Eventually, I managed to take down the antennae. It wasn’t all that difficult once I’d figured out where the stairs to the roof of each police station were, so I could park my car as close as possible.
However, I then spent the best part of an hour finishing that mission, as I had a 5 Star rating upon escaping from the last station, and to complete the mission I had to drop that down to zero. Whilst being chased by another helicopter which seemed to cause my helicopter to veer all over the place.
I knew where one (and only one!) police bribe pickup was (near the very first safehouse you get), but couldn’t remember where any others were. Flying around didn’t really help too much either, as low down you get shot at by the police, and high up you can’t see any pickups. And I didn’t want to land as then the chasing helicopter would shoot me down. Luckily, I remembered there was a bribe outside the golf course, and managed to pick that up, and then remembered there was another near the airport. I snatched that just in time too – my helicopter blew up almost as soon as I landed it and the drop to just two stars and a pipe to hide in meant the police chopper flew off.
Finally, I ran back to my safe house and changed clothes to drop to no stars. Hurrah!