Fast optimization of quasi‐periodic slow‐wave structures with applications to broadband microwave coupler miniaturization

Abstract

This work discusses design optimization of quasi‐periodic slow‐wave structures in the context of wideband branch‐line coupler miniaturization. Size reduction is achieved by using slow‐wave structures as substitutes for conventional transmission lines of the reference circuit. Selection of a specific cell realization as well as determination of a repetition factor of the quasi‐periodic slow‐wave structure is accomplished by optimization‐based theoretical studies. The principal design procedure involves cell optimization using electromagnetic simulations, local response surface approximation modeling of the single cell, and surrogate‐based optimization of the slow‐wave structure composed of cascaded cell approximation models. Surrogate‐assisted design closure is subsequently applied to account for T‐junction effects neglected in the preceding stages of the design process. The final coupler exhibits high‐performance operation over a 31% bandwidth and occupies only one third of the reference circuit area. The component obtained in this work illustrates the highest bandwidth‐to‐size ratio as compared to state‐of‐the‐art structures reported in the literature. In addition, the feasibility of the approach is confirmed by experimental data.