Is this by chance a Radio Shack 8 ft ground rod?
I bought 2 of them. Not bad, but a few weeks later I found virtually
the same thing at the new Home Depot for less than 1/2 the price :-)
Anyway... I drove 2 of them in but in both cases, they had a 2-3 foot
head start (started from the bottom of a hole). I got up on my 8 ft
step ladder at the beginning and used an 8 lb sledge hammer.
It is kind of hard to get them started since after each wack they tend
to >>> boing / wobble <<< around after they are hit. Makes aiming the
next wack a bit difficult. I did it all by myself. I certainly would
be easier with a helper to hold the rod (then I can hit his hands when
I miss :-) )
One thing I have read, and I did once before (I expect to try this
for the 4 more that I have to drive in) is to get a short section of
threaded steel pipe with an ID just large enough to fit over the ground
rod, and put an end cap on the steel pipe then slide it over top of the
ground rod. It will give you a slightly larger target to hit.
Once you get the rod started you just have to keep at it. Take your
time. Try not to miss. You will feel it go in with each wack but you
will also feel it when it hits a rock (very rocky soil around here).
But I kept wacking away and it would start to go down again.. I got
both of them in the full 8 feet. My goal is to have 4 8 footers at the
base of my tower and at least 2 more 8 footers where the cables enter
I would do what you can to go the full 8 feet if you really want them
to do the job. If you really can't go 8 feet then (and you know this
in advance) the next best thing would be to go with twice as many 4
footers (or 6 foot). If necessary buy/borrow/rent/steal:-) a bigger
sledge hammer. I would say 8 pound is minimum. A 12 or 16 pounder
should drive a rod through almost anything short of thick concrete :)
One more thing. The radio shack ground rods had a reasonably nice point
on the ends. The ones at Home Depot did not. You should try to grind a
point on them if you can ESPECIALLY IF YOU HAVE ROCKY SOIL.
It goes without saying. Don't drive in any ground rods if there is
any chance of hitting anything below. Saftey glasses are not a bad
idea too :-)
Pete Rossi - WA3NNA
> I have considered jetting down a ground rod but I don't think
> the contact with the ground would be as good as a driven rod.
I have heard exactly this. In fact, the ARRL made mention of it in a
past issue, that in many types of soils, most of what is left after
using water pressure to make the hole, is stones. All of the con-
ductive earth is washed away, and the worse conductors are left in
contact with the ground pipe. Measured ground conductivity is worse
after using the water method. The other thing is that pipe tends to
clog, then freeze, and split open. I much prefer the 5/8" galvanized
ground rods that are commercially sold. Yes, you have to beat on them
for 10 minutes to getone in, but unlike pipe, they don't buckle under
the beating, and they last.
Remember that a lightning bolt has a surge current of around
8,000 amperes. You have to size and bond your conductors to
handle this load. The duration of the pulse is short, however,
only about 20 coulombs of charge is exchanged in the typical
strike, so the conductors don't have to be large enough to
handle the surge *continously*. Copper conductors equivalent
to #6 solid wire are sufficient.
Lightning is RF, though most of its energy falls below 2 MHz,
so the skin effect must be considered. That's why solid strap
is preferred over round wire. Strap has a larger surface area,
pound for pound, than round copper wires. Copper pipe can also
be used, but it's surface area will be half that of copper
strap with a width equal to the pipe circumfrence. That's
because the *inner* surface of the pipe forms a waveguide
beyond cutoff for the lightning RF currents, and isn't effective
at carrying away the surge. Strap, on the other hand, can fully
exploit *both* surfaces. (Pipe *does* have somewhat lower inductance,
so there is a tradeoff here.)
Woven braid conductors should be avoided for grounding runs because
braid has about 5 times the impedance of smooth solid strap on a
pound for pound of copper basis. There are a couple of reasons
for this. First, the braid strands weave in and out, adding inductance,
and second, because the skin effect tries to force currents to the
surface, while the individual strands keep diving into the middle
of the bundle, the currents try to flow from strand to strand along
the outside of the braid. Since the mechanical connections of one
strand to another are fairly loose, a high resistance path is formed.
Fully *tinned* braid is better, since the strands are now bonded
to each other better. However, solder *will* melt during a strike,
and you should plan to depend only on mechanically bonded connections,
IE clamps or cadwelding.
