A Slow Wave Folded Ridge HMSIW Using Spoof Surface Plasmon Polaritons Structure and Its Application in Coupler Design

In this article, a folded ridge half-mode substrate integrated waveguide (FRHMSIW) combined with the novel spoof surface plasmon polaritons (SSPPs) is proposed to reduce both the lateral size and the longitudinal size. The SSPP structure consisting of subwavelength U-shaped grooves, metal patches, and metal blind holes is loaded into the FRHMSIW. First, the SSPP increases the effective permeability and permittivity in the waveguide, which can decrease the cutoff frequency and hence reduce the lateral size. Second, the SSPP has slow wave (SW) effect, which can reduce the longitudinal size of the waveguide. Analytical formulas of the effective permeability and permittivity are derived based on anisotropic media theory, which are verified by comparing the calculation results with full-wave simulation results. According to the derived analytical formulas, a prototype of the proposed FRHMSIW-SSPP waveguide is designed and fabricated using the PCB process. The measurement results show that the proposed waveguide reduces the lateral width by 78.7% and the longitudinal dimension by more than 74% compared with the classical SIW with the same cutoff frequency. Subsequently, the FRHMSIW-SSPP structure is applied to design a 3-dB coupler. Results show that the coupler based on the FRHMSIW-SSPP can reduce the surface area by 92.5% compared with the classical SIW coupler.