The ornamental design for an antenna, as shown and described.
FIG. 1 is a bottom, left side perspective view of an antenna showing our new design;
FIG. 2 is a top, left side perspective view thereof;
FIG. 3 is a top, rear side perspective view thereof;
FIG. 4 is a top plan view thereof, the bottom plan view being an identical mirror image thereof;
FIG. 5 is a front elevational view thereof;
FIG. 6 is a rear elevational view thereof; and,
FIG. 7 is a left side elevational view thereof, the right side elevational view being an identical mirror image thereof.
The broken lines illustrate portions of the antenna and form no part of the claimed design.
The dotted lines and areas within dotted boundaries immediately adjacent to claimed subject matter in FIGS. 3 and 6 depict portions of the antenna that form no part of the claimed design.