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
>> >
>> >
>>
>>
>>
>>
>
>
