Speed of the wave in antenna As far as I know, when an AC generator is connected to a metal rod, that is an antenna. Charges oscillate in the metal back and forth. But, why does the speed of light come into play? I would expect that the speed of the charge wave depends on the electron mass, for example. Larger the mass of the electron, slower the wave in the metal. However, the length of antennas is related to the EM wavelength, which is calculated on the basis of the speed of light. Does it mean that electrons move with the speed of light in the metal?
 A: The charge oscillations in the antenna are a plasma wave, which obviously have speed different from that of an electromagnetic wave. The length of the antenna is chosen to achieve the effective coupling between this plasma wave and the electromagnetic waves (impedance matching), that is it is related to the wave length of these electromagnetic waves. This latter is related to wave frequency via the speed of light: $\lambda f = c$ - this is the property of space where the wave propagates, not of the antenna.
A: The electron resists any acceleration by emitting photons. This is useful for us for the surface electrons on a metal rod. Although the drift velocity is extremely far from that of the speed of light, the transmission of the applied potential difference occurs at almost the speed of light. All electrons (as long as the source has enough power) are periodically moved back and forth on the surface and emit zillions of photons.
This photon emission occurs just as periodically as the antenna generator reverses its voltage. And the photons have the following technically pleasing properties (exemplified by a vertical rod):

*

*their electric fields all point up/down (are polarised)

*their magnetic fields all point to the right/left

*there is a rotation of the intensities of the fields, i.e. electric field with its plus pole pointing upwards, then magnetic field with north pole pointing left, plus pole pointing downwards, north pole pointing right, and this periodically again and again.

It doesn't matter how I change the frequency of the generator. Only the output power and the power loss of the antenna change. The speed with which the photons carry away the output power is always that of the speed of light (in the medium of air).
