Maybe this might work:
Obstacle Collision Avoidance System
It is used on large power lines in Norway, at the long valley/fjord crossings.
The system works, but they originally intended to broadcast the audio warning on the VHF carrier wave somehow, in order to reach all aircraft no matter what freq they are on.
No wonder, it got sh**canned by the Norwegian CAA, interference issues.
But the biggest issue: If the notams are anything to go by, they go U/S a LOT!