Since you are building your house, you have an opportunity now to
install a *Ufer ground*. Mr. Ufer developed this technique during
WWII for army ammo bunkers. The NEC approves its use for commercial
and home grounding systems. In essence, a Ufer ground is just rebar
in concrete. When the builder is preparing to pour your slab, make
sure all the rebar in the slab is bonded together, either cadwelded
or mechanically clamped, before the pour. And make sure to leave a
convienent attachment point exposed. A rule of thumb for a Ufer
ground is that it takes about 20 feet of 0.5 inch rebar to absorb
8,000 amperes of surge. More is better. The rebar should be embedded
in at least 4 inches of concrete.
The way a Ufer ground works is through two paths. First it forms
a large capacitance to Earth. This is an excellent RF coupling.
Second, concrete's ions generally are more conductive than native
soils, so you have a large number of virtual resistors in parallel
connected to Earth that offer a lower resistance than would a
smaller collection of driven rods. Earth is actually a lousy
conductor. Most currents are dissipated through Earth by capacitive
coupling and arcing from soil grain to soil grain. Concrete is a
better conductor since the grains are tightly bound together.
Quoting Richard Quick to Mark Conway:
This was some excellent information. Thanks Mark for posting this
up. More than a few people here are either building coil systems,
or upgrading to higher power levels. I have always said that
Tesla coils literally have to be hand built from the ground up.
MC> Over on the radio amateur echo somebody was saying that this
MC> method <soaking the soil> is not the best for putting in
MC> ground rods as the soil does not make good contact with the
MC> rod as the water washes the soil away from the rod.
Just a heavy soaking is not going to hurt. The practice that I
think was specifically being advised against was pressurizing
a pipe with water and then using the water flow to assist in
getting the pipe into the soil.
Bruce N9EHA said:
> the ARRL made mention..., that in many types of soils, most of
> what is left after using water pressure to make the hole, is
> stones. All of the conductive earth is washed away.
But, a good soaking brings particulate dirt and clay in close
contact to the conductor.
Then quoting Gary Coffman KE4ZV
> Lightning is RF, though most of its energy falls below 2 MHz,
> so the skin effect must be considered. That's why solid strap
> is preferred over round wire. Strap has a larger surface area,
> pound for pound, than round copper wires. Copper pipe can also
> be used, but it's surface area will be half that of copper
> strap with a width equal to the pipe circumfrence.
This is why I used 15 foot long by 3 inch wide copper strap for
the center of my new ground. Again quoting Gary Coffman:
> Woven braid conductors should be avoided for grounding runs
> because braid has about 5 times the impedance of smooth solid
> strap on a pound for pound of copper basis. There are a couple
> of reasons for this. First, the braid strands weave in and
> out, adding inductance, and second, because the skin effect
> tries to force currents to the surface, while the individual
> strands keep diving into the middle of the bundle, the
> currents try to flow from strand to strand along the outside
> of the braid. Since the mechanical connections of one strand
> to another are fairly loose, a high resistance path is formed.
This is a pretty good argument for using wide smooth strap in the
ground path. I have known for some time that strap performs
significantly better than round wire, and have said that the
widest possible strap is better than the skinny stuff (this is
easily experimentally verified).
One other thing that I thought someone might mention, but
considering the different applications that grounding is used for
perhaps not; the use of salt in RF grounding applications. Rock
salt or water softener salt can be buried around the ground
conductor. A depression in the surface of the ground is left and
the area is given a good soaking before firing. A section of PVC
pipe could be partially buried around the ground conductor,
filled with salt, and then soaked before firing. The other way is
to perforate the end of a grounding pipe before planting it and
then rig up a simple gravity pump with a saturated saline
Salt plumes are fairly inexpensive and easy to build up in the
subsoil. They are non-toxic, with the exception perhaps of a tree
root passing directly through it. The presence of an established
salt plume will really increase the local conductivity. Watering
the ground before firing makes an excellent connection between
the ground conductor and the salt plume.
Richard Quick on the salt pipe RF ground.
This is one of the easiest, cheapest, and most effective RF grounding
techniques for Tesla coilers. This ground is not cost effective for use
24-7, but when used occasionally (daily for a few hours) for RF ground-
ing tank circuits and secondary coils you will get a lot of "bang for
the buck". This ground also improves over time.
