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From: "Mike Underhill" <mike@underhill.co.uk>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 16 Mar 2005 18:37:34 -0000
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Subject: RE: LF: Untuned and tuned loops
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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Dear Andy, Jim, LF Group,</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<ol start=1 type=1>
<li class=MsoNormal style='color:navy'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial'>My strong support for what Jim
has said. The immediate environment always affects all field measurements,
not just magnetic field measurements. </span></font></li>
<li class=MsoNormal style='color:navy'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial'>Marcus’s formula is
correct for free-space and it is in all the books. Wire loss is not an
issue for a typical open-circuit loop. </span></font></li>
<li class=MsoNormal style='color:navy'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial'>In an ‘environment’
near real ground, buildings etc., the ‘physics’ insists that
such a loop gives an open circuit voltage proportional to B = mu * H. The
presence of the environment means that mu is not equal to the free-space
value of mu(0). </span></font></li>
<li class=MsoNormal style='color:navy'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial'>The current in a short-circuit
lossless loop is directly proportional to the H field and it does not
depend on the area. This is not in ‘the antenna books’, but
it is in some physics books dealing with super-conductivity. </span></font></li>
<li class=MsoNormal style='color:navy'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial'>There is a third case and this
is when the loop is resonant. If the loop is loss-less and therefore 100%
efficient and power matched to the load of the measuring detector. The ‘physics’
demands that the antenna ‘capture cross-section’ should be the
same as the capture cross-section of any short dipole. In free-space this
is 1.5 / 4pi square wavelengths. The received power is the (average) power
density times this (small dipole) cross-section area. The free-space equivalent
E and H fields can be found directly from this. This measurement is
irrespective and independent of (loss-less) loop size. The ‘proof’
of this is in the better antenna books, such as Kraus and Balanis. <br>
What the system Q ends up being does not matter for this proof. Some may
have noticed however that it is a matter of considerable (unnecessary) dispute,
as reported in the pages of RadCom. However loop (or any antenna) losses
do matter, because the cross-section area of an antenna is reduced in proportion
to its (in)efficiency. This point is in most of the antenna books. After
real conductor losses have been minimised and factored out we find
practical outdoor Qs of 100 to 350. In a screened room the loop Q can
exceed 600. </span></font></li>
<li class=MsoNormal style='color:navy'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial'>There are similar arguments for
short-circuit, open-circuit and resonant electric field measuring ‘whip’
antennas. </span></font></li>
<li class=MsoNormal style='color:navy'><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial'>Thus you have six choices for
measuring the fields of an antenna in a real environment. Only in free
space do all six methods give the same answer.</span></font></li>
</ol>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>I hope that this is useful and of interest.
It just goes to show that real life can be hard and difficult, but more fun
than just theory on its own. Also it has not “all been done before!”
</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Mike - G3LHZ</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<div style='border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt'>
<p class=MsoNormal><font size=2 face=Tahoma><span lang=EN-US style='font-size:
10.0pt;font-family:Tahoma'>-----Original Message-----<br>
<b><span style='font-weight:bold'>From:</span></b> owner-</span></font><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>rsgb_lf_group@blacksheep.org</span></font><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>
[mailto:owner-</span></font><font size=2 face=Tahoma><span lang=EN-US
style='font-size:10.0pt;font-family:Tahoma'>rsgb_lf_group@blacksheep.org</span></font><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>]
<b><span style='font-weight:bold'>On Behalf Of </span></b>james moritz<br>
<b><span style='font-weight:bold'>Sent:</span></b> </span></font><font size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>16
March 2005</span></font><font size=2 face=Tahoma><span lang=EN-US
style='font-size:10.0pt;font-family:Tahoma'> </span></font><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>16:37</span></font><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'><br>
