A compact directional coupler with tunable coupling ratio using coupled-line sections

A compact directional coupler with tunable coupling ratio is proposed. It essentially consists of a pair of 180-degree long transmission-lines and two varactors, which are used for tuning of the coupling ratio in real-time. The particular coupler considered in this work makes use of the broadside coupling configuration to achieve size miniaturization and is, theoretically, capable of having any levels of coupling. A working prototype based on broadside-coupled coplanar waveguides (CPWs) has been successfully fabricated on a conventional single-layer printed circuit board (PCB), providing a good validation to the proposed concept.