The antenna that works with the 250 doesn't require a ground plane. Here's where my 500 antenna is mounted on my tractor:

I wanted to mount it a ways forward when I was running an EZ-Guide+. When the 500 came out, it was supposed to be better without having the antenna mounted forward but since the antenna was already there and it worked so well, I just left it and it continues to work great. As you can see, the EMT doesn't make much of a ground plane. It just supports the strap that has the mounting stud for the antenna.
I used an ATV for surveying field borders earlier this spring:

 As you can see, there is a small square of metal but its only purpose is to give the magnets built into the antenna something to grip. I raised it just enough to clear the lightbar and it really had no trouble holding onto satellites.
The really high end antennas that are used for RTK base stations will have something of a ground plane built into them. Its purpose is just the opposite of what a ground plane is usually intended to do. Instead of reflecting signals, its purpose is to soak them up and come as close to getting rid of them as possible.