A High- $Q$ Quadruple-Mode Rectangular Waveguide Resonator

This letter presents a high-Q quadruple-mode rectangular waveguide resonator. The resonator has a Q better than 14500 in all the four modes at 3.7 GHz using an aluminum cavity. Two narrowband filters (fourth and eighth order) at 3.7 GHz with a bandwidth (BW) of 50 MHz (1.35%) are realized using the proposed resonator as a proof of concept. Due to the high-Q nature of the resonator, the filters exhibit very low insertion loss which is better than 0.3 and 1 dB for the fourth- and eighth-order filters, respectively. Furthermore, a transversal coupling scheme is also explored to enhance the spurious performance.