so i've been putting off doing a proper mobile install for like two years now and last weekend i finally just did it. got the FT-857D mounted under the passenger seat with the head unit up on the dash, ran the power cables directly to the battery with an inline fuse, the whole deal. antenna is a Tram 1185 on a NMO mount i drilled into the roof.

problem is when im on 2m or 70cm i keep getting this kind of scratchy buzzing noise that comes and goes with the engine RPM. so when i rev it up the pitch of the interference changes. i know this is probably alternator whine or something related but ive read like 15 different pages about it and im more confused than when i started. some people say ferrite chokes, some say a ground loop isolator, some say my ground is bad. i did ground the radio chassis to a bolt on the seat rail which maybe wasnt the best choice.

anyone dealt with this before on a modern truck? its a 2019 F-150 if that matters. just want to be able to use the thing on my commute without the noise driving me crazy

yep that's classic alternator whine, the RPM-varying pitch is the dead giveaway. the seat rail ground is almost certainly your problem — those things are not great electrical grounds even if they feel solid mechanically. what you really want is a short direct ground run to the chassis itself, ideally the same point where the vehicle body is grounded to the battery negative. some guys go all the way back to the battery negative terminal with a dedicated wire and that clears it up completely.

the F-150s in particular can be a little finicky because of all the electronics in them. also worth checking where your coax shield is grounded — if the antenna ground and the radio ground are at different potentials you'll get a loop and that causes all kinds of noise. try touching the braid of the coax to the same ground point as the radio and see if anything changes, just as a quick test before you start buying ferrites and stuff.

had almost the exact same thing on my Tacoma with a 7300 i was running mobile for a while. ended up being a combination of a bad ground AND the power cable running too close to the factory wiring harness under the dash. once i rerouted the dc cable away from all that stuff and put a proper ground right to the firewall it was like night and day. took me an embarrassingly long time to figure out the cable routing part honestly