Surrogate-based miniaturization-oriented design of two-section branch-line couplers

A novel methodology for miniaturization-oriented design of a class of wideband branch-line couplers is proposed. The initial design is chosen from a family of optimized circuits that feature a simplified two-section topology. Compact size of the coupler is attained by using quasi-periodic slow-wave structures instead of conventional lines. Our approach explicitly aims at circuit size reduction by adjusting the number of elements within the recurrent slow-wave structure and its designable parameters to reach the smallest coupler layout possible. This is achieved at a low cost by exploiting a surrogate-based optimization (SBO) process with the underlying model of the recurrent slow-wave structure composed of multiple response surface approximations (RSAs). The SBO scheme incorporates adaptively adjusted design specifications to converge in just two iterations. A rapid fine-tuning procedure is applied to account for T-junction effects omitted during the design process. Our methodology is illustrated through a numerical example supported by experimental verification.