Art Unwin wrote:
Does that mean that a radiator reduces mass with use
because the electrons orbiting around the atom are losing mass?

Since mass and energy are equivalent, I suppose the mass
of the radiator increases with increasing power input.
The increase in mass can be calculated but the average
ham has no way of measuring the increase. No need to
worry about the tower falling down due to additional
mass from energized electrons. :-)

The antenna is charged up to a certain energy level
during the key-down transient state. Since the energy
content of the antenna cannot increase forever, it must lose
energy as photonic radiation and/or as heat during steady-state.

Free electrons in a conductor travel at much less than the
speed of light. Photons are emitted from the electrons at
the speed of light. A quote from:

http://c2.com/cgi/wiki?SpeedOfElectrons

"For example, for a copper wire of radius 1 mm carrying
a steady current of 10 Amps, the drift velocity is only
about 0.024 cm/sec!" i.e. about 0.01 inch/second. Ignoring
random movements, the electrons at our RF transmitter never
reach the antenna. At 10 MHz, the electrons move less than
0.000000001 inch during a 100 nS cycle involving a 100 watt
transmitter, i.e. they mostly oscillate in place.

However, other electrons, traveling at a large percentage
of the speed of light, are quite massive as observed in
particle accelerators and radioactive decay.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com