Novel microstrip square-loop triple-mode bandpass filter for wireless communication

This paper presents a novel microstrip triple-mode bandpass filter. Three modes are achieved by adding a microstrip line to a square-loop resonator. The relation between the length of the inset microstrip line and the modes-shifting characteristic is studied. Odd mode and even mode theory is applied to analysis this new proposed resonator. A triple-mode bandpass filter in 3.5GH-band is designed based on the proposed resonator. Then the filter is fed by in-line microstrip feed lines. Circuit model based on coupling theory is set up. The filter is simulated. Experimental results show that three resonant modes occurred in the passband and a transmission zero is achieved in the lower stopband.