Mode filter for high-power microwaves

A resistive-wall mode filter in which the axisymmetric (TE₀₁ and TE₀₂) modes pass almost without attenuation and the nonaxisymmetric (TE₁₁ and TM₁₁) modes attenuate has been developed for 28-GHz high-power microwave applications. Pyrolytic graphite having an anisotropic resistivity was installed in the mode filter so that the normal direction to the deposition surface of the pyrolytic graphite was in the axial direction of the mode filter. Mode attenuation and return losses in the mode filter were measured for the four modes using a scalar network analyzer with mode converters that convert from the rectangular TE₁₀ mode to the circular modes. Measured attenuation was 2.4± 0.3 dB for the TE₁₁ mode, 5.5±0.2 dB for the TM₁₁ mode, and 0.0±0.2 dB for the TE₀₁ and TE₀₂ modes at 28 GHz. Return losses were in the range of -20 to -25 dB for each mode