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Date: Fri, 28 Sep 2007 10:58:43 +0100
From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: A question of calibration
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There appear to be two meanings to the term "far field" when applied
to antennas.   One is usually reserved for electrically LARGE antennas
and is the point where the wavefront can be considered to be planar.
This distance is usually taken to be
2.D^2/lambda, where D = largest antenna dimension and lambda = wavelength.

For electrically small antennas, the "far field" is beyond where the
magnetic and electric components (which roll off faster than the 1 /
R^2 of the radiation field) can be considered to be insignificant.
This value is usually taken as being
lambda / (2.pi)

Andy  G4JNT


FOr LF field strength measurement

On 28/09/2007, Malcolm Harman <malcolm.fr263@ntlworld.com> wrote:
> John.
>
> Fascinating stuff and all your are papers duly saved. I sort of knew PA0SE
> must be right and while E/H = 377 ohms between the Helmholtz coils, we have
> to go to the far field of an antenna before a "plane" wave is sufficiently
> well formed and once again E/H = 377 ohms. Thanks for the clarification.
>
> 73 Malcolm.
>
> ----- Original Message -----
> From: "John Andrews" <w1tag@w1tag.com>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Thursday, September 27, 2007 8:40 PM
> Subject: Re: LF: A question of calibration
>
>
> > Malcolm,
> >
> >> Hi. Can anyone reassure me.
> >
> > Be reassured. Your meter is measuring the magnetic field, and the
> > calibration setup is primarily generating a magnetic field. The cal is
> > being done under near-field conditions to permit the use of low power and
> > take advantage of knowing the mag field accurately based on geometry and
> > current measurement. The scale conversion to electric field remains valid
> > as long as you agree to take your real measurements under far-field
> > conditions.
> >
> > A further discussion of calibration techniques including a simpler
> > arrangement than Helmholtz coils may be found at:
> > http://www.w1tag.com/LF_FSM.htm
> >
> > John Andrews, W1TAG
> >
> >
>
>
>
>