Microstrip dual-mode triangular patch resonator bandpass filters with two or four transmission zeros

An in-line feed-line configuration for design microstrip dual-mode triangular patch resonator bandpass filters (BPFs) is presented in this paper. By properly arranging the feed-line positions, it generates two or four transmission zeros adjacent to cutoff frequencies of the passband, which improves the selectivity. The computed current distributions of a pair of degenerate modes show slight difference from that of conventional triangular patch resonators. Meanwhile, without coupling gaps between the resonator and feed-lines, the introduced structure can reduce etching uncertainties and improve the passband insertion loss. Two demonstrating filters are optimally designed, fabricated and tested. Both theoretical and experimental results are presented.