Evanescent mode filters using strongly-coupled resonator pairs

A new technique for the realization of evanescent mode filters with in-line configuration and pseudoelliptic response is presented in this paper. The basic element of the proposed filters is the strongly-coupled resonator pair (SCRP) embedded between two outer standard resonators. The SCRP, which consists of a pair of relatively close resonators, is exploited in such a way so as to provide a single resonant node as well as a direct path between the outer resonators, thus realizing a triplet. The resulting transmission zero can be located below or above the passband by properly controlling the coupling mechanisms occurring within the structure. The experimental result of a 9^th order filter employing two SCRPs for the realization of as many transmission zeros validates the proposed approach.