Application of microstrip cavity couplers for wideband bandpass filters

This article presents a design for wideband bandpass filters using a microstrip cavity coupler. The broadband nature of the cavity-coupled designs is fully exploited for this application. Two additional circuits are introduced to improve the stopband performance of the cavity-coupled bandpass filter. A prototype design example is presented with its center frequency located at 6.28 GHz. The design has the merits of wide bandwith of more than 140%, flat group delay within the passband, good insertion/return loss, and stopband attenuation of nearly 30 dB up to 30 GHz. The validity of the simulation results is verified experimentally.