A Bandpass Filter with Compact Size and Extended Stopband Using Closed-Loop Transmission-Lines and Short-Circuited Stubs

A novel 3-pole bandpass filter (BPF) based on microstrip loaded ring resonators (LRRs) is proposed. Each resonator is composed by a closed-loop transmission line and a short-circuited stub. By properly adjusting the impedance and the electrical length of each resonator, the proposed circuit may be made compact (over 93.7% smaller than a conventional ring resonator) and its stopband may be extended simultaneously. Each resonator exhibits an area of 0.0727 λ x 0.079 λ where λ is the guided wave length. A BPF at the center frequency of =1.9 GHz with stopband extended up to 7.8 GHz (=4) is developed showing good agreement between simulation and experimental results.