A dual-mode dual-band bandpass filter with novel perturbation element using open loop resonator

A novel dual-mode dual-band bandpass filter (BPF) is proposed by placing a couple of open-circuited stubs in each corner of the square loop resonator. The two degenerate modes are excited by moving the gaps between the ends of the two open stubs to obtain perturbation effect. No extra perturbation element is needed resulting in flexibility in tuning dual-mode characteristic and a compact structure. The proposed dual-mode dual-band bandpass filter operating at 1.8GHz and 4.6GHz is simulated, fabricated and measured. The 3dB fractional bandwidths are 2.5% and 4.3% for the two passbands respectively. The experimental results agree well with the predicted ones.