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Dick Rollema wrote:
<blockquote TYPE=CITE><b><font size=+0>To All from PA0SE</font></b>
<p><b><font size=+0>Jim, M0BMU wrote:</font></b>
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<blockquote type=cite class=cite cite><font size=+0>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.</font>
<p><font size=+0>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.</font>
<p><font size=+0>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!</font>
<p><font size=+0>Cheers, Jim Moritz</font>
<br><font size=+0>73 de M0BMU</font>
<p><b><font size=+0>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.</font></b></blockquote>
<p><font size=+0>73, Dick, PA0SE</font>
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