The ornamental design for an antenna, as shown and described.
FIG. 1 is a perspective view of an antenna showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The break lines that separate parts of the antenna and the portion outside of these break lines represent indeterminate length and form no part of the claimed design.