The ornamental design for an antenna, as shown and described.
FIG. 1 is a top view of an antenna showing my new design;
FIG. 2 is a bottom view thereof;
FIG. 3 is a is a back edge view;
FIG. 4 is a front edge view;
FIG. 5 is a right-hand side view as seen in FIG. 1; and,
FIG. 6 is a left-hand side view as seen in FIG. 1, the power cord being shown broken away in FIGS. 1, 2, 5, and 6 to indicate it has no particular length.