An X-band waveguide bandpass filter with enhanced stopband using offset resonators

This letter presents a narrowband third-order waveguide bandpass filter (BPF) with extended out-of-band response. The filter is composed of three offset resonators, and each resonator is constructed by shifting half of a rectangular resonator, featuring a very large distance between the first and second resonant frequencies. The BPF was designed with a center frequency (f0) of 9.5 GHz and a passband bandwidth of 100 MHz and was fabricated by using the Computer Numerical Control (CNC) milling process. The experiment results show that the BPF has an average insertion loss of 0.45 dB, a return loss of >16 dB, and a 30-dB stopband suppression up to 1.75 f0. The measured results agree well with the electromagnetic (EM) simulations.