Low-cost optimization of compact branch-line couplers and its application to miniaturized Butler matrix design

A low-cost technique for simulation-driven design optimization of compact branch-line couplers (BLCs) is presented. In the first stage, the coupler cells are individually optimized using a pattern search algorithm. In the second stage the entire coupler structure undergoes a fast tuning exploiting fast surrogate model, constructed from cascaded local response surface approximations (RSAs) of the cells. Accurate, high-fidelity EM analysis of the entire coupler structure is only needed at the tuning phase. The final design of the compact BLC (87.6% area reduction compared to conventional coupler) is obtained at the cost of less than three full-wave simulations. The optimized coupler is then successfully applied in the design of miniaturized Butler matrix.