Rat Race Coupler Design in Fixed Width SIW/Microstrip: Methodology and Validation

This letter introduces a simple and practical solution for designing a compact rat race coupler (RRC) in fixed-width microstrip and substrate-integrated waveguide (SIW) technologies. Unlike the traditional method that requires multiple impedance lines, the proposed approach maintains a uniform impedance and constant cutoff frequency, making it directly applicable to SIW. A significant 26.66% reduction in the coupler’s ring size is achieved. Theoretical design equations are validated through the design of couplers at 5 and 18 GHz in microstrip and SIW, and experimental findings confirm the simulation results.