[Lf] [Fwd: LF: Counterpoise Experiment]

[Andre Kesteloot]

Dick Rollema wrote:

> 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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