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From: Roger Lapthorn <rogerlapthorn@gmail.com>
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Date: Fri, 18 Mar 2011 16:51:33 +0000
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
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Subject: Re: LF: Re: Loop TX antennas at VLF?
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Very many thanks for this analysis Jim.

It does suggest there is some mileage in this approach.  As you say, with=
 the Marconi there are many large losses that make it less efficient at 9k=
Hz so the loop may not be so bad comparing one against the other.=20
=20
73s
Roger G3XBM

Via my 2.4GHz handheld (iPod Touch 4g)

On 18 Mar 2011, at 15:26, "James Moritz" <james.moritz@btopenworld.com> wr=
ote:

> Dear Roger, LF Group,
>=20
>> Just wondering if anyone has done the maths to work out what sort of ER=
P
>> could be expected at 8.97kHz with, say, 100W to a smallish loop antenna=
 in
>> the garden?
>=20
> A 10m x 10m, 100m^2 loop of  "thickish" 3mm dia solid wire would have a=
 resistance at 9kHz of roughly 0.1ohm. With 100W available, 32A antenna cu=
rrent should be possible, assuming negligible tuning capacitor losses. Ind=
uctance would be of the order of 40uH. A tuning capacitor of roughly 8uF=
 would be needed.
>=20
> The radiation resistance of an electrically small loop is:
>=20
> 320 * pi^4 * A^2 / (lambda)^4, where A =3D area, lambda =3D wavelength
>=20
> for 100m^2 at 9kHz, Rrad is about 250 pico-ohms (!)
>=20
> The ERP is then 1.8 * I^2 * Rrad, about 0.45uW
>=20
> So pretty low, but with a bit bigger loop and a bit more power, it would=
 seem to be competitive with small verticals of a similar size. This is pe=
rhaps mainly because of the serious losses present in loading coils that=
 people have been able to make for verticals, combined with high voltage=
 limitations of fairly short wire antennas, and high environmental losses=
 of various types also due to high electric fields. The voltage in this ex=
ample would only be about 70V. So might be worth trying for "back garden"=
 experiments (assuming your antenna masts can support thick enought wire!)=
, although I think it would not be competitive for bigger balloon/kite sup=
ported vertical antennas.
>=20
> Cheers, Jim Moritz
> 73 de M0BMU=20
>=20