Re: Standards for ground measurement

Gary,

Jerry Sevick, W2FMI in the March 1981 issue of QST has described a simple
method to measure ground conductivity, which requires a 100 watt electric
light bulb, a 14.6 ohm 5 w resistor, four small probes and an ac voltmeter.
This method has been used by Doty, Frey and Mills, QST February 1983, who
found problems with ground currents, which was resolved by adding an
isolation transformer.

The measurement is made at the power line frequency (50 or 60 Hz). With a
100 watt transmitter and an RMS RF voltmeter, it seems to me that one
could make measurements at the HF frequency of interest.

I am suspicious of the probe method, since for accurate results the soil
should make uniform contact with the probe. The probe method is certainly
no good for rocky soil. The large apparent conductivity change observed on
wet and dry days, and the change from summer to winter is probably also a
probe problem.

Many commercial techniques are available and described in the literature
[reference Lagg, Earth Resistances, Pitman Publishing, 1964, pp. 206-229].

The method I like and have used is to employ an antenna itself to determine
the effective ground conductivity of the earth beneath it. Erect a
horizontal dipole dimensioned for the band of interest. Erect it at a low
height over the ground (say 1 metre). Do not use end insulators -- use
nylon rope. Determine the resonant length of the dipole (reactance zero),
and measure the resistance at the resonant frequency. This can be done
using a RX Noise bridge right at the antenna using a battery operated
receiver; or by measuring at the input end of a length of quality coax that
is exactly a half wavelength long. The coax should have ferite beads over
the shield of the coax at the antenna end, a W2DU type current balun.

You will have to guess at a suitable dielectric constant appropriate to the
type of soil in the area. Using NEC-2, after some trial and error, you can
find the conductivity that best agrees with the resonant length of the
dipole and the resistance of the dipole. If you get agreement with both
resonant length and resistance you have probably made a good guess at the
dielectric constant.

Regards,

John S. (Jack) Belrose, VE2CV

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>Please excuse if this has been recently discussed, but I am very new (just
>now) to the list. Someone suggested this might be the place to post this
>question.
>
>In various places, I see maps giving very general ground conductivity
>estimates. In checking with my local power company, I've found out how they
>test grounds. Their desire is to test an existing electrical ground at a
>given location. Therefore, their tests are done between three points. One
>point is the site electrical ground rod(s). The other two points are provided
>by them. Two 4 foot rods are driven into the ground 75 feet either side of
>the site ground being measured. The two 4 foot rods are tied in parallel to
>one side of a megger. The other side of the megger goes to the existing
>ground. The resistance is checked at 500vdc. I understand how to convert
>this to Siemens/meter, but I wonder about the method of testing for MY
>purposes (antenna modeling).
>
>Is there a more standardized method of testing ground conductivity? For my
>own purposes, I would think that driving the two rods into the ground and
>measuring between them would be more useful. If so, how much rod, how far
>apart, and at what voltage should the test be run? I'm also assuming this
>should be done in a couple of places in the desired area, in order to be more
>accurate.
>
>Any help with the above would be appreciated. Thank you!
>
>Gary McDuffie, Sr.
>Scottsbluff, NE
>mcduffie_at_hannibal.wncc.cc.ne.us

John S. (Jack) Belrose, VE2CV
Director, Radio Sciences
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
Received on Fri Dec 08 1995 - 20:44:00 EST