I'm sure there are aftermarket options. First of all, I like the antenna. I think it is not only unique to BMW, I like the look. Second, it is a multi-function device. I'm not a technical expert, but I'm sure it functions as your AM/FM antenna also.
Not to mention, if you remove the shark fin, you'll have a hole and likely a mark from the base of the shark fin. Why deal with all that? With all that being said, my wife has the 128 vert. It has a smaller shark fin on the deck of the trunk, but because of the size, she still has a fixed antenna for AM/FM.
Next - 997 or M3? (It's complicated)
Current - 2010 128i convertible (wife's car)
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Someone should invent a sarcasm font...