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

Abstract

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.