A Yagi antenna uses a gamma match. The coaxial braid connects to:

In a gamma match system, the antenna's driven element is typically a continuous piece of tubing mounted directly to the grounded metallic boom. Because the exact center of a half-wave dipole is at a zero RF voltage point, it acts as a neutral ground reference. The coaxial cable's braided shield (the ground side of the unbalanced feedline) must connect to this neutral ground point. Therefore, it is attached directly to the center of the driven element, often by grounding it to the boom at that exact location.