A Directivity-Enhanced Directional Coupler Using Epsilon Negative Transmission Line

In this paper, a high directivity parallel coupled line coupler using the ε negative (ENG) transmission line is proposed. With the aid of even- and odd-mode analysis, it is shown that by using ENG transmission lines in a parallel coupled line coupler, the even- and odd-mode phase velocities can become equal at a desired frequency. Hence, the directivity of a coupler can be infinite theoretically. The coupler circuit model and closed-form relations for lumped elements of this model are presented. In addition, a design procedure for microstrip implementation of this coupler is proposed. Based on the proposed procedure, a 10-dB ENG directional coupler is designed at 2 GHz and validated by full-wave simulations. This coupler is fabricated and measurement results are presented. Experimental results show maximum directivity of nearly 40 dB at 1.95 GHz.