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
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