RE: LF: Untuned and tuned loops

["Mike Underhill" <mike@underhill.co.uk>]

Dear Andy, Jim, LF Group,

 

1. My strong support for what Jim has said.  The immediate environment always affects all field measurements, not just magnetic field measurements.
2. 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.
3. 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). 
4. 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. 
5. 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. 
   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. 
6. There are similar arguments for short-circuit, open-circuit and resonant electric field measuring ‘whip’ antennas. 
7. 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.

 

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!”

 

Mike -  G3LHZ

 

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of james moritz
Sent: 16 March 2005 16:37
To: rsgb_lf_group@blacksheep.org
Subject: RE: LF: Untuned loops

 

Dear Andy, LF Group,

 

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.

 

 

Cheers, Jim Moritz

73 de M0BMU

 

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Andy
Sent: 15 March 2005 12:31
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Untuned loops

 

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.

 

(Someone here wants to measure the field strength of a transmission in the HF band)

 

Andy   G4JNT

 

-----Original Message-----
From: markusvester@aol.com <markusvester@aol.com>
To: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
Date: 2005/03/15 10:58
Subject: Re: LF: Untuned loops

 

Hi Andy

 

 U = omega * µo * H * area 

 

with µo = 4 * pi * 10^-7 Vs/A/m, omega = 2 * pi * f .

73 de Markus, DF6NM

 

-----Original Message-----
From: Andy <actalbot@southsurf.com>
To: rsgb_lf_group@blacksheep.org
Sent: Tue, 15 Mar 2005 09:01:51 -0000
Subject: LF: Untuned loops

Can anyone recall this equation, I know it was mentioned on this reflector before...

 

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.

 

Andy  G4JNT

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