A Compact Tri-Band Passband Filter Based on Three Embedded Bending Stub Resonators

A microstrip tri-band bandpass filter (BPF) based on three embedded bending stub resonators (EBSRs) is proposed in this paper. Three resonant paths that resonate at three different frequencies can achieve three passbands. The lumped circuit models of the proposed filter are given for designing. The filter is extremely compact, whose area is about 0.047λ×0.12λ. There are two transmission zeros located between the first two passbands and a transmission zero between the second and third passbands, which results in good selectivity. For demonstrating the proposed filter structure, a filter at 0.9/2.14/3.6 GHz is designed and fabricated. The measured results are well agreed with simulated ones, which indicate the validity.