[Lf] [Fwd: LF: Counterpoise Experiment]

[Andre Kesteloot]

Rik Strobbe wrote:

> Hello Jim,
>
> 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.
> It is interesting to notice that the counterpoise increased the total
> antenna current by 7.5% (32 Ohm vs 37 Ohm) while it takes 20% of the
> current. Maybe it is a bit simplified, but based on that 20% one could
> assume that the RF ground level was raised by 40cm (20% of 2m).
> In that case the effective height would decrease from 9.5m to 9.1m (-0.37dB).
> The increased current (7.5%) gave an improvement of 0.63dB, so the overall
> gain could be as low as 0.26dB.
>
> It seems that an isolated counterpoise is only worth the effort with :
> - high antennas (so the loss in effective height is minimal)
> - a lot of space (and $$ to buy the wire)
> - very tolerant XYL and neighbours
>
> 73, Rik  ON7YD
>
> At 12:51 5/06/2002 +0100, you wrote:
> >Dear LF Group,
> >
> >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!
> >
> >Cheers, Jim Moritz
> >73 de M0BMU
> >