Mike G3XDV is probably right that the
changes are more likely changes in the "environmental losses" due to >>wet
trees and buildings.
I agree with Alan and Mike on the loss thing... My measurements showed that
most radical changes where related to weather events and changes in
environmental conditions around the antenna system... Most notably was some
graphs I produced showing the input power varying with rain, and even the
sun directly shining on the antenna and loading coil. These graphs showed
that there seems to be a recovery time constant after an 'event' that
changes the antennas environment and this time constant remained fairly
constant for the 6 month measurement period(Fall, Winter and Spring)...
One could explain the variations by considering the dielectric changes in
the system's capacitance I guess.
73 Scott
----- Original Message -----
From: "Alan Melia" <[email protected]>
To: <[email protected]>
Sent: Wednesday, August 03, 2005 3:41 PM
Subject: Re: LF: Soil Conductivity Measurement
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
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