Dear David, LF Group,
Thanks for the QSO on Sunday, and the details of the experiment.
We started off with a reference antenna - A 9m vertical with a 5m top
section ... the poor ground made it very inefficient, with 100 W from the
transmitter generating an unimpressive 125mA of antenna current.
Assuming the transmitter was matched to the antenna and delivering the full
100W, the resistive component of the antenna impedance to get 125mA works
out to a remarkable 6400 ohms, which seems a little suspicious, and would
require a very poor ground indeed! The lambda/4 top load giving antenna
current of 1A at the same power amounts to a resistance of 100 ohms - a
remarkable reduction, if the ground system was the same.
... I expected the 'shortening
effect' to be much greater. Perhaps this was a good indication that as I
suspected the 'ground' under the antenna was either a considerable
distance
from the surface or, operating from a hilltop had an advantage
Or it could be that the effective ground was so close to the actual ground
level that most of the antenna's fields were in the air, and only a minor
shortening effect was present - the antenna might then be modelled as an
open-circuit quarter wave transmission line stub, with the wire as one
conductor and the ground as the other, rather lossy, conductor.
...our final plan to turn the whole thing into a
single horizontal dipole mounted along the ridge of a hilltop had to be
abandoned as one of the 2 fields suddenly acquired a large flock of
sheep.
...Reminds me of the Puckeridge Decca site, with the flock of sheep charging
around in the dark and rain, for their own reasons!
Cheers, Jim Moritz
73 de M0BMU
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