The research done in Topeka (Research and Test Department) regarding antennae ground planes led to the specification of 3'x3'. While this is not law and many units have slightly smaller or slightly larger planes, the goal was to elevate the antenna above all other metal object on the locomotives roof while providing the ground plane for the antenna. This led to maximum distance for transmitting and better reception of radio waves at the locomotive site. Antennas were moved around on the locomotives as various rooftop appliances (beacons, air conditioning, RCE antennas, etc) came and went. Depending on your era of CF7, you could get away with a can mounted directly to the rounded roof of an older CF7, a numberboard mounted antenna or an antenna located behind the air conditioner on a 3'x3' ground plane. These were an oddly shaped with rounded front corners. My personal favorite is the numberboard mounted plane. The antenna hovering above the numberboard makes the CF7 look rugged.