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From: "Dave Brown" <tractorb@ihug.co.nz>
To: <rsgb_lf_group@blacksheep.org>
References: <000a01c80132$aae29df0$83e41a52@enigma> <46FC071D.2090202@w1tag.com> <000601c801ae$38cc27a0$0fee1a52@enigma> <fc7eccec0709280258m35a361dfy75caad028a834d57@mail.gmail.com> <001d01c801c6$a918b420$0fee1a52@enigma> <fc7eccec0709280550x2be11f58k7195c7bd0ffd8fb9@mail.gmail.com>
Date: Sat, 29 Sep 2007 02:31:07 +1200
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Subject: Re: LF: A question of calibration
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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" <andy.g4jnt@googlemail.com>
To: <rsgb_lf_group@blacksheep.org>
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 <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
>> >> >
>> >> >
>> >>
>> >>
>> >>
>> >>
>> >
>> >
>>
>>
>>
>>
>
>
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