Design of a broadband bandpass filter using microstrip stubs-loaded three-mode resonator

A novel microstrip stubs-loaded three-mode resonator is proposed, and a wideband bandpass filter having a midband frequency of 4.1GHz and a fractional bandwidth of 34.2% is designed to meet Japan's UWB low-band spectrum mask. The stubs-loaded three-mode resonator is simultaneously used with half-wavelength resonators and a coupled-line hairpin resonator. As a consequence, the desired wide passband is obtained, meanwhile multiple transmission zeros in the stopband of the filter are produced, which result in a sharp passband skirt and a very wide stopband. The frequency response of the filter satisfies well Japan's UWB spectrum mask.