so i've been running a handheld for like two years now and finally decided to do a proper mobile setup. picked up a used FT-857D and got it mounted under the passenger seat with the head unit on the dash. wiring went pretty smooth, ran the power direct to the battery with an inline fuse, 30 amp, seemed like the right call.

the part im struggling with now is the antenna. i have a mag mount NMO that came with some random quarter wave whip for 2m and i slapped it on the roof for now. works okay but im getting a little RF back into the audio on 10m — like a buzz/hum when i transmit. figured it might be a grounding issue with the mag mount but not sure where to even start diagnosing that.

also wondering if anyone has done a through-hole NMO mount on a pickup truck roof and how much of a pain that was. i do a fair amount of highway driving and the rig is mainly gonna be used for 2m/70cm repeaters and some HF when conditions are good. any thoughts appreciated

the RF into the audio on HF is almost definitely a common mode issue — your coax shield is probably picking up RF and bringing it right into the rig. a few ferrite chokes on the coax near the radio, snap-on types work fine, should knock that right out. also worth checking the mag mount is actually making good contact with the roof, sometimes a little surface rust or paint under there kills your ground plane and makes the feedpoint impedance go wonky which just makes everything worse.

on the through-hole mount question — yeah i did one on my tacoma a few years back and it really isnt that bad if you take your time. drill bit, grommet, run a short jumper to a chassis ground near the mount. the antenna performance difference over a mag mount is noticeable, especially if you park in the sun a lot and the mag slides around. just measure three times before you drill obviously. some guys use a lip mount on the rain gutter if the truck has one, avoids any holes at all.

oh man the 857D is such a good rig for mobile, you're gonna love it on 2m. i ran one for probably four years in my old jeep. the head separation cable is a little short for some dash setups but sounds like you already sorted that part out.

i dont have a ton to add on the RF interference stuff that the previous reply didn't cover but just wanted to say — when i finally did a proper NMO through hole mount it was like night and day compared to the mag mount i had before. honestly just go for it, worst case scenario you have a small hole in your roof which isnt the end of the world if you seal it right. i used a step drill bit and it went through the sheetmetal real clean.