A design of microstrip-disk-type directional couplers

This paper describes a design of microstrip rectangular disk quadrature directional couplers on the basis of a modified eigenfunction expansion method with consideration of frequency dependences of an effective width and an effective relative permittivity of a microstrip disk circuit. Using this approximate method, we can optimize the disk configuration of directional couplers in a reasonable computational time. The validity of the design method is examined by comparison with the numerical results by the FDTD method.