To All from PA0SE
Jim, 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!
Cheers, Jim Moritz
73 de M0BMU
As Jim no doubt knows the "standard" counterpoise, or radial
system, for MF broadcast stations in the USA is 120 radials of a quarter
wave length. This is what is probably required to collect all the current
coming from the vertical.
73, Dick, PA0SE
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