A Tri‐Band Bandpass Filter Loaded With Dual‐Function l‐Shaped Stubs

This paper presents a novel planar tri‐band bandpass filter (BPF) composed of a uniform impedance resonator (UIR) loaded with short‐circuit stubs and a set of stepped impedance resonators (SIRs) loaded with dual‐function l ‐shaped stubs. The UIR with short‐circuit stubs operates in the second passband, while the SIRs generate the first and third passbands. Compared to traditional open stubs, the proposed l ‐shaped stubs not only adjust the position of the third band but also improve its performance. The equivalent circuit is analyzed using odd‐even mode theory. By introducing source‐load coupling at the feed lines, transmission zeros are created between the passbands, enhancing selectivity. To validate the design, a compact tri‐band BPF centered at 2.4 GHz, 4.0 GHz, and 5.0 GHz was fabricated and measured. The results demonstrate excellent insertion loss and return loss performance.