Novel structure and EM-driven design of miniaturized microstrip rat-race coupler

In this paper, a novel structure and design procedure of a miniaturized microstrip rat-race coupler (RRC) is described. Small size of the RRC is achieved by folding the transmission lines of the conventional circuit into its interior, as well as by implementation of the structure on three layers. The final size of the coupler realized for the operating frequency of 1 GHz is only 220 mm², which gives over 95% footprint reduction w.r.t. a reference RRC. At the same time, rigorous optimization of coupler dimensions using variable-fidelity EM models permits rapid EM-driven design closure with the final design featuring 240 MHz bandwidth, equal power split, and obtained at the computational cost corresponding to only 12 simulations of the circuit at the high-fidelity level.