Re: LF: Re: How measuring Ground conductivity ?

[Dick Rollema <dickrollema@casema.nl>]

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"
> <captbrian@ukonline.co.uk>
> To: <rsgb_lf_group@blacksheep.org>
> 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 <m0wye@thersgb.net>:
> >
> > > .
> > >
> > > 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"
> > > <uwe.wensauer@gmx.de>
> > > To: <rsgb_lf_group@blacksheep.org>
> > > 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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