I picked up a Diamond SG7500A dual band antenna to use on my new vehicle. It's a 1/2 wave design so it shouldn't need a ground plane.
It's going to be installed on the roof rack. Since there isn't going to be much if any ground plane I wanted to see how much of a difference it would make. I did two sets of tests, no ground plane, and with a set of ground radials bent at a 45 degree angle. The addition of the ground radials made a very noticeable difference in the SWR scans. The no ground plane scans look OK but seems to be biased a bit towards the higher frequencies. That can be adjusted with a small change in length of the bottom element. There are set screws there for this purpose, however the manufacture claims it's "factory" tuned.
I was hoping the bandwidth might be high enough to cover the Ham bands along with MURS / GMRS. Ah, maybe with some tweaking if the vehicle installation approximates the ground radial test setup enough.