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!
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