Do you need to remove the entire power-antenna, or just the mast? If you're carefull, and the collar isn't too tight or corroded, you may be able to unscrew it with a needle-nose pliers. There's ususally an additional brace at the bottom of the antenna attached somewhere inside the fender. The antenna cable is routed through the inner fender/kick panel, but to to reach the whole unit on the car, or to install one, you'll need to remove the inner fender axcess panel, aft of the front wheel.
As for the mast itself, the antenna in your photo looks like a universal Asian replacement, not an OE Hirshmann, but generally, if you remve the collar and power UP the mast it will spit out the mast, along with it's nylon cord.

Happy Motoring, Mark

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DrDKW