so i spent most of saturday running coax and getting the FT-857D properly mounted under the passenger seat of my F-150. ran the power cables directly to the battery with an inline fuse like everyone says to do, ground strap on the chassis near the radio, antenna is a Tram 1185 on a mag mount for now (eventually want to do a proper NMO mount through the bed but thats a whole other project).

anyway everything works, im getting out fine on 2m and 70cm, but theres this low hum in the audio that changes pitch slightly when i rev the engine. i know this is probably some kind of ground loop thing but ive messed with the ground connections twice now and it hasnt really changed. the hum is there even when im not transmitting so its not RF getting back in or whatever. its more of a receive thing.

anyone dealt with this before? i feel like im missing something obvious. the radio is powered off its own dedicated run so i dont think its sharing a circuit with anything sketchy but who knows

classic alternator whine, happens to almost everybody at least once. what youre hearing is interference getting into the audio chain from the alternator and it follows engine RPM exactly for that reason. the dedicated power run helps with a lot of things but doesnt always kill this completely depending on how the vehicle is wired.

first thing id try is one of those little inline noise filters on the power leads right at the radio itself, the ones that look like a small ferrite choke. sometimes thats all it takes. also check that your coax shield is grounded at the antenna mount and not floating, that can contribute too even if its not the main culprit. and if the hum is coming through the speaker specifically, sometimes just bonding the radio chassis to the vehicle chassis with a short heavy braid right at the mounting point helps more than you'd think. people underestimate how important the chassis bond is compared to just grounding the negative lead.

yeah i had almost the exact same thing when i put a Kenwood in my Tacoma a while back. turned out my ground strap was making contact through a painted surface and wasnt actually a good connection even though it looked fine. scraped down to bare metal and the hum dropped way down. might be worth checking that if you havent already scraped the paint off where the strap contacts the body.