<b><span style='font-weight:bold'>To:</span></b> </span></font><font size=2
face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>rsgb_lf_group@blacksheep.org</span></font><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'><br>
<b><span style='font-weight:bold'>Subject:</span></b> RE: LF: Untuned loops</span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span lang=EN-US
style='font-size:10.0pt;font-family:Arial;color:navy'>Dear Andy, LF Group,</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span lang=EN-US
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span lang=EN-US
style='font-size:10.0pt;font-family:Arial;color:navy'>I did a lot of
experiments with measuring loops a few years back for a project at work. You
can get results that are within a few dB quite easily, provided the loading
effect of the receiver/level meter on the loop is taken into account. However,
you do have to be very careful about nearby conductors, particularly the cables
connecting the antenna and measuring gear. If one of these is anywhere near
resonance, gross errors can be caused, particularly with a small loop. In the
upper part of the HF spectrum, say above about 10MHz, this type of effect can
be quite difficult to avoid. The best bet is to use a battery powered receiver,
with a short lead to the antenna, in the middle of some open ground, in order
to minimize the possible parasitic antenna effects. Using the clamp-on
“EMC” ferrite cores at several points on the cables can also help,
but at least a few turns through the core is needed to get usefully high
impedance. Because of the much longer wavelengths, this sort of thing is not so
much of a problem at LF – although overhead power or telephone lines can
produce large errors. A typical tell tale sign is the signal null is not in the
expected direction, or there isn’t a clear null at all.</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span lang=EN-US
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span lang=EN-US
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span lang=EN-US
style='font-size:10.0pt;font-family:Arial;color:navy'>Cheers, Jim Moritz</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span lang=EN-US
style='font-size:10.0pt;font-family:Arial;color:navy'>73 de M0BMU</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span lang=EN-US
style='font-size:10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal style='margin-left:36.0pt'><font size=2 face=Tahoma><span
lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>-----Original
Message-----<br>
<b><span style='font-weight:bold'>From:</span></b>
owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] <b><span
style='font-weight:bold'>On Behalf Of </span></b>Andy<br>
<b><span style='font-weight:bold'>Sent:</span></b> </span></font><font size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>15
March 2005</span></font><font size=2 face=Tahoma><span lang=EN-US
style='font-size:10.0pt;font-family:Tahoma'> </span></font><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>12:31</span></font><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'><br>
<b><span style='font-weight:bold'>To:</span></b> rsgb_lf_group@blacksheep.org<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: LF: Untuned loops</span></font></p>
<p class=MsoNormal style='margin-left:36.0pt'><font size=3
face="Times New Roman"><span lang=EN-US style='font-size:12.0pt'> </span></font></p>
<div>
<p class=MsoNormal style='margin-left:36.0pt'><font size=2 color=black
face="Times New Roman"><span lang=EN-US style='font-size:10.0pt;color:black'>Thanks
to all respondees - I knew it was a simple formula and was (theroretically) an
exact way of determining field strength. Just need to make sure the input
impedance of the voltmeter is significantly higher than the loop impedance to
avoid any loading effects.</span></font></p>
</div>
<div>
<p class=MsoNormal style='margin-left:36.0pt'><font size=3
face="Times New Roman"><span lang=EN-US style='font-size:12.0pt'> </span></font></p>
</div>
<div>
<p class=MsoNormal style='margin-left:36.0pt'><font size=2
face="Times New Roman"><span lang=EN-US style='font-size:10.0pt'>(Someone here
wants to measure the field strength of a transmission in the HF band)</span></font></p>
</div>
<div>
<p class=MsoNormal style='margin-left:36.0pt'><font size=3
face="Times New Roman"><span lang=EN-US style='font-size:12.0pt'> </span></font></p>
</div>
<div>
<p class=MsoNormal style='margin-left:36.0pt'><font size=2
face="Times New Roman"><span lang=EN-US style='font-size:10.0pt'>Andy
G4JNT</span></font></p>
</div>
<div>
<p class=MsoNormal style='margin-left:36.0pt'><font size=3
face="Times New Roman"><span lang=EN-US style='font-size:12.0pt'> </span></font></p>
</div>
<blockquote style='border:none;border-left:solid black 1.5pt;padding:0cm 0cm 0cm 4.0pt;
margin-left:3.75pt;margin-top:5.0pt;margin-bottom:5.0pt'>
<div>
<p class=MsoNormal style='margin-right:0cm;margin-bottom:12.0pt;margin-left:
