LF: Loop experiments in GM

To:	"LF Group" <[email protected]>
Subject:	LF: Loop experiments in GM
From:	"Dave Pick" <[email protected]>
Date:	Wed, 19 Apr 2006 16:31:47 +0100
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Dear all.

Well I got the loop working pretty well at the GM QTH (20 miles West of Dumfries SW Scotland). It ran N/S and was made of 5mm OD coax cable pulled through two trees 100ft apart. At one end it was about 60ft high and the other about 50 although the top was horizontal (ground higher at the South end). To get the right circumference of about 330ft the bottom span was longer than the top and was about 4 to 5ft high. The tuner was placed in the centre of the bottom span standing on an upturned stacking box (so 2ft off the ground).
The 12nF required to tune the loop was made up of 100 1nF 2kV polypropylene "pulse rated" Cs connected in 33 parallel legs each having 3 series Cs (333pF each leg). That gives almost 11nF, across this was a 2nF vacuum variable C motor-driven from the shack.
Current in the loop was calculated at over 30A and measured at about 35. This means each C was passing around 1A. The frequency did move slightly HF under long periods of QRSS and the heating of the Cs could be felt by hand but they were by no means hot. I guess they have a negative temperature coefficient?
The transformer was made of 3 stacked 58mm 3C85 toroids with 20 turns on the primary and two turns of huge fat "starter motor" type cable as the secondary in series with the loop and Cs. The transformer remained completely stone cold even at 1kW input. I think I over-engineered it a bit after some tales of over heating from the states!
Over the four days I worked 7 countries with "O" reports including OH1TN off the side of the loop.
Later, when the weather improved I got out the trusty bow and arrow again and put up a nice Marconi inverted L with a 120ft top at 65ft high and a 45 degree downlead of about 90ft.
Due to the poor ground and proximity to trees I could only get 2.5A up this wire with 1kW, my best efforts on this site in the past gave over 3A (more top wires, more radials). Nevertheless the Marconi gave exactly the same signal reports with G3NYK, G3KEV and G3AQC with whom I did tests. It was, however, much noisier on receive. Every station I heard, regardless of direction, was better copy on the loop.
I also took the PA0DRT mini-whip with me and on a 20ft pole it was easily as sensitive as the big aerials, slightly less noisy than the Marconi but not as quiet as the loop.

So the loop was easy to erect, no worries about wires touching branches, robust due to the thick cable, performed as well as a similar Marconi on TX and was quiet on receive.
On the negative side the lower element gets in the way and it required a specialist tuner... but now I've got one I could be using loops more often!

I've left both aerials up for the next visit but I know which one is more likely to withstand the gales...

This and the pictures are at http://wireless.org.uk/loopy.htm
--
G3YXM IO92BK Birmingham UK (was IO75WC in GM)

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