Parallel-coupled stacked SIRs bandpass filters with open-loop resonators for suppression of spurious responses

A novel method to develop microstrip bandpass filters has been proposed in this letter. The unit cell elements are adopted to design bandpass filters. This unit cell element is composed of the parallel-coupled stacked stepped-impedance-resonators (SIRs) and the open-loop resonators. Compared to the uniform parallel-coupled line, the unit cell element is shorter. Moreover, the mutual couplings of the open-loop resonators can generate the transmission zero for suppression of spurious responses. The fabricated examples are presented for validating the filter configuration.