[TowerTalk] Ground resistance

[Keith Dutson]

It seems to be well known that planting an antenna over salty water will
greatly enhance radiation.  Is this advantage in the near or far field, or
both.  Would installing radials under a tower with horizontal antennas be
worth the time and effort?

73, Keith NM5G

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Jarvis
Sent: Wednesday, August 09, 2006 6:30 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Ground resistance


John asked:

"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
ground?"

Others suggested using a Megger or Hipot tester to measure ground
resistance.
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
question.

The answer to the question is NO.

The answer depends, of course, on whether you intend the ground system to
dissipate 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
reference, 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 other direction?  The measurement MIGHT see a lower resistance
due to the larger system, but would that data MEAN anything?  I doubt it.

If you're talking about the RF efficiency of a radial system, then the only
way to do it is to use a field strength meter, or a calibrated receiver
using a known antenna at a known distance, and take field strength readings
as you increase the ground system.  When you start getting diminishing
returns, you can stop adding radials.

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
ground system.  If the price of copper doesn't get you, then the pain in
your back will, as you install radials.  You KNOW you won't pull in 120 of
them.  You KNOW that
4 isn't
enough, unless elevated.  Empirical work done by AT&T in the 1930's, and
reported in public literature, showed that 16 to 20 radials resulted in
field strengths within 10% of the results with 120 radials.  So plan for 16,
and increase from there if you have the wire, inclination, and Advil to
support the effort.

Measuring the signal will entail use of instrumentation which will either be
relative in its reading, or will cost you more than the ground system you're
testing.
Even to
rent.

n2ea
jimjarvis at ieee.org


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