Tri-band bandpass filter using two short stubs and an open stub loaded resonator

A tri-band bandpass filter (BPF) using two short stubs and an open stub loaded resonators is presented. The classical even-/odd-mode method is applied to analyze the characteristics of the proposed resonator, which shows that it has two pairs of symmetrical resonant modes. The center frequencies of the first and the third passband can be flexibly controlled by tuning the dimensions of two short stubs and an open stub, whereas the second passband frequency is fixed. To verify the proposed approach, a prototype of tri-band BPF centered at 1.57, 2.4 and 3.95 GHz is designed and fabricated. Measured results agree well with simulated ones.