A novel broadband forward-wave directional coupler design with periodical H-shaped structure

A novel broadband forward-wave directional coupler with periodical H-shaped structure is proposed. The broadband impedance matching of the even- and odd-mode of the proposed coupler can be achieved, and the phase difference between the even- and odd-mode can keep at approximate constant during a frequency range. The equivalent models are constructed to predict the propagation properties based on the Bloch-Floquet theorem. This two-layer structure is fabricated on the Duroid substrate. The coupling level is over than 1.5 dB from 3 GHz to 6.6 GHz and the fractional bandwidth is 75%. The average coupling level and average directivity during 3 GHz to 6.6 GHz are 0.8 dB and 20 dB respectively. Compared to the previous works, the proposed structure can be designed with wider operational bandwidth.