[Lf] [Fwd: LF: Counterpoise Experiment]

[Andre Kesteloot]

James Moritz 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