Tri-band Bandpass Filter Evoloved From Wideband BPF with Two Loaded Stubs For Transmission Zeros

This paper presents a tri-band bandpass filter (BPF) based on a stepped impedance resonator (SIR) loaded with two open stubs. Firstly, a wideband BPF is achieved by using a SIR, by controlling the high order resonant modes through the impedance ratio and electrical length of the resonator. Secondly, a longer stub is loaded to the centre of the resonator, a transmission zero (TZ) and a resonant frequency produced in the lower passband of the BPF. Thirdly, another shorter stub is loaded to the center of the resonator, a TZ and a resonant frequency is achieved in the upper passband of the BPF. Thus the wide passband is splitted into three ones and a BPF with triple passband is realized. The structure is analyzed by equivalent transmission line model. Finally, a tri-band bandpass filter with central frequency of 2.4GHz, 3.8GHz, and 5.2GHz is designed and simulated to verify the structure.