Design of a compact triple-band bandpass filter using triple-mode stub-loaded step-impedance resonator

In this paper, a compact triple-band bandpass filter (BPF) is proposed based on a triple-mode stub-loaded step-impedance resonator (SIR). The proposed SIR is formed by attaching two identical open stubs at the symmetrical sides of a step-impedance microstrip line, which can be explained using the even-odd-mode analysis method. A pseudo interdigital structure is used to realize the compact size of the filter, and two coupling paths are utilized. The three desired passbands can be conveniently allocated by properly choosing the dimension parameters of the stub-loaded SIR. The measured center frequencies of the three operation passbands are 2.2 GHz, 3.5 GHz and 4.6 GHz with a low insertion loss of 1.14 dB, 1.21 dB and 2.20 dB, respectively. Good agreement between the simulated and the measured results is observed.