Before revision: These are usually huge air core coils 2-4 meters high wound on a nonconductive frame, with RF resistance reduced by using litz wire consisting of thousands of insulated strands of fine wire braided together.
After revision: These are usually huge air core coils 2-4 meters high wound on a nonconductive frame, with RF resistance reduced by using thick litz wire several centimeters in diameter, consisting of thousands of insulated strands of fine wire braided together.