Balanced‐to‐unbalanced and balanced‐to‐balanced filtering rat‐race couplers using multilayer substate integrated waveguide cavities

Abstract

This study reports a class of balanced-to-unbalanced (BTU) and balanced-to-balanced (BTB) filtering rat-race couplers (FRCs) based on multilayer substrate integrated waveguide (SIW) cavities. Input and output ports can perform the functions of balanced and unbalanced terminals to implement various operating states. The theoretical designs consisting of the mixed-mode scattering matrix and mode analysis are discussed in detail. Good differential-mode (DM) bandpass filtering (BPF) responses are obtained by two pairs of orthogonal degenerate modes in multilayer SIW cavities. Meanwhile, high common-mode (CM) suppression and port-to-port isolation are realized inherently due to mode distributions of the square SIW resonator. Accordingly, to verify the design concept, two types of BTU FRCs and one type of BTB FRC are designed and fabricated, respectively. Moreover, the measured results agree well with the simulated ones, exhibiting high performances in terms of CM suppression, magnitude/phase imbalance as well as port matching/isolation characteristics.