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Resistivity addresses both electrical flow of AC and DC current. The
preferred unit is the OHM-METER: resistance between opposing faces, of a
cubic meter of soil. The theoretical calculation, for any earth-ground
resistance of any EGS or electrode, uses the general resistance formula:
R = p(L/A)
R = the ground system or single electrode resistance
p = resistivity of the earth [soil], ohm-meters
L = length of the conducting path, meters
A = cross-sectional area of the path, square meters
Thus, soil resistivity is proportionally constant, relating EGS
resistance to the length of the conducting path and the path's area, in
cross-section [to you anatomists, a transverse section through a conic
section of soil]. It MUST be remembered that, TYPICAL, soil resistivity
can range from 100 to 5,000 ohm-cm. Soil resistivity MUST be measured to
effect a working grounding design.
Measurements of soil resistivity follow the Wenner 4-pin electrode
method, using a ground resistance meter. 4 metal pins are driven into the
ground, in a straight line, EQUALLY SPACED. Constant current is
introduced, into the soil, via the testing apparatus, PLUS the OUTER 2
electrodes. The potential drop is measured across the INNER 2 electrodes,
with the resistance meter - almost always part of the test gear -
yielding direct ohm readings, which are plugged into:
p = 191.5*R*A
p = soil resistivity, ohm-cm
R = ground resistance meter reading, ohms
A = distance between electrodes, feet
The calculated soil resistance is that between the surface [of the
ground] and a depth equal to the SPACING OF THE PINS.
Naturally occurring, soil, electrolytes vary markedly, in amount and type
of ion, thus ability to facilitate electron flow. Adding moisture
enhances soil conductivity [or, the more moisture, the LOWER the
resistivity]. However, strata with, or composed mainly of, granite,
sandstone and surface limestone [calcium carbonate] will demonstrate
little, or no, emendation in resistivity/conductivity, by addition of
moisture. This holds, also, for salts [non- acids or bases].
Temperature can impact soil properties, greatly. Soil resistivity remains
fairly stable, with temperature, until freezing, at which point most
moisture will freeze [and you thought physics was hard]. Frozen moisture
INCREASES resistivity.
A grounding rod, driven into earth [you guys with a Freudian bent, pun
intended, must be having a field day] of uniform resistivity will radiate
current omnidirectional. This is referred to as the electrodes 'sphere of
influence' [sounds like something in which the Soviets - forgive my
stumbling tongue - Russians, would be interested]. Ability of electrodes
to disperse current is directly proportional to soil conductivity [or
inversely proportional to resistivity]. If met with high, or low, soil
parameters, designs can be adequately compensated.
An electrode of length L, has a sphere of influence [SOI] with radius,
approximating, length L. If 2 electrodes are spaced too CLOSE, SOI
overlap, reducing, even minimizing, electrode ability to dissipate
current. For maximal effect, electrodes should be spaced a minimum of
twice the length of the electrode.
Rod length [size DOES matter] is an important factor determining
earth-ground resistance. Longer rods [keep it clean], > 10 feet, have a
large SOI, radiating away more current. The overall effect is a
lower-resistance earth-ground, in soil of UNIFORM resistivity. As rod
length increases [OK, now its time for John Holmes], however, SOI reaches
an infinite plateau [so THERE! Holmes fans!] where earth-to-ground
resistance [EGR; not to be confused with exhaust gas ratio] will not be
altered. Extending rod length beyond the infinite plateau point, will do
little to lower EGS resistance.
EGR of an EGS is dependant upon the density of the rod population.
Additional rods decrease resistance of the total system until it, also,
reaches an infinite plateau. This plateau results from overlapping SOI.
Ground systems can be referred to as counterpoises or ground grids.
Adding more rods will have NO effect on lowering ground resistance. To
further reduce resistance, the ground system area must be increased.
