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Counterpoises

Subject:  Counterpoises
  Date:   Mon, 12 May 1997 13:39:50 -0400
  From:   "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:   "Tesla List" <tesla-at-pupman-dot-com>



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-poodle.pupman-dot-com
> Subject: RE: Shorted turns, primary proximity - Add a sheild???
> Date: Sunday, May 11, 1997 1:15 PM
> 
> Subject:  RE: Shorted turns, primary proximity - Add a sheild???
>   Date:   11 May 97 08:53:34 EDT
>   From:   Alan Sharp <100624.504-at-CompuServe.COM>
>     To:   "INTERNET:tesla-at-pupman-dot-com" <tesla-at-pupman-dot-com>
> 
SNIP
> 
> Duane Byland did experiments with copper sheets - very small
benefit,
> and
> using a cointerpoise made from hundreds of copper wires - earthed
in the
> centre
> and laid out as a big star. It gave some benifit and significantly
> reduced the 
> ground currents.
> 
> I would have expected a large surface area to be required to act as
a
> capacitor
> plate against the toriod - but wire seems to work just as well
> presumablely
> because it is "a reflected wave thing". 
> 
SNIP
> 
> Alan Sharp (UK)

Alan,
One reason the counterpoise made from many small wires worked so well
is that it *IS* a much larger total surface area than a sheet of
metal. In fact, the smaller the wire diameter used, the greater the
total surface area for a given mass of copper. The thickness of the
wire should be a couple of skin depths at the frequency of interest. 

The key to making such a "ground" effective is to keep the wires as
far apart as possible. That is why the "radiating" pattern works so
well. Taken to its extreme,  it would be a spherical radiating
pattern and would look for all the word like a porcupine or hedghog
all rolled up, with spikes radiating outward in all directions. The
center of such a thing would be the area of greatest ineffeciency.
That is because whenever two wires actually touch, you have reduced
the available surface area. Also, the wires will tend to shield one
another when they are close. What is the most efficient "spacing" for
such wires? Depends on the frequency. As frequency increases it would
become more efficient.

As has been mentioned by others, we must avoid an either-or
mentality. A counterpoise can often be used together WITH a regular
good RF ground system. The local counterpoise helps to "stiffen" the
ground SYSTEM, which is the TOTALITY of the entire base system. Where
the ground run is somewhat long, the local counterpoise will act in
concert with the larger RF ground system to make the PERCEIVED ground
at the base of the secondary a much better ground.

Those of you with sheet metal available might want to check and see
if YOUR particular system will benefit by adding a local counterpoise
to your RF ground system. You might be pleasantly surprised.

Fr. Tom McGahee