Hello. I’ve installed 3 sector antennas in a tower with some FM stations (4 x 2kw each)
If I scan from one of the RB912, I can see the other two… and also a new “phantom” signal with 100MHz less
Considering that FM has a frequency about 100MHz… can be heterodyning?
I’ve never seen it in wireless networks… but what else can it be?
Regards
The problem in a radio magnetic field.
I also encountered this problem.
Change the location of your antennas or FM antennas 
Using 10MHz wide channels aren’t you?
If you were using 5MHz wide channels you would see the “ghosts” on both sides of your channel.
You can just ignore it…
EDIT:
No sorry.. I see that you are using 20 wide.
Have never seen this on 20MHz wide channels.