[Lf] [Fwd: LF: Counterpoise Experiment]

[Andre Kesteloot]

"mike.dennison" wrote:

> 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!