FM (88 ~ 108 Mhz) is far away from 1090 mhz, and unlikely to affect it unless you have an FM transmitter nearby.
It is the Cell/Moble signal which is prime suspect for interfering with 1090 Mhz as GSM850 and GSM900 are very close to 1099 Mhz.
You can determine existance of these signals by a simple scan described in this post:
Do I Need A Filter?