A novel microstrip forward directional coupler based on an artificial substrate

A novel forward-wave directional coupler with strong coupling level is presented. The artificial coupled-line has some special properties such as equal value of even- and odd-mode characteristic impedances and considerable phase difference between even- and odd-mode. These distinct properties contribute to coupling enhancement and size reduction, respectively. Simple equivalent circuit models for two modes are developed to analyze the characteristic impedances and dispersion relations of the proposed coupler. A practical configuration with seven cells is designed based on the FR4 substrate. The coupling level of -1.6 dB of the proposed coupler can be measured at 2.85 GHz with a significantly compact size.