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Message-ID: <5.1.0.14.0.20020308191310.02953708@gemini.herts.ac.uk>
Date: Fri, 08 Mar 2002 19:54:26 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: Re: LF: loop
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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