I'm working out the science behind mounting an antenna on a vehicle, and I came across the concept of ground planes. This answer says that longer is better but don't worry about it too much, but this article focuses on 1/4 wave ground planes, as do most other articles and products. I'll be mounting a monopole whip antenna like this one.
Can I use a coil for a radial to get a linearly shorter wire and still make a good ground plane? Rubber ducky antennas do this for the main antenna, but I'm interested in using the concept for the ground plane radials. This antenna will primarily be for transmitting on VHF and UHF frequencies, but will receive on many others.
This antenna will be mounted on a tailgate / tire rack, and while there is some room for radials or a sheet of metal for a ground plane or radial, there isn't much more than a foot or so, possibly less. Some directions will have more room than others: the back moreso than in between the tailgates, for example.
I don't want you to get distracted with my particular mounting setup, but here's where I'm planning on mounting it, which is a Smittybilt Atlas bumper and tire carrier. Jeep images are random from a DDG search. There will be some kind of interference with the tire carrier top tube, but I was hoping to mitigate that with a good ground plane. I will cover the tube with some kind of non-conductive material in case the antenna ever makes contact there while I am transmitting.
NOTE: The black part is a bolt. The Antenna is mounted on a small piece of angle iron in an L shape. My first idea was to put a piece of sheet metal as a ground plane perpendicular to the antenna, but when I started looking into ground planes, realized that often they use radials instead.