Inline Substrate Integrated Waveguide Filter With Controllable Mixed Coupling

This paper presents a new mixed coupling structure consisting of an inductive coupling window and a folded coplanar waveguide section. It can be used as a general and independent coupling structure in a SIW filter to produce controllable mixed coupling. Thus, controllable transmission zeros and flexible responses including asymmetric and quasi-elliptic responses can be available for an inline SIW filter without using cross-coupling. To verify and demonstrate the novelties, an inline SIW filter with a 4-poles quasi-elliptic response is designed, fabricated, and measured. The results agree with expectations well. The proposed technique should be efficient for developing high-performance SIW filters in microwave/wireless circuits and systems.