The ornamental design for an antenna, as shown and described.
FIG. 1 is a front elevational view of an antenna showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a front, right perspective view thereof;
FIG. 6 is a front, left perspective view thereof; and,
FIG. 7 is a front, top perspective view thereof.
The claimed design is indicated by the relatively dark portions in the figures. The relatively light portions on the rear of the main portion of the design in FIG. 2 form no part of the claimed design.