There are many types of grounding, and Hams have not been slack in this
area. However, it is best, from a physical and electrical vantage, to
rely on, when possible and feasible, driven rods, water pipes, chemical
wells, ufer grounds and electrolytic rods.
Driven rods are copper or copper-clad steel, pounded into the earth. They
are inexpensive, typically 10 feet long and 5/8" diameter. They are
usually part of grid systems or isolated-equipment grounds. Rods have
these drawbacks: easily degraded by environment, age, temperature,
moisture; easily damaged during installation [scratches expose the steel
surface which is, then, susceptible to corrosion and electric eddy
attack]; resistance increases, steadily, with age. In short, driven rods
are inexpensive, adequate for a SHORT time, in GOOD soil. They WILL fail,
in service.
Water pipes, or mains, are used as electrodes but with the cons of:
difficult to test; impossible to maintain; plastic inserted ANYWHERE -
e.g., O-rings - destroy circuit integrity; cold-water pipes promote
condensation whose moisture encourages corrosion. WATER PIPES SHOULD
NEVER BE USED AS A SINGLE GROUND SOURCE. They are unreliable and can be
destroyed by the most moronic of upgrades - with plastic. Driven rods &
water pipes should always be used TOGETHER and/or with ground-grids, per
NEC 250.
Chemical wells are holes, filled with conductive chemicals, connected to
EGS via copper rods. Common, inexpensive salts are used, e.g., copper &
magnesium sulfate. Unfortunately, these are environmentally hazardous
compounds, restricted by EPA.
UFER grounds are copper-wire grids, incorporated into a structure's
concrete foundation, usually 0000 stranded cable [this is done during
construction, for those of you who have never played in cured concrete].
Obviously, they are impossible to test or maintain. Time and removal of
moisture can alter foundation integrity thus ground resistance.
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Electrolytic rods are 100% copper TUBING, filled with earth salts. To
'activate' them, there must be orifices at top and bottom. The top holes
are for ingress of air, thus are called 'breather' holes. The earth salts
are hygroscopic [they absorb water, a process known as deliquescence -
del ih QWESS ins] becoming electrolytic - capable of carrying electric
charge or current - or forming electrolytic solutions, which solution is
communicated to the back-filled soil by egress - leeching, or migrating -
out the bottom holes. These fingers of electrolytic solution are called
'electrolytic roots', which lower ground resistance by ionizing -
stripping-away electrons, then available for electron flow - surrounding
soil. Common table salt, NaCl, experiences physical change from a solid
to aqueous - in water - state, producing ions:
NaCL => Na+ + Cl-
The generalized equation for change of state of any salt is:
MX, solid => M+ + X-, aqueous
M+ = metal cation [cation = positively charged ion, i.e., an atom
stripped of, at least, 1 electron]
X- = nonmetal anion [negatively charged atom, i.e., at least 1 additional
electron]
Sodium & Chlorine are examples of 2, atomic, species that undergo an
ionic bond: an electron from chlorine is 'stolen' COMPLETELY by sodium,
forming a charged p[article, as opposed to co-valent bonds, where
electrons are shared. Separately, Na and Cl react violently and lethally,
but together, for 1 of the most benign substances we know. Notice that
this salt is composed of a, solid, metal - sodium - and a non-metal -
chlorine - which exists. naturally, in gaseous form.
NaCL electrolytic roots conduct or dissipate electric current. An
advantage of the electrolytic rod is no need for recharging/refilling,
thus, it is maintenance -free. Another advantage is the, dissolved,
ionizing salts LOWER the freezing point of any moisture [this phenomenon
can be effected with ANYTHING - known as a solute - dissolved in ANY
diluent - the thing which dilutes, forming a solution; it is called
'molar freezing-point depression'; molarity is a way of expressing
chemical amounts, in terms of atomic or molecular weight AND a way of
making a solution, viz., which goes in the beaker 1st, solute or diluent.
