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.