so i've been putting off doing a proper mobile install for like two years now and finally pulled the trigger last month, got a FT-7900R going into my F-150 and overall it went pretty smooth except for this one thing thats been driving me nuts

when i'm transmitting, especially if the truck engine is running, the guys on the local repeater keep telling me they hear a whine or buzz in my audio that kind of changes with the RPM. i've read a bit about alternator noise and i think thats probably what it is but im not totally sure how to track it down or fix it. i ran the power leads directly to the battery with an inline fuse like everyone says to do, so i thought that would take care of it

the antenna is a nagoya NMO mount through the roof, coax goes down the A pillar and under the carpet. everything looks clean to me but clearly something isnt right. has anyone dealt with this before? is there something obvious im missing

yeah that's classic alternator whine, the RPM-dependent pitch is the dead giveaway. even with a direct battery connection you can still get it coming in through the coax shield or a bad ground somewhere. the first thing i'd check is your coax grounding at the mount — if the NMO mount isn't making solid contact with the roof or your ground braid to the chassis is iffy, that's usually where it sneaks in.

try this: get a ferrite choke and put it on the DC power leads right at the radio, wrap them through a few times. also put one on the coax feed line close to the radio. that kills most of it in my experience. i had almost the exact same setup in a silverado years back and a couple of type 31 toroids sorted it right out. also double check your coax isn't running parallel to any power wiring under the carpet for a long stretch, that can couple noise in too.

im having the same issue with my ID-5100 in my jeep lol, been meaning to post about it actually. mine only does it when the AC compressor kicks on though which is slightly different i think. gonna try the ferrite thing the guy above mentioned and see if that helps mine too