Wide-band microwave bandpass ring filter

This paper presents a wide-band 2^nd order bandpass filter based on a dual-mode ring resonator. A slot ring is constructed in the ground plane and coupled to quarter-wavelength microstrip feedlines. This way, the coupling gap between the ring resonator and the quarter-wavelength lines can be reduced to the value of the substrate thickness. Thus, the coupling of the resonator can be improved and the bandwidth of the filter is increased. Simulation and measurement results are proposed throughout the paper to illustrate the concept and to validate the idea.