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 partially enlarged front elevational view thereof along the lines 3-3 and 3′-3′ of FIG. 1; and,
FIG. 4 is a partially enlarged rear elevational view thereof along the lines 4-4 and 4′-4′ of FIG. 2.
The broken lines illustrating the unclaimed portions of the antenna form no part of the claimed design. Dot-dash broken lines forming boundaries of the claimed design form no part of the claimed design.