From my reading about the antennas, the airframe itself may be the ground plane. As I reported on my newly energized comm radio, my airframe is behaving like an effective ground plane to the best of my knowledge. Not flight tested of course. The ground plane can be simple wires run out from the center or it can be a disk. I contemplated installing a few radials of copper tape but so many people say that their antenna works fine having it mounted to the airframe itself.