so ive been fighting this for about three weeks now and its driving me nuts. i put a yaesu ft-7900r in my f-150 last month, antenna is a tram 1185 on a mag mount sitting on the roof, coax runs down the drivers side door jamb and under the dash. power is tapped directly off the battery with the supplied cable and a 20 amp inline fuse.

everything works fine when im just sitting still — i can hit the local repeater no problem, audio is clean on both ends. but when i start driving, especially accelerating, i get this weird buzz or hum that comes through on transmit. people on the repeater tell me it sounds like engine noise or alternator whine. im not hearing anything on receive, just when i key up. ive tried adding a ferrite choke on the coax near the radio and it didnt seem to help much.

anyone dealt with this before? i feel like its probably a grounding thing but i really dont know where to start. the radio chassis is just sitting in a cheap bracket under the dash, not explicitly grounded to anything separate.

yeah almost certainly your grounding. the radio chassis needs its own short ground strap directly to the vehicle chassis — not just relying on the power cable ground. i mean a short as possible, like a few inches if you can manage it. braid strap works better than regular wire for this. the mag mount is also a potential issue, those things depend on the vehicle body for their ground plane and sometimes the paint or whatever interferes with a good connection, though if its on bare metal up top you should be okay.

also check if your alternator whine follows engine RPM or just when you hit the gas — if it goes up and down with RPM thats a classic sign of alternator noise getting in. a lot of guys swear by an astatic rd104 noise filter on the power leads for exactly this situation. i had something similar in my old tacoma and it turned out the ground wire was just barely touching a bracket and making intermittent contact. drove me crazy for a month.

mag mounts are kind of notorious for this honestly. i switched to a lip mount on my jeep and the difference was pretty noticeable. also — and this might sound obvious — but is the coax shield actually making good contact with the mag mount base? sometimes the connector or the cable itself is the culprit. i've seen brand new coax from those tram antennas have weird issues right out of the box.