^O mode to be excited by changing the taper dimensions. Simulation results reveal that the TE₂₁^O and TE₀₁^O mode converters, respectively, have a 1-dB bandwidth of 12.8 and 9.3 GHz with an excellent performance. Converters for the back-to-back millimeter-wave measurements were fabricated with computerized numerical control and wire cutting techniques. Vector network analyzer measurements are in good agreement with the simulations.