Two-layer elliptic filters with enhanced bandwidth

A two-layer design of elliptic filters with enhanced bandwidth is introduced. Larger bandwidths are achieved by placing square resonators on different planes to increase the strength of their coupling. Square resonators are used to control the sign of the coupling coefficient (magnetic or electric). A fourth order elliptic filter with a bandwidth of 320 MHz at 2.5 GHz is designed to demonstrate the validity of the approach. A second two-layer 4^th order elliptic filter based on a different coupling scheme and topology is also presented.