36.0pt'><b><font size=2 face=Arial><span lang=EN-US style='font-size:10.0pt;
font-family:Arial;font-weight:bold'>-----Original Message-----</span></font></b><font
size=2 face=Arial><span lang=EN-US style='font-size:10.0pt;font-family:Arial'><br>
<b><span style='font-weight:bold'>From: </span></b><a
href="mailto:markusvester@aol.com">markusvester@aol.com</a> <<a
href="mailto:markusvester@aol.com">markusvester@aol.com</a>><br>
<b><span style='font-weight:bold'>To: </span></b><a
href="mailto:rsgb_lf_group@blacksheep.org">rsgb_lf_group@blacksheep.org</a>
<<a href="mailto:rsgb_lf_group@blacksheep.org">rsgb_lf_group@blacksheep.org</a>><br>
<b><span style='font-weight:bold'>Date: </span></b>2005/03/15 </span></font><font size=2 face=Arial><span lang=EN-US style='font-size:10.0pt;font-family:Arial'>10:58</span></font><font
size=2 face=Arial><span lang=EN-US style='font-size:10.0pt;font-family:Arial'><br>
<b><span style='font-weight:bold'>Subject: </span></b>Re: LF: Untuned loops</span></font></p>
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lang=EN-US style='font-size:10.0pt;font-family:Verdana'> </span></font></p>
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<p class=MsoNormal style='margin-left:36.0pt'><font size=2 face=Verdana><span
lang=EN-US style='font-size:10.0pt;font-family:Verdana'>Hi Andy</span></font></p>
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lang=EN-US style='font-size:10.0pt;font-family:Verdana'> </span></font></p>
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<p class=MsoNormal style='margin-left:36.0pt'><font size=2 face=Verdana><span
lang=EN-US style='font-size:10.0pt;font-family:Verdana'> U = omega *
ľo * H * area </span></font></p>
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lang=EN-US style='font-size:10.0pt;font-family:Verdana'> </span></font></p>
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<p class=MsoNormal style='margin-left:36.0pt'><font size=2 face=Verdana><span
lang=EN-US style='font-size:10.0pt;font-family:Verdana'>with ľo = 4 * pi *
10^-7 Vs/A/m, omega = 2 * pi * f .</span></font></p>
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<p class=MsoNormal style='margin-left:36.0pt'><font size=2 face=Verdana><span
lang=EN-US style='font-size:10.0pt;font-family:Verdana'><br>
73 de Markus, DF6NM</span></font></p>
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lang=EN-US style='font-size:10.0pt;font-family:Verdana'> </span></font></p>
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<p class=MsoNormal style='margin-right:0cm;margin-bottom:12.0pt;margin-left:
36.0pt'><font size=2 face=Verdana><span lang=EN-US style='font-size:10.0pt;
font-family:Verdana'>-----Original Message-----<br>
From: Andy <actalbot@southsurf.com><br>
To: rsgb_lf_group@blacksheep.org<br>
Sent: </span></font><font size=2 face=Verdana><span lang=EN-US
style='font-size:10.0pt;font-family:Verdana'>Tue, 15 Mar 2005</span></font><font
size=2 face=Verdana><span lang=EN-US style='font-size:10.0pt;font-family:Verdana'>
</span></font><font size=2 face=Verdana><span lang=EN-US style='font-size:10.0pt;
font-family:Verdana'>09:01:51</span></font><font size=2 face=Verdana><span
lang=EN-US style='font-size:10.0pt;font-family:Verdana'> -0000<br>
Subject: LF: Untuned loops</span></font></p>
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<div id="AOLMsgPart_2_933454df-b8fe-49bd-8705-56a0ae1e11df">
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<p class=MsoNormal style='margin-left:36.0pt'><font size=2 color=black
face=Verdana><span lang=EN-US style='font-size:10.0pt;font-family:Verdana;
color:black'>Can anyone recall this equation, I know it was mentioned on this
reflector before...</span></font></p>
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<p class=MsoNormal style='margin-left:36.0pt'><font size=2 face=Verdana><span
lang=EN-US style='font-size:10.0pt;font-family:Verdana'> </span></font></p>
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<p class=MsoNormal style='margin-left:36.0pt'><font size=2 color=black
face=Verdana><span lang=EN-US style='font-size:10.0pt;font-family:Verdana;
color:black'>What is the output voltage for an untuned small loop into an
arbitrarily high impedance? It is proportional to H, and presumably F^2,
and I seem to recall it is an absolute value, virtually un-influenced by
losses, conductor diameter etc. Therefore can be used for accurate H
field (and hence far field radiation) signal strength measurements.</span></font></p>
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<p class=MsoNormal style='margin-left:36.0pt'><font size=2 face=Verdana><span
lang=EN-US style='font-size:10.0pt;font-family:Verdana'> </span></font></p>
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<p class=MsoNormal style='margin-left:36.0pt'><font size=2 color=black
face=Verdana><span lang=EN-US style='font-size:10.0pt;font-family:Verdana;
color:black'>Andy G4JNT</span></font></p>
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