In message
<OFDEF1DD24.6EF50883-ON88257A99.005ECCE0-88257A99.005FC1FD@US.Schneider-E
lectric.com>, dated Tue, 16 Oct 2012,
ralph.mcdiar...@schneider-electric.com writes:
I've read that, depending on antenna size, 3 metre distance is within
what is called the "near field" and if so, then I assume spectrum of
radio emission wouldn't correlate well to what would be measured at 10m
or 30m.
It's not so much antenna size, in principle, as antenna type and
wavelength. The 'far field', where the ratio of electric to magnetic
field strength is constant at 377 ohms, is established at and beyond
about 3 wavelengths from the source antenna, if it's small compared with
the wavelength, but it may not be.
Nearer the source than 3 wavelengths, the ratio of field strengths is
not constant and depends on whether the antenna is electric or magnetic,
and how big it is. There is a 'transition region', between about
one-sixth of a wavelength and 3 wavelengths; the ratio has a maximum or
minimum at one-sixth wavelength. Closer than that is 'near field', and,
for a small antenna, the ratio varies with distance from the source but
is predictable by simple formulas.
I have to say that 'doctors differ' over those numbers. In checking
references, I find that at least one 'doctor' considers that the 377
ohms impedance is established at somewhat less than a wavelength from
the (small) antenna. I suppose it depends on whether you settle for 337
+/-3.7 or 377 +/- 74.
--
OOO - Own Opinions Only. See www.jmwa.demon.co.uk
The longer it takes to make a point, the more obtuse it proves to be.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
-
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