The energy (E) in a single photon is E = h X f where h = Planck's Constant and f = the frequency of the individual photons in a stream of electromagnetic waves.

So if a radio transmitter is outputting electromagnetic waves (photons) of a given frequency (F), then it seems the power would be the energy per photon X the number of photons/second. Therefore the power would be proportional to the square of the frequency.