Miniaturized planar branch-line coupler with asymmetrical π-shaped structure

This paper presents a new design methodology for microstrip branch-line couplers utilizing asymmetrical π-structures. Design equations of the coupler with sub-optimum performance are analytically derived. The branches of the original hybrid coupler are decomposed in to high and low impedance sections with the low impedance section being reduced to an equivalent π-network. The proposed approach features compact size, planar structure and low-loss. The developed planar branch-line coupler occupies 46.28% of circuit area compared to the conventional structure. Design method is illustrated with a microstrip branch-line coupler operating at 2.44 GHz. The isolation is 40dB at 2.44GHz. The impedance bandwidth of 38.6% is obtained centered about the design frequency.