Solution of field distribution in stripline directional coupler

In design of the waveguide-stripline-coaxial line directional coupler, the solution of the field distribution in the stripline is essential. Using the Schwarz-Christoffel transformation function, the perfect stripline is transformed into a parallel-plate capacitor, and the field distribution in the stripline is derived. The coupling coefficient, directivity and the relationships between coupling coefficient and geometric dimension are obtained by the coupling theory of small hole. The comparison between the calculated coupling coefficient and the measured one of couplers manufactured by us shows little difference as 1.04 dB.