Blimey !
That is dramatic. And there is a minute wobble up to about 10 times
the distance as well - which would probably give 2% - 3% error.
So, in a way, one could almost say the RSGB figure "is right" to go so
far out:-)
Andy
On 28/09/2007, Dave Brown <[email protected]> wrote:
> It's instructive to look at plots of the variation of wave impedance
> with distance from the source.
> These plots (acknowledgements to John Middleton and Marconi Insts,
> "Engineers EMC Workbook")
>
> http://homepages.ihug.co.nz/~tractorb/zfreespace.jpg
>
> are for 47.7 MHz to scale the distance for lambda/2.pi to 1 metre, but
> the principle remains the same for any frequency. The selection of
> lambda/2.pi for the transition between near and far field regions
> makes much more sense when you see it plotted this way. Note the
> impedance scale is on the extreme right.
>
> DaveB, NZ
>
>
> ----- Original Message -----
> From: "Andy Talbot" <[email protected]>
> To: <[email protected]>
> Sent: Saturday, September 29, 2007 12:50 AM
> Subject: Re: LF: A question of calibration
>
>
> > 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 <[email protected]> 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" <[email protected]>
> >> To: <[email protected]>
> >> 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 <[email protected]> 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" <[email protected]>
> >> >> To: <[email protected]>
> >> >> 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
> >> >> >
> >> >> >
> >> >>
> >> >>
> >> >>
> >> >>
> >> >
> >> >
> >>
> >>
> >>
> >>
> >
> >
> > --
> > No virus found in this incoming message.
> > Checked by AVG Free Edition.
> > Version: 7.5.488 / Virus Database: 269.13.32/1033 - Release Date:
> > 27/09/2007 11:06
> >
> >
>
>
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