Design of high‐performance hybrid branch‐line couplers for wideband and space‐limited applications

Abstract

This study presents a new design method of high-performance broadband equal-split branch-line couplers with compact footprints. Size reduction as well as wideband operation are achieved here by using cascaded slow-wave cells in place of conventional transmission lines that are basic building blocks of a reference structure. The initial stage of the design process involves numerical optimisation of the reference coupler equivalent circuit model to enhance its performance and account for practical design limitations. The next stage concerns determination of a repetition factor of the recurrent slow-wave structure that provides sufficient bandwidth and phase characteristics to be a suitable substitute for a conventional line. The subsequent steps include cell optimisation using fine electromagnetic simulations, local response surface modelling of the single cell, and surrogate-based optimisation of the recurrent slow-wave structure composed of cascaded cell approximation models. Additionally, minor T-junction and proximity effects that deteriorate the performance of the compact coupler are compensated by means of surrogate-assisted design closure. The proposed method is demonstrated by two numerical examples. The compact couplers obtained in this study illustrate the highest bandwidth-to-size ratio when compared against state-of-the-art wideband couplers from the literature. Experimental verification of one of the examples confirms the reliability of the proposed design approach.