Antennas *can* be mounted on the head of a tripod, despite the fact that most antennas use pole-mounts. The trick is to mount the pole to the tripod head.
Here's a HyperLinkTech 15dBi Parabolic antenna attached to a pole that has been mounted on a tripod head. The pole is standard poolside equipment aluminum tubing, and it's been mounted to a screw with a 1/4 x 20 thread. I then removed the mounting screw from the head of the tripod (leaving just the nut) and screwed the pole on to the head.
Here's a closeup: 
