Directional Coupler Compensation With Optimally Positioned Capacitances

An accurate design synthesis for the phase velocity compensation in coupled line microstrip couplers by means of parallel capacitances is presented. In contrast to previous approaches, an a priori arbitrary placement of the capacitances along the coupled line structure is considered. By optimizing these positions, the directivity-bandwidth performance is significantly improved. Cases with two and three capacitances are treated extensively. The findings are used to generalize the compensation scheme to any number of capacitances. Throughout the analysis, the parasitic even-mode capacitance is taken into account using a realistic model. Several design examples are presented. Simulation results are confirmed by measurements. They compare favorably with those reported previously.