Coupling Analysis of Adjacent Substrate-Integrated Waveguides Based on the Equivalent Transmission Line Model

Abstract

This article presents a method for analyzing the coupling between two adjacent substrate-integrated waveguides (SIWs) sharing a common row of metallic cylinders, which is based on the concept of equivalent circuit and wavenumber calculation. When the specific sizes of two adjacent SIWs are known, the scattering parameters of the shared common row of metallic cylinders can be derived, and the equivalent transmission line model of the SIW coupling structure is extracted. The even-and odd-mode wavenumbers in both transverse and propagating directions are calculated for this equivalent circuit. For an ideal forward-wave directional coupler, the transmission and coupling coefficient are then derived directly from the wavenumber and the length of the structure. Based on the proposed method, three SIW directional couplers with different coupling levels are designed and fabricated for verification. Good agreements between the calculated and simulated coupling parameters are observed. The proposed method provides a simple and accurate approach to estimate the coupling between two closely spaced SIWs sharing one common row of metallic cylinders and is expected to use for the design of millimeter-wave SIW-based circuits and systems.