A Single-Ended-to-Balanced Impedance-Transforming Branch-Line Coupler With Arbitrary Power Division Ratio

In this paper, a new single-ended-to-balanced (SETB) impedance-transforming branch-line coupler with the arbitrary power division ratio is presented. By using the matrix transformation, the standard scattering matrix and corresponding odd- and even-mode matrices are deduced from the mixed-mode scattering matrix. A simple configuration composed of four quarter-wavelength and two half-wavelength microstrip lines is proposed to achieve desired functions of the SETB branch-line coupler with the arbitrary power division ratio and arbitrary terminated impedances. The parameter selection and design principle are given. Finally, three prototypes with different power division ratios and terminated impedances are designed to verify the theoretical prediction. Impedance matching, port isolation, and mode conversion suppression are all obtained successfully for all the three fabricated SETB branch-line couplers. The proposed configuration also offers other attractive advantages including simpler configuration and design method, lower insertion loss, and good compatibility with the planar circuit technique.