Impedance-Varying Broadband 90° Branch-Line Coupler With Arbitrary Coupling Levels and Higher Order Harmonic Suppression

We propose a new design methodology to broaden the bandwidth of a single-section 90° branch-line coupler (BLC) with arbitrary coupling level. The method used here to broaden the BLC bandwidth is to replace conventional uniform transmission lines that would never be feasible in many implementations with nonuniform lines governed by a truncated Fourier series. Based on how impedances are profiled, the fractional bandwidth of a single-section 90° 3-dB BLC is extended to 57%, and the realization of broadband BLCs with predefined coupling levels is also possible. Furthermore, higher order harmonics are suppressed by enforcing BLC S-parameters to match the design requirements only at a given frequency range. A systematic design approach based on the even-odd mode analysis is first presented. Then, three examples of 3-, 6-, and 9-dB BLCs are demonstrated at a 3-GHz center frequency. The obtained theoretical response, supported by full-wave electromagnetic simulations and measurements, justifies the design concept.