Envelope-to: dave@picks.force9.co.uk
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From: "Alan Melia" <Alan.Melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
References: <20050803185205.54655.qmail@web25708.mail.ukl.yahoo.com>
Date: Wed, 3 Aug 2005 23:41:30 +0100
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Subject: Re: LF: Soil Conductivity Measurement
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Hi Mike (and Mike), I have considered doing some longer term measurements
here. I did spend several weeks in my early career doing 4 probe measuements
on silicon wafers and ingots. I have this nagging suspicion that although
ground resistance changes with the weather, it probably doesn't make as big
an effect as one might think. Part of the reason for this thinking is the
way we have managed to get the "ground loss" down at a number of different
sites, with totally different soils. Mike G3XDV is probably right that the
changes are more likely changes in the "environmental losses" due to wet
trees and buildings. Also some measurements that Finbar did that showed
ground loss increasing after rain. I am not sure now whether we thought to
"tweak" the wires to shake off any water film. It took about 30 minutes
after precipitation stopped for the loss measurement to return to "normal".
Insulator loss was not a problem as be were using a very low voltage source,
and a bridge. Finbar is on rock, he jokes about planting out the garden
using dynamite to make the planting holes.....but he is right on the sea
edge.

Bill Ashwell has measurments suggecting that there is an effect on loops
where the bottom wire is close to the ground. Most of the states-side big
loops have the bottom wire above head level I believe.....once its got to be
off the ground, this is problably the most convenient, for the rest of the
family at least.

To Brian G3YKB, could it be that your insertion of coils killed the
basically low impeadance / high current nature of the loop and introduced a
significant voltage on the loop. this would then drives a current through
the lossy (capacitive) environemt of the foliage. The US stations have found
that provided the impedance is kept low ( Lawrence runs in the order of 40
amp RF at 400 w) there is no appreciable loss to the trees, and little needs
to be done to insulate, or space the antenna from the trees. These loops are
big ...up to 400 feet circumference, and stretched over 70 foot high trees,
using no insulators.

Cheers de Alan G3NYK