It's got to unscrew. Spray some WD40 around the base of the rod and let some
3in1 oil run down bottom of aerial rod in the hope it will find its way to the bolt. Leave for a day or two and apply a few times. Just protect paint with a cloth while spraying, although don't think it does any harm. Try not to get WD40 on the windscreen as makes it a bit smeary.Paul J