Re: [IRCA] Measuring D-C resistance of ground rods to earth

["Chuck Hutton" <charlesh3@xxxxxxx>]

Charles:

You might want to do a Google for "three point method" and "fall of
potential". These are the two standard methods for measuring grounding
resistance. You should easily find tutorials from manufacturers of testing
equipment.

EWE's are at or near the top of the list of antennas that are picky about
having a good ground. Were it my EWE, I'd follow standard RF grounding
practice and put it at least 3 8' ground rods separated by at least 5 feet.
With average soil or better, that should suffice. With poor ground, buy a
silver mine and install it under the antenna.


Chuck

-----Original Message-----
From: irca-bounces@xxxxxxxxxxxxxxxx [mailto:irca-bounces@xxxxxxxxxxxxxxxx]
On Behalf Of Charles A Taylor
Sent: Friday, July 13, 2007 12:13 PM
To: irca@xxxxxxxxxxxxxxxx; ABDX@xxxxxxxxxxxxxxx
Subject: [IRCA] Measuring D-C resistance of ground rods to earth

DX Crew, Mr and Mrs. America, all the ships at sea and pesky CEs
who are snooping on this reflector:

I wanted to get a good handle on the D-C or A-C resistance to
ground of a 8-foot driven ground rod here on the coastal
plains of North Carolina.

I first took the gutless route and measured the D-C resistance
from an 8-foot rod driven 7½ feet into the ground, using the
house ground and neutral back to Progress Energy lines as the
reference.

I measured (with a multimeter) 1.4 kilo-ohms with the probes
connected one way, and a down-scale reading the other way.
That wasn't getting me anywhere, and besides it doesn't
account for earth currents that flow from one ground to the
other. These would render invalid what ever reading I do
get.

When I measured A-C voltage between house ground and the rod,
I measured about 15 millivolts (0.015 volts). Not helpful, but
it goes to show how RFI can get into a system that has two
or more grounds.

Using a D-C power supply sourcing 15.5 volts, I sank current
into the rod via the same multimeter on D-C current scale.

With the power supply bringing the ground system with one
polarity. I measured 51 mA of current; polarity reversed,
52 mA.

Dividing the supply voltage of 15.5 volts by the average of
the two D-C currents got me

300.9 ohms.

Note that 300.9 will be on the high side of just such a ground
because this one rod is driven 1 foot from the side of our
house and so blocks access to earth on one side of the rod.

Since the R-F resistance of a rod in earth is likely to be
lower because the rod is also capacitively coupled to earth
and that capacitance is in parallel with the D-C resistance.

Which brings up the point that a EWE antenna could, in
principle, be terminated just on a rod, barefoot.

The problem is to get a handle on the R-F resistance of an
8-foot ground to earth.

I'm thinking about firing up my Heathkit DX-35 CW transmitter
and repeating this process at 1,600 kHz with about 10 watts.
(it's crystalled for 1,600 kHz, and I have some low-range
R-F ammeters.

Anyone else have any experience at this?

Perfesser Chuck




   Charles A Taylor, WD4INP
  Greenville, North Carolina 



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