A novel planar tri-band bandpass filter using stub-loaded resonators

Abstract

This letter presents a novel planar tri-band bandpass filter (BPF) using a set of stepped-impedance resonators (SIRs) and a short-circuited stub-loaded uniform impedance resonator. The former is designed to operate at the first and third passbands and the latter at the second passband. Corresponding equivalent circuit is analyzed by using the even-odd mode theory. Skirt selectivity is obtained due to source-load coupling. For validation, a tri-band BPF with compact size is designed, fabricated, and measured, its center frequencies are located at 2.4, 3.5, 5.15 GHz. Measured results show good performance on insertion loss and return loss.