Efficient design optimization of compact dual-band microstrip branch-line coupler using response features

In this paper, computationally efficient design optimization of a miniaturized dual-band microstrip branch-line coupler is presented. Our optimization approach relies on suitably extracted features of a highly nonlinear response of the coupler structure under design. By formulating the design objectives in terms of the feature point locations, we carry out iterative optimization of the linear model of the features, embedded in the trust-region framework. Due to only slightly nonlinear dependence of the features on the designable parameters of the circuit, the optimized design satisfying prescribed performance requirements is obtained at the low computational cost of only 24 high-fidelity EM simulations of the structure.