Wideband Two-Resonator Filters with Increased Working Attenuation Slope and Extended Stop Band

This paper presents a novel design for wideband microstrip filters using planar two-resonator structures. These structures offer a combination of sharp working attenuation slope and excellent frequency selectivity across a wide stopband. The authors explore the formation mechanisms of six zeros and ten poles of working attenuation within these structures, enabling the implementation of highly selective filters with relative bandwidths ranging from 50% to 100% or even wider. The squareness factor of the working attenuation, determined by levels of minus 30 dB insertion loss and minus 18 dB return loss, is found to be less than
$\mathrm{K}_{30/18} < 1.3$
for the proposed two-resonator structures.