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RE: LF: Untuned loops

To: [email protected]
Subject: RE: LF: Untuned loops
From: "james moritz" <[email protected]>
Date: Wed, 16 Mar 2005 16:37:02 -0000
Importance: Normal
In-reply-to: <[email protected]>
Reply-to: [email protected]
Sender: [email protected]

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: [email protected] [mailto:[email protected]] On Behalf Of Andy
Sent: 15 March 2005 12:31
To: [email protected]
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: [email protected] <[email protected]>
To: [email protected] <[email protected]>
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 <[email protected]>
To: [email protected]
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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