A tunable directional coupler with a wide tuning range of coupling ratios

A tunable microwave 90-degree directional coupler with a wide tuning range of coupling ratios is introduced in this paper. The architecture of this device is designed based on a fully symmetrical structure. Two varactor diodes loaded to the side of the device are used as the tuning elements. With different biasing voltage, the output power ratio between the two output ports (Port 2 and Port 3) can be easily tuned. During the tuning, very good isolation and low return loss are always kept. In the theoretical discussion, the general design equations are presented. Also, a sample circuit working at 1 GHz was made and tested to verify the design theory. The measurement results of the prototype show a good agreement with the design theory.