WD40 is grease in a solvent carrier. Over time the grease builds up. Although WD40 provides the grease cutting and lubrication you seek short term, long term the grease builds up and exacebates the problem.

The correct thing to clean the antenna, as recommended by Hirschman who make them is a cleaning pad soaked in silicone spray. I don't know if they still do this but older cars used to come with a little Hirschman cleaning pad in the glovebox.

You don't want to spray silicone near your car as any drop of this stuff on the paint is very very bad news. Spray a little bit on a folded over paper towel or something well away from the car, then clean the antenna a couple of times with it.

- Richard Sexton