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 |S
₁₁
| <; -16.6 dB and |S
₃₃
| <; -16.75 dB, the measured relative bandwidths are 68.84% and 56.67%, respectively. Within the above passband, the isolation (|S
₃₁
|) is higher than 20.61 dB. The size is 0.034 λ
_g
²
. Good agreement is obtained between the experimental and simulated results.