The ornamental design for an antenna, as shown and described.
FIG. 1 is a front elevational view of an antenna embodying the design of the present invention, the rear elevational view appearing the same as the front elevational view.
FIG. 2 is a left side elevational view of the antenna shown in the prior Figure, the right side elevational view appearing the same as the left side elevational view.
FIG. 3 is a plan view of the antenna shown in the prior Figures; and,
FIG. 4 is a perspective illustration of the antenna shown in the prior Figures.