A novel microstrip forward directional coupler using defected ground structure

A novel forward-wave directional coupler with high coupling level is demonstrated. The artificial coupled-line has a considerable phase difference between even- and odd-mode. The distinct property contributes to the coupling enhancement and size reduction, respectively. A 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 nine cells is designed based on the Duroid substrate. The coupling level of 0.67 dB of the proposed coupler can be measured at 2.6 GHz with significantly compact size.