so ive been running a 2m/70cm dual band mobile in my truck for about three months now and the antenna situation is driving me a little crazy. right now i have a mag mount NMO base stuck on the roof just behind the cab and its been fine but i keep reading that a true ground plane matters a lot on VHF and im not sure the mag mount is actually giving me a decent one. the truck is a crew cab so theres plenty of metal up there but something feels off.

did a bit of testing last weekend driving around town on the local 2m repeater and the audio reports i was getting were pretty inconsistent, sometimes full quieting and other times the repeater was telling me my signal was breaking up. i dont think its the radio itself, its a Kenwood TM-V71A which i bought used but it tested fine at home. could be the coax run too, i just kind of routed it through the door seal which i know is not ideal.

curious where other folks with trucks ended up mounting their antenna and how you ran the coax. i really dont want to drill a hole if i can avoid it but maybe thats just what has to happen

the door seal thing is almost certainly your problem, or at least part of it. that pinches the coax and over time can nick the shield or center conductor just enough to cause weird intermittent issues. you might not even see damage if you look at it but the SWR can get ugly and you lose a surprising amount of signal. i ran mine through the firewall grommet when i installed in my F-150 and it made a big difference, takes about an hour if you have the right tools and a fish tape.

on the antenna placement question, roof center is actually pretty good on a pickup if you can get it there. the ground plane concern is real but a big steel cab roof is plenty of metal for VHF, mag mounts work fine on that surface as long as the mount itself has good contact. id clean the paint contact area with some scotchbrite if you havent already, sometimes theres a clear coat layer under there messing up conductivity. also worth checking the PL-259 or whatever connector you have at the radio end, those can loosen up from the vibration of driving.

yeah i went through this exact same thing last year, honestly just drilled the hole and put in a proper NMO mount and never looked back. the mag mount was always sliding around a tiny bit when i took corners and i was paranoid about it flying off on the highway. with the drilled mount i know its solid and the coax comes down through the headliner which looks clean.

one thing i will say is that on 70cm the antenna placement matters more than people think because at those frequencies even small changes in height or ground plane shape can shift things. if you have a dual band antenna make sure its actually resonant on both bands, some of the cheap combos are kinda mediocre on 440.