Re: Counterpoise?

Tesla List wrote:
> Original Poster: Esondrmn-at-aol-dot-com
> In a message dated 12/4/98 1:26:36 AM Pacific Standard Time,
> writes:
> <<
>  Thanks to everyone that helped with my previous really long post. :)
>  I ran into another question....
>  I have been reading the archives and there was mention of using a
>  counterpoise instead of an earth ground for a coil.
>  This is of interest to me since I am currently building this for use in
>  schools and such where I may not have a good RF ground set up. I will
>  build one for my home, but I may not be able to guarante a good one
>  elsewhere. The first run away from home is scheduled in late January and
>  it gets real cold arround here that time of year. Any sugestions? I have
>  asked to be located near an exit from the building to I can attempt to get
>  a ground.. I may be able to get access to a water pipe as well.
>  My first thought is to use some more flashing or screening laid out in the
>  snow with maybe some rock salt thrown on it to help conduct (melt the
>  snow a bit). Or see if I can get a decent ground from a water pipe.
>  Any help appreacted, driving ground rods into frozen ground doesn't sound
>  like fun to me.
>  Travis
>   >>
> Travis,
> A counterpoise is a good alternative to an earth ground for a small coil.
> did not say how much power you are running.  If the supply is 12 or 15 kv at
> 30 or 60 ma, I would think a counterpoise would work well, especially for a
> portable, indoor type coil.  A couple of chunks of mesh metal screen or
> fencing maybe 5 ' x 5 ' each should work well.  Just lay them on the
floor off
> to one side or directly under the coil, connected together and to the
base of
> the secondary.  Someone should be able to calculate just how many square
> of conductor is required for this based on the power supply size, but I
> not know how to do it.  Sounds like a good math project for someone.
> Ed Sonderman

	A counterpoise usually consists of a radial network of wires starting
from the base of a vertical antenna, and extending out to a radius of at
least the antenna height.  The purpose is to reduce the losses due to
current flowing in the ground.  Where the ground resistance is high
enough it is common practice to mount these wires on short poles above
the ground.  In the case of a Tesla coil the ground does two things.  It
certainly provides a return for the current which flows through the
coil, and for that purpose a wire  screen of radius equal to the height
of the coil should do a commendable job.  (I guess really it would be
better to extend it beyone the range of the longest streamers.)  The
second purpose of the ground is safety - to keep any part of the coil
from having high power line voltage on it if for any reason the power
transformer inadvertently  becomes connected where it shouldn't.  A
counterpoise won't take care of that at all, so some sort of fairly low
resistance ground is needed to make sure the operator and his friends
stay alive in case something fails.