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Date: Fri, 28 Sep 2007 13:50:54 +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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Yes, it would be "certainly true" that 10 times the recognised
distance is in the far field. It certainly provides a huge safety
margin!

Most texts use 2.pi, although I did once see 4.pi quoted.

Andy  G4JNT


On 28/09/2007, Malcolm Harman <malcolm.fr263@ntlworld.com> wrote:
> Hi Andy.
>
> Interesting, have a look at
> http://www.rsgb-spectrumforum.org.uk/radiation_theory.htm where via the
> 500kHz section (so presumably meant for electrically small antennas) it says
> "It is generally well known that the far-field is predominantly a radiation
> field; this is certainly true when the distance is greater then 10 x
> lambda/2pi." So there seems to be a factor of 10 difference from your
> equation.
>
> So using the RSGB paper, far field equates to 955 metres. Since the CCIR
> curves for LF/MF propagation give 3mV/m at 1km for 100mW erp and is within
> the distance ground conductivity has much effect, that's where I have been
> measuring.
>
> 73 Malcolm
> (G3NZP)
>
> ----- Original Message -----
> From: "Andy Talbot" <andy.g4jnt@googlemail.com>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Friday, September 28, 2007 10:58 AM
> Subject: Re: LF: A question of calibration
>
>
> > 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
> >> >
> >> >
> >>
> >>
> >>
> >>
> >
> >
>
>
>
>