Dr. Slick wrote:


I was going to ask you to define "far-field", and i thought maybe
people defined this as a number of wave-legnths away, but if it's
nebulous like a lot of RF topics, then i would certainly understand.


It's not nebulous at all. There is no boundary fence in the ether with a
sign saying "Here endeth the near field", but you *can* draw your own
lines.

That is not at all unusual in physics and engineering, nor is it limited
to RF problems.

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. Coming closer to the
antenna, you enter the transition zone where you start to see measurable
and calculable deviations, but you can set an engineering criterion to
say you're still close enough to far-field conditions (a 1 degree E-H
phase difference is one example, but people can and do set different
criteria for different purposes).

Equally, you can begin in the near field and work your way outward.
You'll notice some strange behaviour of the E and H fields very close-in
(all of which turns out to be completely predictable if you try hard
enough) but farther out they get their act together and settle into
their correct far-field relationship.

The lack of ready-drawn boundaries doesn't make any of those ideas
"nebulous". They are exactly as clear - or exactly as nebulous - as the
way you choose to think about them. There's still the same rock-solid
physics underneath.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek