Millimeter-Wave Tunable Notch Filter Based on Waveguide Extension for Plasma Diagnostics

A millimeter-wave tunable notch filter using several extensions of the fundamental-mode rectangular waveguide has been developed to protect plasma diagnostics techniques from stray gyrotron radiation. The design utilizes the resonant behavior of the TE₃₀₁ mode excited by cutoff coupling holes in a symmetrical smooth H-plane extension of waveguide. Proper choice of the shape and sizes of the coupling elements allows one to tune the operating frequency, notch depth, and notch band. Low-power experiments with the six-cavity filters show a maximal notch depth of below -120 dB at 70 and 170 GHz, and a notch depth of below -90 dB at frequency bands of 70 ± 0.05 and 170 ± 0.2 GHz. The filter design is rather simple and can be advanced into a higher frequency band up to the 400 GHz.