Notch filters are the most important mm-wave components to protect front-end receivers from intensive gyrotron stray radiation in fusion plasmas. Here we describe a notch filter design with 105 GHz center frequency and 200 MHz rejection bandwidth. The design is based on a fundamental rectangular waveguide with cylindrical cavities coupled by narrow iris gaps, i.e. small elongated holes of negligible thickness. We use numerical simulations to study the sensitivity of the notch filter performance to changes in geometry and in material properties within a total bandwidth of ±10 GHz. The typical insertion loss in the passband is below 1.5 dB, and the attenuation in the stopband is approximately 40 dB.
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