I've got a Diamond SG7900A NMO dual-band antenna mounted on my truck roof, and I'm seeing some strange SWR readings. On 2 meters I'm getting 1.3:1 across the band, but on 70cm it jumps to 2.5:1 at 440 and over 3:1 at 450 MHz. The antenna is center-mounted with good ground plane contact. Using about 15 feet of LMR-240. Anyone seen this before?

Sounds like the antenna is a little long for 467MHz and just about right for 448MHz - you might trim it a little at a time to balance the SWR between the frequencies. But first, double-check all your connections and make sure the mount has solid electrical contact.

To function properly, your antenna mount needs to be grounded to your vehicle's metallic chassis - otherwise you'll get sky-high SWR. Even though you say you have good contact, I'd verify with a multimeter from the mount to a known good ground.

Had a similar issue on my mobile setup. Fluctuating SWR is caused by changes in conductivity of the surface under the vehicle and objects in close proximity - loose coax connections are the main cause of inconsistent SWR readings. Check your PL-259 connections first.