Oversized Multimode Waveguide Filter for Circular TE Mode High Power Transmission Lines

A design method of oversized multimode waveguide filter, which can deteriorate the unwanted modes without affecting the operating mode, is proposed to restrain the trapped mode resonances for the circular TE _{01} mode high power transmission lines. In order to separate the undesirable modes from the TE _{01} mode, a deformed circular waveguide is introduced. The eigenmodes in the waveguide are deduced theoretically by perturbation analysis, and the propagation performances of some common modes are studied. The analysis demonstrates that a clear mode distinction can be obtained between the operating TE _{01} mode and the others by the deformed circular waveguide, especially for the TE _{02} mode that shares a highly similar field pattern with TE _{01} mode. According to the analysis, a mode filter is designed in which four radiation slits are introduced to couple the power of parasitic mode out of the waveguide while maintain the transmission of TE _{01} mode. A prototype of the mode filter is designed, fabricated, and measured in Ka -band. The input and output radiation field is measured to qualitatively demonstrate the performance of the proposed waveguide filter. Transmission measurement results further show quantitatively that the parasitic modes are filtered efficiently, while the operation TE _{01} mode suffers little attenuation.