Collinearly fed dual-mode dual-band bandpass filter based on a quadruple-mode half-circular ring resonator

A collinearly fed dual-band bandpass filter (BPF) with good selectivity and wide upper stopband using a single quadruple-mode half-circular ring resonator (Q-HCRR) is presented. The two passband with two transmission poles in each passband are created by using the two degenerate modes of the Q-HCRR. Meanwhile, four transmission zeros (TZs) are properly tuned for the rejections between the two passbands and in the upper stopband. The filter not only has not any perturbation element inside the ring, but also has collinear feed structure. A demonstrated filter is fabricated and measured, good agreement is achieved between simulated and measured results.