Microstrip bandpass filters using cross-shaped triple-mode resonator

Compact three-pole microstrip bandpass filters (BPFs) are presented. The basic block is a triple-mode resonator that consists of a conventional half-wave resonator, a shunt open-circuited stub and a shunt short-circuited stub. The half-wave resonator, the shunt open-circuited stub and the shunt short-circuited stub control each of the first three resonant frequencies. Then, two BPFs using the resonator are designed, fabricated and tested to validate the design concept. For the narrow-band BPF, from 2.4 to 2.6 GHz, the measured return loss is greater than 15 dB and the measured insertion loss is less than 1.7 dB. For the broadband one, from 1.25 to 3.9 GHz, the measured return loss is greater than 12 dB and the measured insertion loss is less than 1.5 dB. The group delay variation is 0.45 ns.