A Broadband Forward-Wave Directional Coupler Using Periodic Y-Shaped Ground Via Structures With Arbitrary Coupling Levels

A broadband forward-wave directional coupler loaded with periodic Y-shaped ground via structures is proposed. Broadband impedance matching of the even and odd modes can be approximately achieved. The proposed coupler can provide an almost constant phase difference between the even and odd modes over a wide frequency range. Therefore, a forward coupler with broad bandwidth can be expected. Based on the Bloch-Floquet theorem, the Bloch impedances and the electrical lengths of both modes can be predicted with the aid of our simple equivalent models. Also, a complete design procedure for arbitrary coupling level is provided. A test sample is fabricated on a low-temperature co-fired ceramic substrate with coupled-line length 2.02 λ_g. The coupling level over 1.5 dB can be measured from 4.5 to 10.4 GHz, which corresponds to the fractional bandwidth of 79.2%. To our knowledge, the proposed coupler is the smallest design with such a broad bandwidth.