The ornamental design for the antenna element, as shown and described.
FIG. 1 is a perspective view of the front, right and bottom sides of an antenna element showing our new design;
FIG. 2 is a perspective view of the rear, right and bottom sides thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The evenly broken line in the figure drawings represents unclaimed environment only, and the dash-dot line represents unclaimed boundary line. The evenly broken line and the dash-dot line form no part of the claimed design.