Directivity tuning of microstrip couplers

A method for tuning the directivity of directional couplers by simultaneously adjusting the phase velocity and the impedance level of the coupler is presented. It is shown that by using three symmetrically positioned capacitors, the solution of the compensation problem is under. Here, the additional degree of freedom is used to compensate the effect of typical manufacturing tolerances. A theory for the compensation with three capacitances is developed. Experimental proof is provided with a conventional mechanical tuning scheme. Measurement results show good performance, thus confirming the approach.