The aerial is built in to the rear windscreen, like the heated rear window element. The part that usually fails is the diversity antenna. Link to the part number for it below:

http://bmwfans.info/parts-catalog/E60/Europe/520d-M47N2/R-N/browse/audio_navigation_electronic_systems/single_parts_f_antenna_diversity/

There's always a few available on EBay.