Hi Charles.
If your aerial is the V-Tronix/Hawk model then I think Brian is right. Just above the mounting for the "close-hauled" indicator arms is a collar which retains the windvane. It is possible for the vane to "escape" the collar,or in an older installation, for the plastic to become brittle, break, and allow the vane to rise spectre-like, up the whip, with the result you observed!
I used two methods to cure it. Initially a few well-placed turns of electrical tape just above the vane to prevent it rising up, but when that failed due to sun and wind drying out the tape I came upon a more permanent method.
I extracted from the top of my can of gas lighter fuel ( which of course we all have on board to refill our gas-fired cooker lighter wands!)a couple of likely sized nozzles, and took a ride up the mast so armed together with a bradawl. It was the work of a minute or two to ream out a suitable nozzle and place it as a friction fit in place of the tape just above the vane. As a belt and braces I also placed a nozzle over the tip of the aerial in place of the original cap(which had also gone missing).
275/30 "Cascada"