Microstrip dual-mode bandpass filter design using pie-section truncated semi-circle and quarter-circle resonators

New dual-mode microstrip bandpass filters (BPFs) using pie-section truncated semi-circle and/or quarter-circle resonators are presented. The even-mode and odd-mode frequency responses of these two resonators are investigated. Compared to the four-parameter optimisation of the conventional stub-loaded dual-mode resonator, only three parameters need to be adjusted and optimised for the proposed resonators. Based on these two novel resonators, three BPF examples are designed, fabricated and tested. In addition, the technique of source-load cross coupling to improve performance of the filters is adopted. The good agreement between simulated and measured results validates the design concept.