A Flat-Passband Microstrip Filter With Nonuniform- $Q$ Dual-Mode Resonators

A compact lossy microstrip bandpass filter is realized with flat passband and high selectivity, using the method of nonuniform Q. Two dual-mode resonators are utilized. The Q-factor of odd mode can be properly reduced by loading resistors over the symmetric plane of each resonator, with the even-mode Q unchanged. The geometric and resistance parameters are determined by the derived formulas and optimization. The 0.2 dB bandwidth of the fabricated prototype is improved by 62% when compared with the uniform-Q counterpart. The equivalent Q-factor is as high as 970, which is extracted from measurement.