A Symmetrical Broadband Tight-Coupled Directional Coupler With High Directivity Using Three-Folded-Coupled Lines

In this brief, a novel symmetrical broadband tight-coupled directional coupler with high directivity using three-folded-coupled lines is proposed and implemented. The directional coupler is consisted of two simple coupled lines and two three-folded-coupled (TFC) lines, which realizes advantages of broadband, tight coupling, and high directivity. The TFC line is simplified as two-level coupled lines. Electrical lengths of coupled lines and precise mathematical equations for ${S}$ -parameters of the proposed TFC-structure coupler are calculated based on the method of even- and odd-mode analysis. The gap between the simplified two-level coupled lines is also considered and discussed. For demonstrative purposes, a broadband coupler using TFC lines is designed and fabricated at about 2 GHz with a power-dividing coefficient of ${k} = 0.6$ . The measured fractional bandwidth (FBW) of the proposed coupler is about 54.0% as the return loss $(|S_{11}|)$ and the isolation $(|S_{41}|)$ are both higher than 20 dB. The fabricated directional coupler also shows advantages of tight coupling (about 3 dB) and high directivity (the highest directivity is about 33.1 dB).