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A novel forward-wave directional coupler with periodical patterned ground structures is proposed. The input impedance of the even and odd mode of the proposed coupler can be matched, and the phase difference between the even and odd mode is also strongly enhanced. The equivalent models are constructed to predict the propagation properties based on the Bloch-Floquet theorem. A completed design procedure is also proposed based on the equivalent models. This two-layer structure is fabricated on Duroid substrate. 0.5-dB coupling level can be measured at 1.5 GHz, and the coupler length is 0.5 λg. Compared to previous studies, the proposed coupler can be designed at a lower frequency range with a smaller size.