W‐band waveguide filter based on large TM 120 resonators to ease CNC milling

A W‐band waveguide bandpass filter, based on coupled TM 120 resonators, fabricated using CNC milling, is presented. TM 120 resonators are superior to conventional TE 101 resonators, in terms of having a larger size and reduced aspect ratio, which facilitates the fabrication process as well as allowing larger tolerances in the milling process. This type of resonator also offers a high unloaded quality factor, and ultimately yields a small insertion loss for the filter. A W‐band filter, with a centre frequency of 100 GHz and a fractional bandwidth of 6%, is designed using four TM 120 resonators and constructed from three brass shims which directly fit to the waveguide flanges. This filter is measured to have a median passband insertion loss of 0.5 dB and a return loss better than 13 dB across the whole passband.