A general analysis and new designs for the hybrid-ring directional coupler

General expressions are presented for the scattering parameters of the hybrid-ring directional coupler, assuming only one-axis symmetry and isolation of output arms. Under the condition of matching of input arms, the existing designs fur 3-dB as well as those for arbitrary power division follow as special cases. A general design method for 3-dB couplers is evolved, based on which a number of new designs are presented. The calculated frequency response of the new designs has been compared with that of the existing ones. A new design using λ/5 section and having an overall length 13 A/10 is found to be superior to the 5 λ/4 and 7λ/6 designs, and by virtue of its shorter length, is expected to have an insertion loss less than that for the common 3 λ/2 design.