okay so i finally got around to putting the FT-7900R in my F-150 and ive been chasing this annoying noise for like two weeks now. its kind of a whine that changes pitch with the engine RPM which i know usually means alternator noise but ive already run a separate ground directly to the battery negative terminal and it didnt really help much. the power leads are about 12 gauge going straight to the battery with an inline fuse, antenna is a Diamond NR770HB on a lip mount on the trunk lid — well i mean the tailgate i guess since its a truck. the noise is worse on 2m than 70cm for some reason which is kinda weird to me. i dunno if its the mount location or the coax or what. anyone dealt with something like this before

yeah that RPM-related whine is almost always alternator interference getting into the coax shield or the power leads, sometimes both. a few things worth checking — first, is your coax shield actually grounded at the antenna mount? lip mounts can be tricky because you're relying on that metal-to-metal contact with the lip of the tailgate and sometimes there's paint or coating in there breaking the ground path. try running a short jumper from the mount to bare metal and see if that changes anything. second thing is you can try a ferrite choke on the coax right at the radio end, just a couple turns through a type 31 mix toroid. ive also seen people have luck with a noise filter inline on the power leads but honestly fixing the antenna ground is usually the first thing to nail down.

i had almost the exact same setup in my Silverado, Diamond antenna on a lip mount and the noise was driving me crazy. turned out for me it was the coax — i had nicked the shield a little bit when routing it through the door jamb and wasnt getting a clean ground through the mount anyway. replaced the coax run completely and spent some time scraping paint off where the mount contacts the truck and the difference was night and day. the 2m being worse than 70cm thing is a little odd but i wouldnt overthink it, just start with the antenna ground and work from there