The Strongly-Coupled Resonator Triplet

This paper introduces an innovative structure, namely the strongly-coupled resonator triplet (SCRT), which is suitable for the realization of compact pseudoelliptic evanescent mode filters with inline configuration. The SCRT can be seen as the combination of a strongly-coupled resonator pair (SCRP) and a third resonator next to the pair. Compared to the SCRP, which at the passband frequencies generates a pole and a transmission zero, the new SCRT is capable of generating an additional passband pole, thus leading to the design of more compact inline filters. Similarly to the SCRP, the basic structure for filter design purposes is obtained by embedding the SCRT between two outer resonators. The coupling mechanisms and functional properties of this basic structure are explained and validated by the experimental results of a manufactured prototype.