Improving isolation of slot-coupled directional couplers

A suitable method is introduced to improve the isolation of slot-coupled directional couplers. The proposed coupler consists of two trapezoidal-shaped patches that are coupled through a trapezoidal-shaped slot in a common ground plane. The inherent phase velocity differences between the even- and odd-modes of the slot-coupled directional coupler are compensated around the centre frequency of the coupler by introducing some slots to the trapezoidal-shaped patches. A design graph is derived by sweeping some dimensions of the coupler. A hatched area is also added to the design graph that indicates the appropriate coupler dimensions which lead to the equal even- and odd-mode phase velocities around the centre frequency. To demonstrate the effectiveness of the proposed coupler, an 8 dB coupler operating from 1 to 3.3 GHz is designed and constructed. The measurement results are in good agreement with the simulation results. The results show that the isolation of the designed coupler has improved significantly in comparison with the other designed coupler with no cut slots in the patches.