Just to clarjfy, a ground plane for a com antenna, like the comant CI 122, should be installed horizontally with some kind of conductive material. Copper paint, aluminum sheet, mesh. It doesn't care. I've used Reynolds wrap in an emergency and it worked. The optimal size is the radius of the ground plane should be the length of the antenna. The is usually unreasonable in our aircraft, so as big as possible. Square or circle. Inside or outside as long as there's continuity between the ground plane and the base of the antenna. Also, once the ground plane starts up the side of the airplane, it is not effective, so don't bother with the sides trying to get the optimal size.