so i've been running a Yaesu FT-7900 in my pickup for about three weeks now and 2 meters is working great, SWR is like 1.3:1 all across the band which i'm happy with. but 70cm is sitting around 2.1 to 2.4 depending on where in the band i am and i cant figure out why. the antenna is a Diamond NMO mount dual band whip, i think it's the X50 or something close to that, mounted on the cab roof with a through-hole NMO so the ground plane should be solid.

i trimmed the antenna to what the instructions said but maybe i did something wrong? or maybe the coax run is the issue. i used RG-58 which i had laying around and the run is probably 12 feet from the antenna down through the cab to the radio. i've seen some people say RG-58 is fine for these distances and others say use LMR-400 or at least RG-8X. starting to think maybe thats it but wanted to ask before i go buying more cable.

also just to mention, the radio does work fine on 70cm, i can hit the local repeater no problem, just the SWR reading is bugging me and i dont want to damage anything running it like this.

RG-58 over 12 feet shouldn't cause a SWR issue like that on 70cm, higher loss yes but it won't reflect power back and inflate your SWR reading that dramatically. More likely the coax connector at the antenna base or wherever you terminated it. Even a tiny amount of moisture or a bad solder joint at PL-259 will do exactly what you're seeing — fine on 2m, noticeably worse on 70cm because the wavelength is less forgiving of connector issues. I'd pull the coax connections, inspect them closely, and if you soldered the PL-259 yourself just redo it. Also double check the NMO to antenna connection, sometimes those can work loose especially with vibration from driving.

The 2.1 to 2.4 range you're seeing on 70cm is honestly not terrible for a mobile install, won't hurt the radio, but if you want it tighter then start with the connectors before spending money on new coax.

yeah i had almost the same thing happen with my Kenwood mobile last year. drove me nuts for like a month. turned out to be one of those cheap PL-259 connectors that came with a coax kit, the center pin wasnt making solid contact. swapped it out and 70cm cleaned right up. the X50 is a solid antenna btw, probably not the antenna itself.