A REDUCED-SIZE BRANCH-LINE HYBRID-RING USING COUPLED LINES

To miniaturize the hybrid-ring, a design method is presented herein fo r the circuit in which there is an electromagnetic coupling between th e transmission lines in folded transmission lines. A 3-dB branch-line hybrid-ring is treated. Analysis is carried out for the two cases of h omogeneous and inhomogeneous circuit configurations in which each sect ion has only one coupled line. It is found that in the circuit with a homogeneous configuration, perfect match and complete isolation hold a t the center frequency as in the conventional type; additionally, the bandwidth becomes narrower as the coupling becomes stronger. In the in homogeneous structure where the phase velocities of the even and odd m odes are different, there is a shift in characteristics. An equal powe r division can still be accomplished if the degree of coupling and the phase shift in the coupled lines are appropriately chosen. The overal l characteristics differ little from the conventional type. To confirm these findings, an inhomogeneous circuit is constructed and the resul ts are found to agree with the theory.