Thanks!
The box is 7.5 x 5 x 1.5 inches, roughly 0.125 inches thick, with the large surface facing up. This is sitting on top of an aluminum robot chassis about 8 inches by 6 inches, elevated by the 1.5 inch thickness of the box of course.
I'm glad you prompted me to look up ground planes in reference to antennas. I didn't realize the importance of it. If I understand properly it reflects signals back onto the antenna, and so has to be λ/4 the size, so that the full length of the wave can reflect onto the antenna assuming its at the center. I'm going to infer that then I want to make sure the antenna is at least this far from any edge?
I'm not dead set on this configuration of antenna. If it would help I could use two antennas, a "clover" antenna, or a dipole, or mount the antenna further up, though this would extend the wire needed to reach it.
To give a little more detail, the most common use of my rover will be on sand, sometimes wet sand, the dunes are typically less than 2 or 3 feet tall, and the height of the track rover only has the antenna about 8 inches off the ground. The rover is either controlled by a computer or a hand held transmitter, typically about 4 feet off the ground. Under normal use the rover may tilt as much as 20 degrees, and it will usually be within 100 feet of the control source.