FW: [TowerTalk] 2 4' vs 1 8' ground rod

[Bob Wanderer]

(On Behalf Of Dana Roode)

All things being equal, theoretically
the same amount of grounding contact
would be achieved by double the number
of short (halved) ground rods as
compared to longer ones.  It is more
likely however that more (perhaps much
more) than double the number of halved
rods will be necessary to achieve
similar ground impedance/resistance (in
Ohms) to that achievable with full-sized
rods for the following reason.

The potential (and usually very real)
problem is that earth stratum typically
varies with depth.  Usually the upper
layers (strata) are the newest and have
the least compaction, density, etc.
This can mean that less than ideal
connection (bonding) to the earth occurs
nearest the surface.  The reason ground
rods are usually => 8 feet is for the
purpose of gaining access to the denser,
more compacted earth reached with them
(there are 20-foot rods in common use).

One theory of grounding envisions that
surrounding each vertical ground rod, a
"sphere of influence" (actually a
cylinder) exists with a diameter equal
to the rod's length.  This theory also
calls for rods to be spaced their length
(8-foot rods, 8-feet separation) in
order that their cylinders of influence
don't overlap, gaining the most useable
earth contact, per rod, for the purpose
of dissipating lightning strike energy.
A good RF ground is not necessarily a
good lightning ground, but a good
lightning ground is always a good RF
ground.

One method of achieving greater depths
in hard ground is though the employment
of hydraulic boring.  With the ready
availability of high-pressure washer
machines, hydraulic boring is quite
possible, even for the amateur.  Using a
piece of 1/2" pipe as an extension
connected to the nozzle, most compacted
earth will succumb to this boring
technique.  Of course a rock strata
probably will not.  The pressurized
water, depending upon volume, will
usually lift the debris out of the hole,
allowing insertion of a ground rod
subsequently.  The hole will be larger
than the rod, but back filling it with
the muddy debris and/or other soil
enhancements (manganese sulfate-Epson
Salts, Bentonite, etc.) with plenty of
water to make a good slurry can result
in a very good contact with the earth
after it has "dried".

Auguring holes quite a bit larger than
the ground rods' diameters (say, 6-8
inches) and then backfilling and tamping
for compaction around the rod, is
another method that is in common use.
Soil enhancements such as above are
typically employed for backfill as is
"conductive" (carbonized) concrete, a
relatively recent technique.

Finally, low-impedance interconnection
between multiple ground rods and to the
tower base is imperative to realize the
entire contribution of earth grounding
afforded by a ground rod system.  Using
=>1.5" copper strap, (bare) MCM-1000
copper cable, or =>3/4" copper tubing
for the interconnections is not
overkill!  Moisture-free, high
conductivity connections to the rods
(and tower) are mandatory!  Ideally,
exothermic welding ('Cad Weld',
'ThermoWeld') is recommended, however
there are clamping systems employing a
moisture-repelling, high conductivity
paste (such as 'Jet Lube') that will
suffice.

For more information on the subject,
your attention is directed to PolyPhaser
Corporation's web page
(www.polyphaser.com) and other similar
references.

73,

John Campbell (ex-WA6RCY)

PS As I (Bob Wanderer AA0CY) remember the "sphere of
influence" theory, the spacing was 2x the rod length (8' per
rod), so 8' rods would be spaced 16' apart.  In a PolyPhaser
modification of this theory, the distance apart should be
twice the distance the rod is in good conductive soil.  So
if 4' of the 8' rods were in such soil, the spacing would be
8'.  The problem, of course, is that it is very difficult to
ascertain this information.  For that reason, I recommended
that the spacing be 1.5 x rod length, so 8-footers would be
spaced 12' apart.  Nevertheless, you probably would not have
any issues if you used 1x.  FYI, both John and I spent
several years at PolyPhaser as Technical Consultants.


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