SIW Directional Coupler with Improved Isolation for X-band Applications

This paper presents the design of a high-isolation directional coupler for X-band applications, utilizing substrate integrated waveguide (SIW) technology. The coupler features a simple structure, compact size, and ease of integration with other planar circuits. Typically, the S-parameters of a directional coupler are determined by the dimensions of the SIW and the aperture (or hole) of the coupler. In this study, we introduce additional via lines to modify the SIW and the coupler aperture, aiming to achieve high isolation. First, two via lines are embedded in the center, converging into two central vias that form the coupler's aperture. The power ratio within the coupler is controlled by adjusting the width of the aperture and the overall width of the SIW. Specifically, the width of the SIW at the aperture position is affected by adding vias on the two outer sides of the SIW. By incorporating these vias, we can effectively manage the power distribution across the four ports while ensuring sufficient isolation among them. The proposed design achieves an insertion loss of 3.3 dB, a coupling factor of 6 dB, and an isolation factor of 28.6 dB at 10 GHz. Experimental results demonstrate that the coupler maintains S_41 less than -20 dB over a 30% fractional bandwidth, ranging from 8.6 GHz to 11.6 GHz.