From: MarkusVester@aol.com
Message-ID: <130.94f4a82.299b9b91@aol.com>
Date: Wed, 13 Feb 2002 05:36:01 EST
Subject: Re: LF: Loop comments (again)
To: rsgb_lf_group@blacksheep.org
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Hi Dave 'YMC,

>  ...There is a small earth
>  coupling loss which reflects to a series loss resistance.  The actual  
mechanics of
>  this I have never understood, but the effective resistance of this is no 
more than
>  0.5 ohms. 

There are two causes for ground-induced loss in a magnetic loop:

- eddy currents in the ground. These are hardly avoidable and their losses 
increase with ground conductivity, so "bad LF ground" is good for loops. Of 
course, some distance (ie. height) helps, whereas using multiple turns makes 
no difference.

- capacitively coupled losses if the high potential end of the loop comes 
close to ground. Properly balanced feeding from the top, low inductance and 
multiple series capacitors can reduce these losses.

A similar situation occurs in magnetic resonance imaging, where small receive 
loops ("surface coils") are often placed close to the patient's skin to 
achieve maximum sensitivity, and the eddycurrent noise resistance of the 
tissue sets a fundamental limit in achievable SNR.

Back to LF: If you had an (single-turn) loop of say 200 m^2 and 0.5 ohms, the 
series radiation resistance would be around 50 microohms and the efficiency 
10^-4 (-40dB). On the other hand, a 10 m high top-loaded vertical radiates 
with approx. 33 milliohms, roughly the same 10^-4 at 300 ohms loss.

The latter may be somewhat extreme though. In most cases, the losses of 
amateur vertical antennas seem to be on the order of 30 to 100 ohms (probably 
containing a large part of capacitively coupled losses rather than direct 
ground connection resistance). - A different point is the directional 
characteristic of a loop, which may sometimes become a handicap for TX.

If you live on very high-impedance ground, an earth loop (a long wire laid 
out on the ground, shorted at the remote end and spanning up its area 
underneath due to the skin depth of the earth return current) may also be a 
viable option.

73s and all the best
Markus, DF6NM