The ornamental design for an antennabase, as shown and described.
FIG. 1 is a front perspective view of an antenna base illustrating my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a front perspective view of the antenna base coupled to an optional antenna.
In FIGS. 1-8, evenly-sized broken lines illustrate features that form no part of the claimed design. In FIG. 8, long-short broken lines illustrate an optional environment of an antenna that forms no part of the claimed design.