Re: School demo

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



Tesla list wrote:
> 
> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Pyrotrons-at-aol-dot-com>
> 
> Hello....
> 
> (snips from Jim Lux)
> > Actually, I beg to differ, the important capacitance is between the top
> > load and the counterpoise, supporting the RF current flow through the
> > secondary inductance.
> 
> Your description sounds great to me, I might have been mistaken when I said
> the coupling is from counterpoise to earth. Given both views, maybe what's
> happening is a little of both? I mean, there definately IS capacitance
> between a counterpoise and ground........whether or not this is the
> "capacitor" doing the work or not, I don't know.

I think so... I'm doing some modelling right now, assuming flat plates,
etc. (of course, Terry could modify and crank up E-field "XP" to do a real
analysis)
> 
> 
> I do know that I was able to draw hot, bright sparks (typical of discharging
> any capacitance) from the counterpoise to a real live RF ground. That tells
> me there is a lot of voltage there, and that tells me this is not the best
> thing to do when running a coil at a school. I set this experiment up
> intentionally to prove that a dedicated RF ground is much better than a
> counterpoise.

> Granted, a bigger counterpoises were working better. At the time I was using
> a smallish (2.5 foot square) counterpoise for a 3" coil running about 500VA.
> MUCH larger counterpoises (more aluminum foil ; ) were making a good deal of
> difference................but, it's been my experience that you have to use a
> HUGE counterpoise to lower the voltage enough to *at least* not arc out and
> get you. All of the counterpoises I've tried have had corona all over the
> edges. Lots of voltage.


2.5 ft square is going to be way too small.  I'd suggest that you need the
radius of the counterpoise to be no less than the height of the top load
above it.


> 
> My point: I don't think any *practical* size of counterpoise will ever be
> safe. Of course, one stretched out over a mile will work excellent......but
> that's not practical ; ) I'm keeping in mind that running a coil for the
> public (especially at a school) requires a practical setup usually. That's
> why I'm not really "for" the counterpoise in this situation.

And here's where the Efield modeling comes in: The field from topload to
"ground" is pretty non uniform and drops off fairly fast as you move
radially out.  There is probably a fairly reasonable size that keeps the
voltage at the edge of the counterpoise low enough.

Something else to consider.. If there IS significant RF voltage at the edge
of the counterpoise, then sharp edges or points (chicken wire ends) would
aggravate the arcing.
> 
> Take care,
> 
> Justin Hays
> KC5PNP
> G-3 #1150
> Email: pyrotrons-at-aol-dot-com
> Website: www.hvguy-dot-com