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From: "mike.dennison" <mike.dennison@ntlworld.com>
To: rsgb_lf_group@blacksheep.org
References: <5.1.0.14.0.20020605144754.00ba4008@pb623250.kuleuven.be>
Subject: Re: LF: Counterpoise Experiment
Date: Thu, 6 Jun 2002 09:56:25 +0100
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I also recently tried a counterpoise, consisting of two wires 18m long (the
length of my top section) under the antenna and about 1m high. The
difference was that I resonated it. It gave me slightly worse results than
my earth stakes, but I assumed that more wires and lower loss coils would
help. Like Rik, I noted that a reduction in effective height may be a slight
problem. This is an area for more experimentation, but I agree with Jim that
the inconvenience of low wires may make it difficult to justify.


Mike, G3XDV
http://www.lf.thersgb.net
====================

ON7YD wrote:
> keep in mind that an elevated counterpoise will reduce the effective
height
> of the antenna (by "pulling up'' the RF ground level.
> So a part of the improvement could be lost again due to the lower
radiation
> resistance of the antenna.

M0BMU wrote:
> >Over the weekend I put a temporary counterpoise under my antenna, to see
> >how much effect it would have and make some rough measurements. The
> >counterpoise consisted of 11 parallel insulated wires about 45m long,
> >spaced about 1.2m, to make a rectangle 45m x 12m. These were supported at
> >a height of about 2m above the ground, and virtually filled the garden.
> >The antenna was my usual inverted L, currently at a mean height of about
> >9.5m and 40m long. Due to the position of the antenna in the garden, the
> >layout is asymmetrical, with the counterpoise extending 3m to one side of
> >the antenna, and 9m to the other side.
> >
> >With no counterpoise, the antenna loss resistance at 136kHz was 37ohms.
> >With the counterpoise as above, Rloss dropped to 32ohms, a reduction of
> >about 14%. With antenna current of 5A, 1A (ie 20%) of RF current was
> >returned through the counterpoise. Removing alternate counterpoise wires
> >to increase the average spacing to 2.4m led to Rloss of 35ohms, and 12%
of
> >the antenna current flowing in the counterpoise. Reducing the
counterpoise
> >to 45m x 6m with 1.2m spacing of wires, located centrally under the
> >antenna, led to Rloss of 34ohms and 12% of the antenna current in the
> >counterpoise.
> >
> >So a small reduction of loss was achieved by the counterpoise - it would
> >seem likely that, if the area of the counterpoise was increased and the
> >spacing of the wires reduced, a large reduction in loss could be
achieved.
> >The counterpoise acts like a screen between the field of the antenna and
> >the lossy ground - however, since only a small fraction of the antenna
> >current flowed in the counterpoise, it is clear that my counterpoise was
> >only intercepting a small fraction of the total field of the antenna, so
a
> >much greater area would be required to produce a substantial efficiency
> >improvement. If this greater area was available, a similar increase in
> >efficiency could probably be more easily obtained by increasing the size
> >of the antenna top loading, or a modest increase in height. In my case, a
> >much more practical way of obtaining the same improvement in radiated
> >power would be to increase the TX power by 14% - it really is very
awkward
> >having your whole garden covered in wires at head height!