Stepped-impedance directional coupler with enhanced isolation using interdigital capacitance compensation

A novel and successful stepped-impedance coupler with enhanced isolation is presented. By incorporating the distributed interdigital capacitance at the ends of the coupled section in the planar structure, the even- and odd-mode phase velocities can be equalised so as to enhance the isolation. Measured results show an enhanced isolation of 47.5 dB at the centre frequency 2.4 GHz and a size reduction of 30% when compared with the conventional parallel-line coupler. It is also shown that the coupling frequency flatness is as broadband as 3.1 GHz.