A fully planar broadband rat-race coupler with stepped-impedance and coupled microstrip lines

A broadband rat-race coupler is proposed with planar configuration. Stepped-impedance and coupled transmission lines are utilized. Due to the symmetric structure, it is easy to design such a component by the even-/odd-mode analysis. A prototype is optimized to demonstrate our idea. The central frequency is 1.37 GHz with a 52% relative bandwidth. The return loss and port isolation are both better than 15 dB, the maximum magnitude imbalance is less than 0.5 dB, and the phase imbalance is within ±10°. Since the proposed circuit is fully planar, requiring no metallic tapes, plated through-holes, etched ground, bonding wires and multilayer structures, it is easy to be integrated into a low-cost microwave system.