[TCML] Tesla Coil Ground Systems

[<sparks@xxxxxxxxxxxxxx>]

I usually just read posts on the list here, but having had many years
experience in the AM broadcast industry, and as a long-time Amateur
radio operator, I may be able to add something to the topic at hand.

-----

>From an RF engineering perspective, a Tesla coil is simply a radio
transmitter connected to a very short and inefficient, grounded-base
vertical antenna.  We want the antenna to be inefficient, since we don't
actually want the energy to be radiated.  Instead, we want it all to
"pile up" on the top load and exit as long, bright and beautiful sparks.
Nevertheless, any antenna, no matter how low the efficiency, will
radiate some portion of the applied RF, and that is what the ground
system and/or counterpoise has to address.

The RF energy that is radiated from the secondary coil (acting as a
short vertical antenna) "wants" to get back to the source of the
energy - the secondary coil.  The proper return connection is the bottom
end of the secondary coil winding.  Normally this RF energy is
intercepted by the earth and flows back in the direction of the coil.
Since the earth has resistance, the voltage drop caused by the return
currents flowing through the earth resistance causes voltages to appear
on anything that is connected to the earth and within the radiated field
of the Tesla coil.  That is why we must lower the effective earth
resistance and connect the base of the secondary winding to ground as
well as we can.

Is the coil is operated out of doors, well away from any buildings or
other things which may interfere with the radiated RF field, the return
currents will appear in the ground in a large circle which is centered
on the Tesla coil.  This circle theoretically spreads out until the
entire earth is covered.  In practice, the majority of the return RF
current is intercepted by the earth within a circle having a radius of
about 3 to 4X the height of the top of the secondary coil above the
surface of the earth.

Note that this is for a perfectly conducting earth, which most of us do
not have, unless we have floated our Tesla coil in a metal boat on the
ocean.  For the rest of us landlubbers, the radius of the circle will
increase in size as the ground resistance becomes higher.  Therefore we
must make use of various strategies to "grab a firm hold" on the earth
to return the RF current to the base of the coil.

Driving one or more ground rods deep into the damp ground is one way,
laying a ground screen or multiple ground radials is another.  Ground
rods should be long enough to get well into the conducting region on the
local soil.  This may require a rod of 6 to 8 feet in length.  Note that
the NEC calls for 8 foot ground rods for standard electrical service.
This is because in most cases, it does require a rod of that length to
get a good ground connection.  In cases of high ground resistance, the
NEC requires more than one rod, in fact, it requires using as many
ground rods as needed to get the requisite low ground resistance.

If multiple ground rods are used, they should be spaced apart by at a
distance that is at least twice the length of the ground rod itself.  In
other words, using 8 foot ground rods, they should be spaced apart by at
least 16 feet.  This is because a ground rod has a circle of influence
in the soil with a radius equal to the length of the rod, i.e., an 8
foot rod influences a cylinder of soil with a radius of 16 feet.
Therefore two 8 foot long ground rods spaced 16 feet apart will just
have their cylinders of influence touching each other.

The wire connecting the ground rod or rods to the base of the coil
should be of large surface area.  Copper tube is as effective as solid
copper wire due to the skin effect at RF frequencies.  Flat metal strap
can also be used if desired, it is even better than round wire as it has
more surface area than the equivalent weight of copper used to make
round wire. Copper or aluminum will work very well, so if you have
access to some aluminum wire or strap, use whatever you can get.  You
can even use steel electric fence wire, but since the resistance is
higher than copper or aluminum, you will need to use a larger gauge wire
or use more of it to achieve the same low resistance.

The wire should be run as straight as possible, directly from the ground
rod to the base of the coil.  Avoid any sharp or right angle bends as
they greatly increase the inductance of the wire and hence the RF
resistance of the wire, making the ground system less effective.  Each
ground rod should have its own wire connecting it back to the base of
the coil.  A sweeping bend is OK.

Note that ground rods have two different functions.  They are used to
provide a return for the radiated RF energy back to the Tesla coil
(probably the most important function) and they also serve to ground the
system to ensure operator and equipment protection with regard to the AC
mains system by preventing RF energy from getting back into the mains.

A ground screen consisting of metal sheets, plated, rolled-out aluminum
foil, chicken wire, or anything conductive may also be used.  Since the
return RF field is strongest in the area close to the base of the coil,
you should take the approach the AM broadcasters do when constructing a
big AM broadcast antenna, and "copper plate the earth" under the base of
the antenna (Tesla coil) with as much metal as possible.  Make sure that
it is all connected together and connected to the bottom of the
secondary winding!

Ground radials are also effective at returning the RF current to the
base of the coil.  If you are short on wire for your ground radials,
remember that it is better to have many short radials than just a few
long ones.  They need not be longer than about 3-4X the height of the
secondary is above the earth.  After you pass about 16 to 24 radials,
you pretty much have reached the point of diminishing return.  Doubling
the radials from 16 to 32 will be less effective that going from 6 to 16
radials.  Note that you can also use a mix of long and short radials
together.

Ground radials can be buried or laying on top of the ground.  RF-wise,
they will work better if they are not buried, but the difference is, for
the most part, slight, for the radial length likely to be used by the
average coiler.

Note that is if OK to use combinations of these - you can use ground
rods, ground screens and radials in any combination.  The more, the
better!

A counterpoise is a special case of a ground radial system.
Counterpoises are used in AM broadcasting where it is difficult,
expensive, or otherwise impossible to install a buried radial system.
Broadcasters bury their AM radial systems to prevent damage from mowing
machines and as theft prevention from copper thieves.  A temporary
radial system for a Tesla coil will most likely be laid on top of the
ground for the duration of the session.

A true counterpoise, to operate properly, must be elevated slightly and
completely insulated from the earth at all points.  It must have enough
wires in it to properly screen the earth under the counterpoise from the
radiated RF energy.  This prevents RF losses in the earth and allows
effective and low-loss return of the RF current to the base of the
antenna or Tesla Coil.  If a counterpoise connects to the earth at any
point other than at the center, it will have losses due to the RF
current being partially short-circuited through the earth and bypassing
the counterpoise.  When this happens, the counterpoise becomes a form of
a ground radial system.

Tesla coilers are unlikely to ever encounter a true counterpoise system.
For most of us, the closest thing will be to construct a ground radial
system where insulated wires are laid on top of the earth.

If a counterpoise or insulated radial wires are used, it is vitally
necessary to use some sort of ground connection to the earth for power
mains safety for the operator and equipment.  An elevated counterpoise
outside may have one or more ground rids sunk in the earth at the center
of the counterpoise.  The rod(s) are then connected to the center of the
counterpoise.  This provides a DC/power mains ground for the system and
allows the counterpoise to take care of the RF return to the
antenna/Tesla coil.

In the case of running a Tesla coil indoors or upstairs, I would suggest
installing as much of a ground screen as possible, particularly under
the base of the coil and extending out as far as possible.  This will
likely be limited by the dimensions of the room.  Connect a ground wire
to the center of the counterpoise and the Tesla coil, and run the wire
UNDER the counterpoise away from the coil.  Connect the wire to an earth
ground, preferably NOT via the ground pin of the mains outlet.  Since
this wire is simply an earth safety connection, the gauge of the wire is
not critical as if the ground screen is effective, the safety wire will
have little RF current flowing through it.

73,

Ralph   W5JGV - WD2XSH/7

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