Miniaturized Circular Substrate Integrated Waveguide Bandpass Filter With Flexible Mixed Coupling and Wide Stopband Rejection

This brief presents a series of miniaturized circular substrate integrated waveguide (CSIW) bandpass filters (BPFs) with compact size and wide stopband performance. The proposed BPFs exploit the sub-mode CSIW resonator with arbitrary angle and construct a class of second-order BPFs with mixed electric and magnetic coupling scheme. The relevant electromagnetic characteristics and operating mechanism have been analyzed in detail. In this instance, the transmission zeros (TZs) of the second-order BPFs can be positioned on the left or right side of the passband, and they can be flexibly adjusted. For validating the designed concept, a third-order sub-mode CSIW BPF has been designed, simulated, fabricated and measured as a specific example. The prototype exhibits a center frequency of 10 GHz with a 3-dB fractional bandwidth of 10%. Two TZs are generated in the vicinity of left and right passband. The upper stopband rejection can reach to
$2.09f_{0}$
up to 20 dB. The simulated and measured results have demonstrated decent agreement.