"Ian White, G3SEK" wrote in message ...
....
In the idealized far field, the E and H fields are orthogonal, in phase
and have a ratio of 377 ohms. At any finite distance, you know that
you're not in the idealized far field, but until you come quite close to
the antenna you can't measure the difference in any way, so ideal
far-field conditions are a very good assumption.

(rest deleted...)

Again, just to make things clear, this is fine in freespace (or good
approximations of freespace), but when there are conductors and
dielectrics scattered about, it can be significantly disrupted. Of
course, all this is explained quite nicely by exactly the same
physical understanding that Ian mentioned for the fields in the
vicinity of the radiating structure. It's "only" a solution to a
bunch of differential equations and boundary conditions. Things are
just a lot simpler in freespace.

From a practical standpoint, it can be helpful to have an idea when
it's safe to say you're in the far field and when you're in the near
field. Somewhere in my files I have some articles about that
(awaiting better organization so I can actually find them! ;-). I
think the one I liked best was in "RF Design" magazine perhaps five
years ago. The estimate takes into account not only how far from the
antenna you are, but also how large the antenna is.

Cheers,
Tom