Compact cascaded quadruplet bandpass filter (CQ-BPF) using folded loop resonator with via-hole

A compact cascaded quadruplet bandpass filter (CQ-BPF) using short-circuited loop resonators is proposed for size reduction. The short-circuited loop has a resonance at a half wavelength by utilizing a via-hole and is modified to a folded structure to reduce a loop size. Comparing with the conventional loop resonator, the proposed resonator shows a size reduction of about 81%. The frequency responses of the proposed BPF have also been calculated via transmission and reflection rational functions. The measured insertion and return losses at the designed center frequency of 2.12 GHz are about 2 dB and 15 dB, respectively. The group delay is less than 1.55 ns. The full wave simulated results are well agreed with the measured ones.