It is best to select a low spot or natural drainage area that is as
close as possible to the base of the Tesla coil. Try to get the inital
placement within 15 - 20 feet of the Tesla lab. You will need the
TWO, 8 FOOT COPPER CLAD STEEL GROUND RODS, (good)
TWO, 8 FOOT LONG BY 3/4 INCH DIAM. HARD COPPER WATER PIPES (better)
ONE, 8 FOOT LONG BY TWO INCH DIAMETER GALVINIZED STEEL PIPE (best)
TWO HUNDRED POUNDS OF ROCK SALT OR WATER SOFTENER SALT
ONE HUNDRED POUNDS OF COARSE SAND AND A FEW BUCKETS OF GRAVEL
A FOUR FOOT LENGTH OF VERY LARGE DIAMETER PIPE (10 INCHES DIAM. MIN)
ONE ROLL OF FOUR INCH or SIX INCH WIDE ALUMINUM STRAP (gutter flashing)
ELECTRICAL OXIDE INHIBITOR
HOSE OR OTHER CLAMPS FOR THE ROD OR TUBE CONDUCTORS
DIGGING AND TRENCHING TOOLS
Start by digging a hole at least four feet deep that will accept the
large diameter pipe. It is advisable that a two foot or three foot
diameter pipe be used. PVC, corrugated culvert, concrete, iron,
really any type of pipe may be used depending on what is available.
Once the hole is dug, case or line the hole with the pipe, then fill
with water. Do not allow too much of the casing pipe to remain above
ground level, a few inches is OK. Next, work (or drive) the rod (or
tube) conductor(s) into the earth at the bottom of the cased and water
filled hole. It is important that as much conductive rod or tube as
possible be in contact with the earth. Place the vertical conductors
near the edge of the cased hole. If two vertical conductors are used
then place them on opposite sides of the hole.
Once the vertical conductors are in place, trench a path back to the
Tesla work area. Make the trench wide enough to accept the four inch
or six inch smooth alumium strap (gutter flashing). Make sure the
trench is below the sod level.
Smear the top(s) of the vertical conductors with a light coat of
electrical oxide inhibitor. This compound is used to prevent
corrosion whenever electrical connections between disimilar metals
are made. Once smeared with inhibitor, wrap one end of the aluminum
strap around the vertical conductor and clamp into place with a hose
(or other) clamp. Verify the connection with a VOM. If two verticals
are used then use two separate lengths of aluminum strap. Place the
strap into the prepared trench back to the Tesla work area and back-
fill the area in.
Next pour a few inches of gravel into the bottom of the cased ground
hole. Begin to backfill the cased hole by alternating shovels of salt
with half shovels of sand and gravel. Continue until the hole is filled.
The ground is left dry when not in use. Before firing coils, fill the
ground hole with water. The water will dissolve the salt which then
migrates downward to form a conductive "plume" in the subsoil. The salt
will require periodic replenishment. Over time, and with continued use,
the salt plume will make contact with bedrock or the water table.
The vertical conductors will corrode fairly quickly in the presence of
the moist salt. After a few years new vertical conductors should be
driven in and connected alongside the old ones. The alumium strap that
is used to make the run between the lab and the ground pipe will also
corrode and will need to be renewed after a few years.
Improvements to this ground may be made by substituting copper strap
for aluminum strap in the construction. Copper will last longer and
will make a better connection to the verticals. As mentioned above
the best connection is braised. Another improvement can be made by
clamping a conductive mesh screening to the vertical conductor in
the cased ground hole. This will allow a greater surface area of
contact with the salt water. The best mesh is either copper, steel
or stainless steel harware cloth. Do not use aluminum screening.
The purpose of the sand and gravel is to prevent caking of the salt
which will prevent water from passing through the ground hole. When
the salt is replenished it is a good idea to remove some of the sand
amd gravel or otherwise mix the sand and salt together. Dumping in
a large quantity of salt all at once without some inert filler will
cause a salt block to form that is difficult to break up or dissolve.
The theory of operation is pretty simple. The casing pipe prevents
the salt from migrating sideways in the surface soil where grass,
bushes, and tree roots would be poisoned. The salt dissolves and
travels downwards into the moist subsoil, while at the same time
spreading out laterally. When the "plume" contacts the water table
a connection is made. If the soil is very dry and shallow the plume
will desend to the bedrock and will then expand laterally which forms
an underground plane. In either case the ground that is formed is
high quality, and is ideal for even very high powered Tesla coiling
From: Richard Quick
Subject: Tesla's RF Ground
In re-reading the first fifty pages of The Colorado Springs Notes
(CSN), I have retraced again the construction and measurement of
the grounds that Tesla used there. In discussing this material
will quote Tesla's actual notes whenever possible.
