so ive been trying to sort out my dual band mobile setup in my F-150 for the past few weeks and im getting inconsistent SWR readings depending on where i put the antenna. right now its on a lip mount on the rear of the cab roof and im seeing around 1.4:1 on 2m which isnt terrible but on 70cm it jumps all over the place sometimes 2.5:1 and sometimes almost flat. antenna is a Tram 1185 which i know a lot of people use without issues.

i ran the coax down through the door jamb and into the cab, used some split loom to protect it. the radio itself is a Yaesu FTM-300 mounted under the dash. grounding is to the chassis via the radio mounting bracket.

not sure if this is a coax issue or a ground plane issue or what. the truck has a lot of plastic body panels near the rear which i wonder if thats affecting things. anyone run into something similar on these newer trucks with less metal to work with

yeah the plastic panel thing is almost certainly your problem on 70cm. the ground plane on these modern trucks is just not what it used to be. a lot of guys go to a mag mount on the roof center just to test and if the SWR flattens out that tells you its definitely a ground plane situation not the antenna or coax. lip mounts can be finicky too, the connection between the mount and the vehicle body matters a lot more than people think.

also worth checking if your coax is getting pinched anywhere in the door jamb run. even a small kink can do weird things especially at UHF frequencies where everything is more sensitive. what coax are you using? if its RG-58 id swap it out for LMR-240 or even RG-8X, makes a real difference on a run that length.

i had almost the exact same problem on my tacoma. ended up being that the lip mount wasnt making good electrical contact with the roof edge, there was a thin coat of paint plus some factory sealer in there. took a piece of sandpaper and scraped down to bare metal where the mount contacts the roof and night and day difference. kinda annoying to do but it fixed it.