Miniaturized Planar 90° Hybrid Coupler with Unchanged Bandwidth Using Single Characteristic Impedance Line

A planar branch-line coupler which is entirely composed of transmission line of single characteristic impedance equal to the system reference impedance is presented. The low impedance series arms of the conventional coupler are replaced by their equivalent structures with 50Omega lines. A transmission-line model is derived to obtain the design curves. The main advantage of the present design is that the bandwidth of the coupler remains unchanged. In addition, the area of present coupler is only 50% and also the second harmonic is suppressed by at least 45 dB as compared to the conventional design. Full-wave EM simulation is used to confirm the design approach for the prototype coupler operating at 1.2 GHz.