The minimum ac resistance of slabs and tubes

It can be shown that the minimum ac resistance of a tubular conductor does not occur when the tube is solid. Derivations of and reasons for this behavior are presented. Knowledge of this effect can result in more efficient operation of high-power ac equipment where skin effect losses are important. Examples in the power field are overhead lines, underground cables, and isolated phase bus; examples in the microwave region are resonant cavities, high-power waveguides, and high-power coaxial lines.