Compact tri-band bandpass filter using tri-section stepped impedance resonators

A compact tri-band bandpass filter (BPF) using tri-section stepped impedance resonators (SIRs) with short loaded stubs is presented. In this letter, the second passband frequency of the tri-band filter can be flexibly controlled by tuning the tri-section SIRs, and the first and the third passband frequencies of the tri-band filter can be obtained by adjusting the parameters of the tri-section SIRs and the short loaded stubs. To validate the design, a tri-band bandpass filter is fabricated and measured. The measured results are in good agreement with the electromagnetic simulation. The circuit only occupies 0.11λg×0.12λg, where λg is guided wavelength at the lower passband center frequency.