On the first day that Tesla began coiling inside the building
at Colorado Springs (June 15, 1899), he had two metal to earth
connections available, the water pipe and the lightning arrestor
ground. Tesla notes:
"Sparks went over the lightning arrestors instead of going to
ground. This made it necessary to change the connection to the
ground, separating that of the secondary of the oscillator from
the ground of the arrestors. By connecting the secondary to a
water pipe, and leaving the ground of the arrestor as before, the
sparks ceased. This indicates a bad ground on the arrestors. THE
LATTER WORKED EXCEEDINGLY WELL. The ground connection was made by
driving in a gas pipe about 12 feet deep and gammoning coke
around it. This is the usual way as here practised."
First note that arcing was occuring from the earth connection
over the arrestors. This shows that the lightning arrestor
ground, an iron gas pipe driven twelve feet into the earth, was
insufficient for even a low power test. Tesla clearly recognizes
the differences between the two grounds he has available. The
capitalized sentence above was in italics in the original. His
setup is brand new. His tune is rough.
The following day, June 16, 1899, he had workmen going full tilt
on a dedicated RF ground for his experiments.
"A new ground connection was made by digging a hole 12 feet
deep and placing a plate of copper 20" x 20" on the bottom and
spreading coke over it again, as customary. Water was kept
constantly flowing upon the ground to moisten it and improve the
connection but in spite of this the connection was still bad and
to a remarkable degree. It is plain that the rocky formation and
dryness is responsible and I think that the many cases of damage
done by lightning here are partially to be attributed to poor
earth connections. By keeping the water constantly running the
resistance was finally reduced to 14 ohms between the earth plate
and the water main."
Tesla clearly notes the indivuality of the water pipe and the
earth plate, just as earlier he noted the difference between the
arrestor ground and the water pipe. I will call the earth plate
the "dedicated RF system ground" or simply "system ground".
In text following the quote above, Tesla next refers to using a
"sensitive device" to determine the presence of a ground current
around the lab. I have used resonate pickup coils with a small
neon indicator bulb on the air terminal, or even a low pressure
gas tube to detect ground currents.
Apparently the water was turned off that night, for on the
following day, June 17, 1899:
"Measurements of resistance between ground wire and water main
showed the surprising fact that it was 2960 ohms, and even after
half an hour watering it still was 2400 ohms, but then by
continued watering it began to fall rapidly. Evidently the soil
lets the water run through easily and being extremely dry as a
rule it is very difficult to make a good connection. This may
prove troublesome. The water will have to be kept flowing con-
tinuously. The high resistance explains the difficulty, from a
few days before, of getting the proper vibration of the second-
ary. The first good ground was evidently at the point where the
water main feeding the laboratory connected to the big main
underground and this was several hundreds of feet away. This
introduced additional length in the secondary wire which became
thus too long for the quarter of the wave as calculated. The
nearest connection to earth was as measured about 260 feet away
and even this one was doubtful."
OK, Tesla has said a mouthful. First his measure of resistance
when the water is off overnight skyrocketed. Though the water
expense was unbudgeted, it ran 24-7. (The bill was finally paid
when the wood used in construction was sold after the building
was dismanteled.). This shows that Tesla was determined that no
expense was to be spared in obtaining the lowest resistance
ground connection possible.
Tesla then notes that the first good ground occurs on the water
main at the junction to the laboratory connects. He notes at the
end of the quote that even that ground point is doubtful,
possibly because his equipment is powerful enough to push the
center of the "true electrical ground point" further up the main.
Tesla also notes that the ground path leading to the true
electrical ground point must be considered as a parasitic
conductor length in all secondary calculations. This distance
between the base of the secondary coil and the "true ground"
affected his ability to determine the resonate frequency of the
grounded coil and kept him from establishing a sharp tune in the
system. Add the fact that the location of the true electrical
ground point on the pipe may not be stable, and would possibly
move farther away with increasing power levels, meant that the
water pipe would be completely unsatisfactory for the system RF
To jump ahead to page 125, the section notes indicate that
stationary standing waves were observed on the water pipe, and
the exact electrical distance from the ground plate to the
electrical node on the pipe was determined to be 550 feet. This
would be an unpleasant amount of uncoupled conductor to add to
any secondary coil.
In conclusion, Tesla recognized the need for a dedicated RF
ground in his coil systems. His specifications were such that the
true electrical ground point for the system ground had to lay as
close as possible to the base of the secondary coil. He
recognized the need for a highly conductive pathway adapted for
low frequency high-voltage RF.
> THE COLORADO SPRINGS NOTES, 1899-1900
By Nikola Tesla... Hardcover, 440pp, Published by NOLIT, Beograd,
Yugoslavia, 1978. Prefaced and annotated by Aleksandar Marincic,
Assoc. Prof. of EE Beograd Univ. and advisor to the Nikola Tesla
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12