The current paper presents a wideband forward-wave directional coupler. Periodic shunt structure is employed to design the proposed coupler. Using even- and odd-mode analysis, it is shown that even- and odd-mode characteristic impedances are equal over a wide frequency range. Moreover, an almost constant phase difference between the even and odd modes at this frequency range is obtained. The introduced coupler is numerically investigated by commercial software. A prototype of the coupler is made using a single layer substrate and it is successfully tested. The measured results agree well with those obtained by simulation. It is shown that the measured coupling is 0.35 dB with 1 dB flatness over broadband frequency range from 4.36 GHz up to 12.43 GHz corresponding to 96% fractional bandwidth.

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