Trans-directional coupler with adjustable coupling coefficients and reconfigurable responses

The study presents the theoretical and experimental study of a trans-directional coupler with adjustable coupling coefficient and reconfigurable responses. The proposed coupler is composed of coupled lines, capacitors, varactor and inductors. By connecting different inductors through switches for the circuit implementation, and using varactor as compensation capacitor, the coupling coefficient of the proposed coupler can be easily controlled. It also features balanced output ports power distribution, high isolation, good return loss, and consistent output ports phase difference. In addition, when no inductors connected, the proposed coupler is reconfigured to be a crossover. Closed-form design equations are derived by utilising the even-odd mode method. Based on these analytical equations, an experimental prototype is designed, fabricated and characterised. The measured results show that the coupling coefficient of the coupler can be adjusted from -3 to -40dB with good isolation and return loss performances.