Improving isolation of slot-coupled directional couplers with weak and tight couplings

A suitable method for improving the isolation of slot-coupled directional couplers with elliptical-shaped slot and patches is introduced. It is shown that the even- and odd-mode phase velocities depend on the width of the patches and slot of the slot-coupled directional couplers. For couplers with tight couplings, the even- and odd-mode phase velocities can be effectively equalised by cutting some parallel slots into the patches, while for couplers with weak couplings, the even- and odd-mode phase velocities can be equalised by cutting some teeth into exterior part of the slot in the common ground plane. For various slot lengths and tooth depths, various design graphs are derived by sweeping some dimensions of the coupler. On the basis of the design graphs, three couplers with tight, moderate and weak couplings are designed and simulated. The designed coupler with tight coupling is also constructed and tested. The results show that by employing the proposed method, the isolation of the slot-coupled directional couplers has improved significantly.