so anyway i've been working on this install for probably three weekends now and i think i finally got it dialed in but now there's this new problem that showed up and im not sure if it was always there and i just didnt notice or if i did something recently that caused it.

background: FT-7900R mounted under the dash of my f150, NMO mount through the roof with a Tram 1185 dual band antenna. grounds look good, i used the chassis ground near the mount and ran the power directly to the battery with the recommended fuse close to the battery terminal. been pretty happy with it overall.

the issue is when i key up on 70cm i get this buzzing in the audio that kind of pulses with engine RPM. classic alternator whine i guess? but here's the thing — on 2 meters it sounds totally clean. no noise at all. only on 440. ive read that UHF can be more sensitive to this kind of thing but i dont really understand why it would only show up on one band and not the other. is this a filtering issue or could it be the antenna or feedline? running LMR-400 which seemed like overkill for a mobile but i had a bunch of it left over from another project.

anyone dealt with this before? kind of at a loss here

the alternator whine thing on UHF but not VHF is actually pretty common and you're right that UHF tends to pick up more of that garbage. a few things worth checking — first, how is your coax routed? if it runs anywhere near the firewall or close to any of the engine harness wires that can cause problems, especially on the higher frequencies. LMR-400 is good stuff but it's only as good as the path it takes.

second thing, and this is the one people overlook, is the antenna mount ground. even if your chassis ground for the radio itself is solid, if the NMO mount isn't making a really clean connection to the roof metal you can get all kinds of weird RF behavior. i'd pull the mount and clean everything up with some sandpaper if you haven't already, make sure you're getting bare metal contact.

the other thing you could try is a cheap noise filter inline on the power leads, the little toroid choke types. sometimes that cleans it right up. had almost the exact same issue in my tacoma a few years back and it ended up being the power wire running too close to the HVAC blower motor wiring of all things.

im probably not much help here since i just got my tech in April and haven't done a full mobile install yet but this is exactly the kind of thing i've been reading about before i do mine so thanks for posting this. curious what you end up finding out