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We have a 2kW TC, and we have used a screwdriver driven into the ground
with water poured around, and it worked fine in dry desert soil.
A counterpoise 3 feet square about 1/2 inch above a concrete floor would
have a capacitive reactance of about 1k ohms. Not too bad, but the
closer to the concrete and the larger the area, the better.
---Carl
On 4/9/2015 4:29 PM, Jim Lux wrote:
On 4/9/15 10:36 AM, Billy Watson wrote:
As understood in the N.E.C. concrete is considered as ground.
An office on slab w/ground water issues, would be considered as an
ideal
ground.
The NEC grounding/bonding requirements are all about line frequency,
which is a bit different than what we want for a tesla coil.
The NEC is looking at handling kiloamps from things like a power line
short or a lightning impulse. You'll never have that kind of energy
in a tesla coil (or, if you do, you'll have plenty of experience by
then).
Ufer grounds are virtually required in all new construction. No using
water pipes any more.
However, for a TC, what you're really looking for is a lowish
impedance return path for the RF current from the topload and sparks.
The important thing is that the currents be contained *near* the
coil. If your ground connection is a 30 foot wire to some building
ground point, then the current has to flow down that wire, radiating
fields where you don't want them.
If you lay the counterpoise on the concrete floor, then any currents
that aren't coupled directly to the counterpoise and flow through the
concrete will be capacitively coupled to the counterpoise
(counterpoise is one capacitor, the concrete is the other).