When you add ethylene glycol to your radiator, you are depressing the
freezing-point of your radiator's contents [rust & all. The stuff ALREADY
in the poor cooling system is the diluent, the anti-freeze is the solute,
and together - with rust, scum, crap.. - they form a solution. This is a
MOLAR solution, because you poured the solute INTO the diluent. If your
end-point is, say, 1 gallon of solution, you could alter the final amount
OF SOLUTION, by pouring solute into diluent or vice versa [don't worry
about all this, just BACK FLUSH the damn cooling system and you'll be
fine!]. With unfrozen moisture, you have a better EGS.
THe proper way to install electrolytic rods is to auger a hole, place the
rod into the hole, then back-fill with bentonite, a neutral pH, natural
clay [and a FABULOUS bowel cleanser. Nutritionists use it all the time].
Bentonite insulates the rods from corrosive soil.
Design begins with a site survey, including soil resistivity, at several
depths, plans for the site, topographic analysis and a boring [no, not
dull] core sample. The survey will ascertain if physical barriers, e.g.,
rocks, high-resistivity soil or even power-lines, will affect resistivity
of the earth-ground. Following are some equations and simple examples.
EGR for a single electrode - driven or electrolytic rod - is calculated
from:
R = p/2 * pi(l(2l/r)*(1+2K))
R = resistance, ohms
p = mean soil resistivity, ohm-cm
l = length of rod, cm [obviously not 1 of YOURS!]
r = radius of rod, cm [as above]
K = co-efficient of area length-to-width ratio at depths 0, .1*SQR
(area), .17 *SQR (area). K varies
from 0.9 to 1.4. Calculation of K is a little beyond this
monograph. Nomograms and
graphs, for finding K, are in myriad texts, etc.
The many geometric configurations also plague design, e.g., degree of any
bends, conductor length, etc. Formulae for differing configurations are
found in the IEEE Standard-142 'Green book'. Hand calculation is VERY
tedious but software makes it a pleasure [now go find, and pay, for the
software. I'm up for some pirating...]. Some software requires only soil
resistivity and ground system dimensions. A soil resistivity model is
computed. The designer then uses a 3-dimensional co-ordinate system to
program geometric arrangements, premised on site dimensions and
incorporating buried conductors, size, grids, et al. Matrices are used to
compute EGR. You fool with differing arrangements until reaching, or
approaching, your resistance design goal.
The typical personal telecommunications system - shack - requires an
earth-ground resistance less than 10 ohms. Electrolytic rods are more
reliable, stable in harsh environments, preferred in low earth-ground
resistance, improve over time [with migration of more electrolytic roots
into the soil]. Assume a 30 ft x 30 ft counterpoise, of conductor buried
2.5 feet deep [6" below the frost line]. Rods are placed on each corner
of the counterpoise and 1 rod between each base-station/transceiver and
the base of the tower to which that transceiver's coax runs. The radio
gear and legs of all towers, at the base, are connected to the rod
located between the shack and tower. This intentional placement provides
maximal lightning protection. This EGR should remain, longer than the
radio gear itself. Now we may use grounding fundamentals to finish the
job. Assume 10 ft electrolytic rods - at least 20 ft apart, i.e., 2L -
taking maximal advantage of the rods' SOI. Ground conductors are buried
below the frost line then connected to the rids in the counterpoise,
insuring system integrity. This grounding system is stable, reliable,
safe, maintenance-free, of protracted longevity, without recurring costs.
JAMES CLERK MAXWELL: ONCE MORE WITH GUSTO
Known as the finest physicist of the 19th century, Maxwell was the first
to combine 2 forces, viz., electric and magnetic. Few people have had
such impact on RF electronics. His work, on field relationships, are a
never-ending gift for understanding current flow and EM-fields. Maxwell
was proud, reclusive and brilliant.
Next time, we look at Maxwell, through the eyes of his biographer, and
close friend, Lewis Campbell [with help from William Garnett].
After many hours of Easter and Passover wining & dining, I suggest we all
take time to rest & digest.
73 de WF3W
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