Re: Electromagnetic wave and photon spin

On Thu, 20 Sep 2006, rds wrote:

In the near field, you can have a faster drop-off of either E or B,
depending on the details of the antenna. But the near field is not
radiation, in that the energy doesn't travel away from the antenna without
coming back.

In, for example, an electric dipole antenna, the electric field in the
near field drops off as r^3, and in the far field, both E and B drop off
as r^1.

Is that the defenition of near field?

You could define the far field as where E and B drop off as r^1, for all practical purposes.

It wouldn't be a good definition of near field, since depending on the details of the antenna, one can arrange to have the most important component of the electromagnetic near field being either E or B and dropping off as r^N, N > 2.

If you have a strong mathematical background, "multipoles" and "multipole expansion" are the magic words. In particular, compare the usual electrostatic multipoles (note that they usually written down for the electrostatic potential, not the fields) and the wave equation multipoles in the small r limit.

That in the near field the energy
returns to the wire or radiator? This would be the inductive power
transformer field correct?

Yes.

Is this near field radiation still a part of
or the source of the readiated energy or are they two distinct energies?

This is stored-and-recycled energy; it isn't radiated. Thus, one can call the far-field the "radiation field" and the near field the "inductive field", although one then risks some confusion, since the radiated energy is also travelling outwards in the near field. As to how separate they are, since the far field radiated energy wouldn't be there if it wasn't for the inductive stored energy in the near field, I think it depends very much on exactly what you mean by "separate".

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html

.

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