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
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