Wideband Bandpass Filters Based on Eighth-Mode Substrate Integrated Waveguide and Microstrip Resonators

A class of wideband bandpass filters (BPFs) with two eighth-mode substrate integrated waveguides (EMSIW) and two microstrip resonators is presented in this brief. The quarter-wavelength microstrip resonator is connected to the hypotenuse side of EMSIW resonator, which is different from these filters with two EMSIW resonators sharing vias or iris windows. By introducing the quarter-wavelength microstrip resonators, the design flexibility can be improved with a compact size. Based on the proposed topology, a BPF centered at 4 GHz is designed and fabricated with fractional bandwidth (FBW) of 25%. The measured insertion loss within the passband is only 0.9 dB, while the return loss is better than 14.1 dB. To further improve the roll-off skirts and attain a better out-of-band rejection, a transmission zero (TZ) is generated with the mixed electric and magnetic coupling between two microstrip resonators. The measured FBW of the BPF with one TZ is 26.6%, whose center frequency is also at 4 GHz. The measured stopband rejection above the passband is better than 34 dB from 5.02 to 8.31 GHz. Good agreement between simulated and measured results validates the feasibility of the proposed BPFs.