Message-ID: <001d01c801c6$a918b420$0fee1a52@enigma>
From: "Malcolm Harman" <malcolm.fr263@ntlworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <000a01c80132$aae29df0$83e41a52@enigma> <46FC071D.2090202@w1tag.com> <000601c801ae$38cc27a0$0fee1a52@enigma> <fc7eccec0709280258m35a361dfy75caad028a834d57@mail.gmail.com>
Date: Fri, 28 Sep 2007 12:56:51 +0100
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Subject: Re: LF: A question of calibration
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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
>> >
>> >
>>
>>
>>
>>
>
>