Hi jim,
Okay, so my setup is on my window ledge, which isn't very big, but big
enough. It's not big enough to throw a 6x6 foot piece of chicken wire
though. Does the counterpoise have to actually be under the coil, or was
that just for demonstrational purposes? If I could put it in the front
yard and run a wire through my window, that would be best.
Thanks a lot!
Brandon
On Feb 6, 2010, at 12:59 PM, jimlux <jimlux@xxxxxxxxxxxxx> wrote:
Brandon Hendershot wrote:
Hi Jim,
Could you explain the concept of "counterpoise" for me or provide a
link to some documentation? I've never heard of anything like it...
Thanks btw,
Brandon
counter-poise, literally, "against" "weight" or "balance"
It's an artificial ground plane for your coil.
in this context.. think of your topload as one plate of a capacitor.
what's the other plate? We talk about "free space", but really, it's
the surface of the floor (ground) under the tesla coil, since it's
closest.
The secondary is a series LC circuit.. the secondary coil being the L
and the "top load and ground" being the C. Since it's a series circuit,
all the current that flows in the L has to also flow in the C. So,
imagine current flowing out of the top of the secondary winding into
topload, through space, into the ground, flowing to wherever the wire
from the bottom of the secondary is, and back to the coil. (in
practice, there's other capacitances involved, e.g. from one segment of
the winding to another segment of the secondary winding..aka "self C";
but that's by the by)
Our goal is to make sure that the RF current in that big capacitor
(topload to ground) goes where we want it to (as easily back to the
secondary L) and NOT to somewhere we don't want it to (consumer
electronics in your house).
Now, with a BIG coil (or a transmitting antenna), the thing sits on the
"ground" and you work on making the connection to "ground" as good as
possible.. rods, radial wires, salt water, etc.
But for a smaller coil (say, up to 6 feet tall), you can make a
"artificial ground"... say you made a big aluminum pizza pan 30 feet in
diameter around your coil. Aluminum is a good conductor, so the RF
current will flow in the pizza pan, rather than somewhere else. Turns
out that chicken wire, or similar, works just about as well.
And, as for how big it should be: if you plot out the electric field
distribution, you'll see that most all of the field is within a circle
with radius equal to the height of the coil above the ground
plane/counter poise.
Now.. for safety's sake, you don't want the voltage on that counter
poise to be significantly different than "real ground" on which you
might be standing, or to wiring in your primary side. So, you could
connect the counterpoise to the "greenwire" ground (which is bonded to
"earth ground" or to a "ground rod". Note well that this "safety
ground" connection is NOT for RF. If the counter poise is laying on the
real ground, the capacitance to the actual earth is huge, so in RF
terms, it's already connected.. but you want a "DC" or "AC line
frequency" connection for safety. You could put a big RF choke in
series with the connection and it would exactly the same (and prevent
any stray RF from getting into your electrical system.
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