Optimization Procedure of Four-Port and Six-Port Directional Couplers Based on Polygon Equivalent Circuit

A new optimization procedure for four- or six-port couplers is proposed. The optimization is based on the equivalent polygon network that is constituted by a kernel made by a polygon with N sides, as the number of the coupler ports, with N(N - 1)/2 susceptances placed at the sides and the diagonals. The kernel susceptances must have proper values to represent a coupler with the excited input port matched, the other N/2-1 ports insulated, and the output N/2 ports sharing the same power. The procedure permits to optimize the coupler in two steps. In the first step, only the diagonal susceptances are optimized varying the geometrical parameters of the coupling region. In the second step, four or six irises are placed at a proper distance from the coupling region to synthesize the desired susceptances on the polygon sides. The optimization permits to exactly obtain the power division to the output ports at the design frequency. Finally, the optimization procedure is applied to the transitions between the coupler and the connecting standard flanged waveguide. The proposed optimization is efficient because a few geometric variables must be optimized. Moreover, with the equivalent circuit approach, the synthesis of the coupler and the connecting transitions is two separate tasks.