[TowerTalk] Ground resistance
jimjarvis at verizon.net
Wed Aug 9 07:29:37 EDT 2006
"Is there any simple way to measure the impedance of the ground radial
system as you install them to know when you have enough of them in the
Others suggested using a Megger or Hipot tester to measure ground
While it is true that one can measure resistance between two rods in the
earth, and from that determine earth conductivity....that wasn't the
The answer to the question is NO.
The answer depends, of course, on whether you intend the ground system to
charge, and in the event of a lightning strike, peak current, OR, if you're
looking for a radial field which will be effective as part of an RF antenna.
But in both cases, the DC measurement is either misleading or irrelevant.
If you have radials and rods in the ground, you could measure the resistance
of the ground system, through earth, to a reference ground rod, a known
distance away. When you run a radial and rod system closer to your
the earth path resistance will go down. But that's because you introduced a
shorter measurement path. Suppose you put in a very effective ground in the
direction? The measurement MIGHT see a lower resistance due to the larger
but would that data MEAN anything? I doubt it.
If you're talking about the RF efficiency of a radial system, then the only
to do it is to use a field strength meter, or a calibrated receiver using a
at a known distance, and take field strength readings as you increase the
system. When you start getting diminishing returns, you can stop adding
As Dave Robbins said, "WAY, yes. Simple, no."
My answer is, "why bother?"
As a practical matter, you will be limited by resources in constructing the
system. If the price of copper doesn't get you, then the pain in your back
you install radials. You KNOW you won't pull in 120 of them. You KNOW that
enough, unless elevated. Empirical work done by AT&T in the 1930's, and
public literature, showed that 16 to 20 radials resulted in field strengths
of the results with 120 radials. So plan for 16, and increase from there if
the wire, inclination, and Advil to support the effort.
Measuring the signal will entail use of instrumentation which will either be
in its reading, or will cost you more than the ground system you're testing.
jimjarvis at ieee.org
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