so i finally got around to doing a proper mobile install in my truck, FT-7900R dual band, mag mount for now until i can drill through the roof which my wife is NOT thrilled about. anyway i ran the coax from the antenna through the door seal down under the floor mat toward the back seat and up under the center console to the radio. seemed like a clean install honestly i was pretty proud of it.

but now im getting weird SWR readings, like on 2m its sitting around 2.1 which isnt horrible but its higher than it was when i just had it sitting on the roof with the coax going straight in the window. on 70cm its even worse, jumps around when i touch the coax near the center console area. ive checked the PL-259 connections twice and they look fine to me.

anybody dealt with this before? wondering if running the coax under the carpet is causing some kind of issue or maybe the door seal is pinching it somewhere. the coax is just some RG-58 i had lying around, maybe thats part of the problem too

RG-58 on 70cm is gonna hurt you, thats probably a big part of it. the losses on that band with RG-58 over any real length add up fast and you can get weird reflected power behavior especially if theres any slight damage to the coax. if you ran it through a door seal theres a really good chance you kinked or compressed it somewhere even if it looks okay from the outside, any sharp bend or crush point in the coax will mess with the impedance and you'll see exactly what you're describing, readings that shift when you flex or touch the cable.

i'd swap it out for LMR-240 or at minimum RG-8X and route it more carefully, try to avoid the door seal entirely if you can. some people use a thin flat coax made for door/window routing, Comet and a few others make them, they're not perfect but way better than crushing regular coax in a door jam. also double check that your mag mount has a decent ground plane, on a modern truck with a lot of plastic and composite the ground situation can be surprisingly bad even when it looks like it should be fine.

yeah i had almost the exact same thing happen when i first set up my IC-2730 in my car. turned out the coax was getting pinched right where the door seal met the A pillar trim, looked totally fine but when i pulled the trim off there was this one spot where it had basically a 90 degree kink in it. replaced about 3 feet of coax and routed it differently and SWR dropped right down.

the RG-58 thing the other guy mentioned is real too though, i wouldnt use it for 70cm installs personally