On miniaturization isolation network of an all-ports matched impedance-transforming Marchand Balun

A technique for reducing the isolation networkʹs size in a Marchand balun needed for perfect all-ports matching and isolation is proposed in this letter. The proposed isolation circuit is realized using a coupled-line phase-inverter in place of the bulky 180(degree) line section that has been previously proposed. Analysis of the proposed circuit yields the required relationship between the coupling coefficient and the electrical length of the coupler. Based on the design equations, the circuit is experimentally demonstrated at 1.8 GHz and has shown excellent results.