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
>
>
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