I've gotten interference on a FM transmitter before from a 60Hz battery charger
: yes, the interference comes through the 12v lines. One way it can happen is if you don't have good batteries; that was our problem. Batteries tend to absorb a certain amount of line noise. Once we replaced our batteries (which we tested and saw that they couldn't hold a charge), the problem went away.
The 60Hz hum (which is quite low frequency) started showing up in broadcasts, and no one could figure out what it was. Once we determined it to be 60Hz, we knew it was coming from the AC somewhere, and from there it was easy.
Essentially the radio (receiver or txm) is using the 12v to bump the speaker, and oscilates it for the transmission
. If there's already a bit of oscilation to the current
, that can sneak through. Filters vary by radio; some are more and less effective, depending upon the inbound frequency.