Message-ID: <001601c10cae$921cfa60$b48c01d4@g4jnt>
From: "Andrew Talbot" <G4JNT@thersgb.net>
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: Near field effects
Date: Sat, 14 Jul 2001 22:47:22 +0100
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No !
This is a completely different matter altogether.  The 'near field' here
relates to arrays and antennas much larger than a wavelength and refers to
the point where the wavefront is planar, and true inverse square law
radiation holds.  At a distance of approx 2 D^2 / lambda

It has nothing nothing whatsoever to do with the 'near field relating to the
reactive fields of a small antenna.   Means that as the antenna gets larger,
the far field gets greater as the square of the antenna dimension.  To
illustrate the practical implications of this in an amateur context,  for a
typical size 0.6 metre dish at 10GHz you have to go out as far as 24 metres
before gain measurements become meaningful.
------
I looked up the equations relating to a small antenna, in the excelent
antenna book simply called   'ANTENNAS'   by L V Blake and originally
published in 1966 - thrown out by our works library !  .  Sure enough it
shows my memory is fading - the E field of a short monopole does indeed fall
off as 1/R^3 as I correctly remembered, but gives the magnetic field roll
off as 1/R^2 as quoted by John earlier today not the 1/R^6 I initially
thought.

However, this is for a short, basically E field, antenna.   My gut feeling
is that a magnetic antenna such as a loop would behave in the opposite way
and the H field would roll off much faster than the E field.

What we want is someone who really understands how to apply the basic
calculations from first principles - there may be one or two in the world
who can do it, they write these text books !!   .

Andy  G4JNT


Hi All,
Trying to find out more about near field effects, I have read that the
transition from near to far field happens at the "Rayleigh distance",
sometimes called the "far field distance". An estimate for this distance is
given by the formula (2 d^2)/(lambda) where d is the maximum dimension of
the radiating structure. (See for example:
http://www.ee.surrey.ac.uk/Personal/D.Jefferies/antennas.html)

For a quarter wave vertical this gives a value of one eighth of a
wavelength, not too different from the 1/2Pi formula quoted elsewhere.
However for a typical amateur 10 metre vertical, the formula gives a
ridiculously small value of less than 0.1 of a metre at 137 kHz.
It seems that the Rayleigh distance is commonly used when considering
microwave antennas, dishes, horns and the like.

Can anyone explain to me why it is not appropriate for our proportionally
small antennas? In particular are the Near Field effects of our small
antennas over-estimated when we suppose them to be still significant out to
lambda/(2Pi)? Is the latter formula only appropriate to professional
antennas?

73, John, G4CNN
email: computernetworks@go.com
web page: http://www.g4cnn.f2s.com


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