Design of a quasi-millimeter-wave UWB bandpass filter using microstrip dual-mode ring resonator and half-wavelength resonators

In this work, a novel quasi-millimeter-wave ultra wideband (UWB) bandpass filter (BPF) is designed by using a microstrip dual-mode square ring resonator and two folded half-wavelength stepped-impedance resonators. The transmission line equivalent circuit of the filter is analyzed first based on the even- and odd-mode method, and design formulas are derived for determining the frequencies of reflection zeros in the passband and transmission zeros in the stopband, as well as the 3dB edge-frequencies. Then by using the obtained formulas and circuit simulators, variations of the frequency response of the filter versus the circuit parameters are investigated. Finally, a compact UWB microstrip BPF with a midband frequency of 25.5 GHZ and a fractional bandwidth of 22% is designed, and the desired wide passband, sharp skirt property, and wide stopband are realized simultaneously. The FCC´s UWB spectrum mask is met quite well.