A New Tri-Band Bandpass Filter Based on Stub-Loaded Step-Impedance Resonator

A tri-band bandpass filter designed based on a stub-loaded step-impedance resonator (SIR) is proposed. The resonant behavior of the stub-loaded SIR is analyzed. By properly controlling the impedance ratio K
₁
, the length ratio (α) and the length ratio (r) of the stub-loaded SIR, the center frequencies are designed at 1.575, 2.4, and 3.5 GHz, respectively, corresponding to the GPS, WLAN and WiMAX applications. Furthermore, a 0° feed structure is used to provide at least one transmission zero near passband edge of each passband, resulting in high selectivity. Experimental results show good agreement with the simulated results.