Owen,

I like the IRE definition, which according to W8JI is: "The total
power radiated in all directions divided by the square of the net
current causing the radiation".

That definition draws a distinction between Rrad and the resistive
component of the feedpoint impedance; it makes the Rrad of a folded
dipole 75 ohms, not 300 ohms; and it avoids some of the errors folk
make in assuming that "folding" a vertical can reduce ground losses.

73,
Steve G3TXQ