The ornamental design for an antenna, as shown and described.
FIG. 1 is a first perspective view of an antenna comprising a base, a clock, two dipole antennas, and a closed loop UHF antenna;
FIG. 2 is a second perspective view, from a second angle of the antenna of FIG. 1;
FIG. 3 is a side view of the antenna of FIG. 1, the bottom of the antenna is flat, therefore there is no need to show the bottom; additionally, the opposite side view of the antenna is not shown since the antenna is symmetrical; and,