Dear Mal, LF Group,
At 18:38 08/03/2002 +0000, you wrote:
...This first observation puts some doubt on some previous tests on Decca
sites with a small vertical in the vicinity of a large ground radial
system, where the small vertical was supposed to radiate better than the
main antenna...
Actually, included in the the conclusions of the experiments I did at the
Puckeridge Decca site was that the main Decca antenna was about 160 times
more efficient as a radiator than the small vertical.
I was putting less than 1W into the Decca mast, and about 300W into the
small antenna; the levels had been calculated to produce the same radiated
signals from both antennas, but the small antenna proved to be slightly
more efficient than calculated, and the Decca mast slightly less efficient.
So the signal strength from the small antenna was slightly higher. But the
point was that the difference in signal strength was the same irrespective
of the distance or direction over which it was received - effectively
showing that both antennas had much the same radiation pattern.
Incidentally, on receive, it was almost impossible to tell the difference
between either antenna.
There was no detectable interaction between the two antennas - I had
arranged things so that the antenna not in use was de-tuned. I also tried
transmitting into the small antenna while the big antenna was resonated,
but the current in the big antenna was only a few mA even then, so there
probably would not have been significant interaction even without
de-tuning. However, the two antennas were 120m (ie 400feet) apart. Part of
the reason for this was so that the small antenna would be off the Decca
antenna ground system - it had it's own grounding consisting of 4 rods.
With Mal's 50ft spacing, greater interaction would be expected - especially
since in Mal's case, the dimensions of both the antennas are of the same
order as the spacing between them. You can easily de-tune the antenna by
shorting or open-circuiting the loading coils.
Cheers, Jim Moritz
73 de M0BMU
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