Single-Layer Planar Wideband Rat-Race Coupler Using a Shorted Parallel-Coupled Multi-Line Section

In this brief, a novel single-layer planar wideband rat-race coupler using a shorted parallel-coupled multi-line section is demonstrated. The proposed coupler consists of three transmission lines and a shorted parallel-coupled six-line. The performances of wider bandwidths and compact size are achieved just by employing a shorted parallel-coupled multi-line microstrip structure, which has a high coupling factor. Explicit design equations of the proposed coupler were derived using the equivalent approach and the even/odd-mode analysis. For demonstration, a 3-dB broadband rat-race coupler operating at 2.2 GHz was designed. Under the criterion of |S11|33|31|) is higher than 20.61 dB. The size is 0.034 λg2. Good agreement is obtained between the experimental and simulated results.