Size-reduction of 3 dB microstrip forward-wave coupler using defected ground structure

A novel 3-dB directional coupler using defected ground structure is demonstrated. The input impedances of even- and odd-mode are perfectly matched and the phase difference between even- and odd-mode is also enhanced. Equivalent models are developed to predict the characteristic impedance and dispersion curve of the proposed structure. A tested sample is fabricated on the Duroid substrate. 3.4 dB coupling level of the proposed coupler is measured at 1.5 GHz. Compared with the microstrip forward coupler, the proposed coupler can achieve about 95 % size reduction.