Compact microstrip square-loop dual-mode bandpass filter design with wideband response

In this paper, a dual-mode bandpass filter using square-loop resonator is proposed for wideband applications. By introducing two capacitive stepped-impedance inner arms along the square-loop resonator, a wide passband response is achieved. In addition, the degradation of the insertion loss over the passband can be enhanced by a pair of tight coupling feed lines located at two perpendicular positions of the square ring. In order to demonstrate the performance of the proposed filter structure, a prototype filter is designed at 5 GHz with 74% fractional bandwidth. Good agreement is obtained between simulations and measurements.