I'd have a little fun with this just to see what happens. Start leaving something on your antenna when you leave the car, like a bow, a balloon, an AA flyer, maybe even a small sack with a cookie in it. Put a wad of holiday tissue paper under the door handle (maybe add a bow there too). If the problem is just undirected vandalism, perhaps the culprit will realize they're dealing with another human being and feel enough shame to stop.