A dual-mode bandpass filter that allows for triple-band operation is proposed in this letter. The proposed filter uses a square patch as a resonator. A cross slot is located inside the patch, a pair of slits is applied to the edge of the patch, and a pair of stubs is applied to the feed structure. By conducting parameter study and current distribution analysis, it was found that f1 is formed by the slot placed inside the patch, f2 is formed by the slit, and f3 is formed by the stub. In addition, the slit formed a transmission zero between f2 and f3 to convert the dual-band performance into a triple-band one. The three passbands were centered at 1.96, 2.94, and 3.34 GHz, with fractional bandwidths of 14.2%, 7.5%, and 10.9%, respectively. The measured minimum insertion losses of the three passbands were 1.24, 1.35, and 1.62 dB, respectively.
Youngje Sung (Thu,) studied this question.