Negative‐order ridge substrate integrated waveguide coupled‐resonator filter

A negative‐order resonator (NOR) based on a ridge substrate integrated waveguide (RSIW) structure and its application to a coupled‐resonator filter (CRF) is investigated. The NOR is realised by etching an interdigital slot on the ridge surface of the original RSIW to achieve the composite right/left‐handed (CRLH) characteristics, which makes the resonator exhibit the unique property of negative‐order resonances. The −1st‐order resonance can be generated with a much lower resonant frequency than that of the original RSIW resonator with the same size. Thus, miniaturisation can be achieved by using this NOR structure. Besides, the non‐radiative property of the CRLH RSIW structure would help in avoiding radiation loss and electromagnetic interference to the other circuits, which is a better choice in guided‐wave applications than other open radiative CRLH structures. Electric coupling is adopted between the neighbouring resonators in the filter design. This CRF shows the advantages of a low‐profile, compact size, non‐radiation as well as easy integration with other planar circuits.