There has been some recent discussion about outdoor antennas and grounds.
No one has mentioned what a good ground RESISTANCE is. Here is my two
cents
worth.
Since last fall I have been measuring ground resistance from four TEST
ground rods in my back yard. The rod specs are as follows; one 3/4" dia
X
10 foot copper clad steel, one 5/8" dia X 8 foot copper clad steel, one
1/2"
dia X 30 inch copper tubing (some people call it pipe) and one 5/8" X 24"
copper clad steel.
This is what I read today for each of these rods using a 3 point method
with
a B&K Model 309 ground resistance meter.
10 footer has around 10 ohms
8 footer has around 23 ohms
30 incher has 166* ohms
24 incher has 175* ohms
In addition a new set of readings has been taken either daily or every
other
day since last fall. Along with this am tracking ground temperature at
the
surface and at 1 foot, 2 feet, 3 feet and 4 feet down in the soil, using
type K thermopcouples.
Each rod is also terminated with a length of #14 AWG copper wire and
brought
into my basement to a central area. The five thermocouples also come in
to
the same central area.
None of these rods are connected in any way to the electric service ground
which is completely separate.
The astericks above for the 30" rod and the 24" rod indicate that there is
considerable change in resistance due to frozen soil. This was a mild
winter (with only a few below zero days). My readings show that the soil
froze (frost line) down between one and two feet. The resistance readings
increased as the soil froze and lowered again when the frost thawed. For
example the resistance for the 24" rod increased from 166 ohms to 359 ohms
due to frozen soil. I was hoping for a colder winter (more sub zero days)
where I had suspected an even greater change in both of the short rods.
These two rods are purposely short to trend as the frost line gets deeper
in
the soil.
I intend to trend both resistance and temperature for all the rods for
another couple of years and at time will share the data with you.
In conclusion I would always recommend a 10 foot rod for a good ground
(around 10 ohms in black dirt/clay, moist soils, higher in dry, sandy,
rocky
soils). These values will vary with type of soil and moisture content.
If
a second rod is ever used space it 10 to 15 feet from the first. As a side
note; some electric utility companies who require very low resistance
grounds around substations and generating plants have driven (when
required)
special rods to a depth of 90 (ninety) feet!
Tom Jasinski
Shorewood, IL