A novel wideband microstrip branch-line coupler with compact footprint

In this paper, we present a novel structure of a compact wideband branch-line coupler. The reduced component size is obtained by using cascaded slow-wave structures as substitutes for the main lines of the coupler, and meander lines to shorten the high-impedance branches. We exploit a surrogate-based optimization concept to design the coupler at a low computational cost. This involves electromagnetic models of the slow-wave structure elementary cells and their cascades, as well as local response surface approximations of both types of sub-components. The final design operates over a 50% bandwidth with excellent performance, and occupies only 35% of the area of the reference structure. The overall cost of the design process corresponds (in terms of CPU time) to less than three EM simulations of the entire compact coupler.