At 13:56 15/11/2005 +0100, you wrote:
Hello Bryan,
The earth resistance is frequency dependent. Measurement at 50Hz says
nothing about the value at 137kHz. For more info consult ON7YD's web
site: http://www.qsl.net/on7yd/136khz.htm.
I measure the earth resistance by inserting an RF bridge between the
bottom end of the loading coil and the earth connection. Then I tune the
aerial/earth system to resonance in the LF band. The bridge then
measures the real part of the impedance of the system (so a bridge that
cannot measure reactance is sufficient, for instance a simple homemade
noise bridge).
From the value so obtained I subtract the loss resistance of the coil.
This has been determined by measuring its inductance and Q.
The remaining value is the earth resistance by a good approximation. Of
course loss and radiation resistance of the aerial itself are also part
of the measured resistance. But these contributions are so small as
compared to earth and coil resistance that they can be ignored.
I find for my location an earth resistance at 137kHz of about 22 ohms in
winter. During summer it is somewhat higher and also when the earth is wet
with rain.
73, Dick, PA0SE
At 11:52 15-11-05, you wrote:
Hello Bryan,
Yes. 10s is twenty or thirty meters.
I designed and built a bit of kit for the purpose of doing some
archeological ground resistance surveys, (although in the end we were
able to use professional
equipment from the Kent Archeological Society). I tried a frame with four, 4
inch nails spaced 0.33m apart, in a line, and aerated the the back garden
with it. I found it quite a good random number generator from about 10 ohms
to 60 ohms.
I remade the frame with two nails spaced 1 metre apart, one measuring and
one driven, and using a long cable placed the other two probes well outside
the area being surveyed. This gave a much more consistent reading of about
10 ohms, except near the garden path where the reading was higher. This is
the configuration usually employed by archeologists as it is supposed to
"see" deeper into the soil.
My measuring current was 1mA and the polarity reversed at 66Hz.
You are right that the reading doesn't seem to vary much with probe
spacing - a fact which still amazes me.
My, very limited, knowledge comes mostly from the excellent book "Seeing
beneath the soil" by Anthony Clark.
But as John has already observed, the parameter that we are measuring has
only to do with the ground rod effectiveness, and is not a measure of RF
losses.
73
Hugh M0WYE
----- Original Message ----- From: "captbrian" <[email protected]>
To: <[email protected]>
Sent: Monday, November 14, 2005 8:53 PM
Subject: Re: LF: Re: How measuring Ground conductivity ?
What does s stand for as in 10s of meters ? *(Presumably metres)*
I thought distance apart made no difference in homogenous soil ?
Bryan (Floating in a boat on sea-water )
Quoting Hugh M0WYE <[email protected]>:
.
However I got similar readings to you - around 10 ohms - with moist,
garden
soil.
I found the most repeatable readings were when one current and one
voltage
probe, 1m apart, were seperated by 10s of meters from the other pair - as
is
used for archeological ground resistance surveys.
73
Hugh M0WYE
----- Original Message ----- From: "Uwe Wensauer" <[email protected]>
To: <[email protected]>
Sent: Monday, November 14, 2005 4:31 PM
Subject: LF: How measuring Ground conductivity ?
To discuss
Measure ground conductivity.
I placed two stainless- steel rods ( 3mm dia ) whithin a distance of 7
Meters apart abt 80cm deep into soil
Powered with 50 Hz.
In series with the electrodes is a 10 Ohm resistor. In order to measuring
current, I measure the voltage drop across resistor.
Depending on weather I got 8 Ohm / m = 125mS
Is this measurement setup ( proceeding ) correct ?
Please correct me.
Uwe, DK1KQ
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