Design of WR5 Waveguide Bandpass Filter using Oversized Spherical Resonators

This paper reports on the design of a WR5 band (140–220 GHz) waveguide bandpass filter based on four oversized spherical resonators operating at TM211 mode. Compared to the fundamental mode TM101, the spherical cavities based on TM211 mode offer appealing advantages including a higher unloaded quality factor (Qu) and a larger size. The filter is designed for 3-D printing technique, and a larger size facilitates the fabrication process and allows larger tolerances in the printing process. The filter is designed to have a generalized Chebyshev response with a centre frequency of 205 GHz, a fractional bandwidth of 4.4% and a passband return loss of 20 dB. An extracted pole resonator is introduced to produce a transmission zero at 215 GHz, for improved rejection at the upper stopband. Simulation results of the filter show good performance, demonstrating that the proposed design could find useful application in the 3-D printed millimetre-wave waveguide filters.