I'll hazard a guess that you're not the original owner. It's probably something installed by a previous owner.
You can repair that antenna (if you like) by ordering a new antenna and just using the whip portion. Just remove the screw from the side of the ball of the antenna (in you photo) and take it apart. Remove the whip of the new antenna and install it on your rig. That's what I did. You can get a new antenna on amazon.
http://www.amazon.com/gp/product/B00029X1JG?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00