SIW Triplets Including Meander-Line and CRLH Resonators and Their Applications to Quasi-Elliptic Filters

Abstract

This article presents a design technique of quasi-elliptic filters by applying two novel planar triplets, which eliminates the need for negative couplings. A composite right-/left-handed (CRLH) resonator, together with a meander-line resonator, is investigated first in this article. These two resonators demonstrate a significant difference with respect to their phase shift characteristics that are completely opposite below and above their resonant frequencies. With such a feature, two meander-line and CRLH-loaded substrate integrated waveguide (SIW) triplets are proposed, which can produce a transmission zero (TZ) above and below the passbands if an inductive cross coupling is applied. Therefore, all couplings would be positive regardless of the location of TZs. The applications of the two triplets to quasi-elliptic filters, which are realized by parallel and cascaded triplet topologies, are then illustrated. Two filter prototypes operating at the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$X$</tex-math> </inline-formula> -band are designed and fabricated for validation. They show outstanding electrical performance in terms of high selectivity, low loss, and compactness. It indicates that a joint application of conventional shunt <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> resonant nodes and a CRLH node is a viable option in filter design.