Design of an ultra-wideband compact 90-degree 3dB coupler for image-rejection up-converter

In this paper, the design of a novel 90-degree 3dB coupler with ultra-wide band and compact side is presented. The proposed coupler is formed by two elliptically shaped microstrip lines, which are broadside coupled through an elliptically shaped slot. Results of simulation and measurement show that the designed coupler exhibits excellent performance. The coupling coefficient is 3±1dB and phase difference between the two output ports is 90°±5° from 1.1 GHz to 2.1 GHz. The designed coupler has been applied in an ultra-band Ku-band communication system as a key component of the image-rejection up-converter, where more than 20 dB image rejection is obtained.