It is sometime since I studied the matter, but there is a formula
that describes the energy assocaited with an antenna, and it
is made up of two parts, for the near field and for the far field.

That part for the far field has the length of the antenna in
terms of fractions of a wavelength.

That part for the near field shows the local stored energy
which is indicative of the antenna system being an oscilatory
store of energy which must dissipate as heat if it is not
reflected back down the feeder.

I cannot, for the moment, locate that formula. Can anyone
else whilst I search through the various EM tomes here?