EM-driven topology evolution for bandwidth enhancement of hybrid quadrature patch couplers

A broad operational bandwidth is one of the key performance figures of hybrid patch couplers. Due to the lack of systematic design procedures, bandwidth enhancement is normally obtained through manual modifications of the structure geometry. In this work, an optimization-based topology evolution for EM-driven design of patch couplers with enhanced bandwidth has been proposed. The method exploits a novel spline-based EM model where the spline control points (also referred to as knots) are defined in a cylindrical coordinate system. The model has been optimized in an unattended manner using a gradient-based algorithm embedded in a trust-region framework. Two design cases with 13 and 21 design parameters have been considered. The optimized structures are characterized by symmetrical bandwidth (defined for the maximum of 0.5 dB imbalance between S21 and S31) of 23.4 % and 31.6%, respectively. The obtained coupler topologies have been favorably compared with state-of-the-art patch broadband couplers reported in the literature.