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>



> 
> For a school demo, I would use nothing other than a "dedicated" RF ground,
> one that is totally separate from everything else.
> 
> Using nothing more than a 4' x 4' square of chicken wire for a ground
> (sometimes called a "counterpoise") is going to be dangerous sir.
> 
> How it works, is that it capacitively couples the bottom of the secondary
> coil to ground. (the square of chicken wire is one "plate" of the capacitor,
> the insulator in between is the air, the other "plate" is the earth)


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.  Putting the counterpoise there reduces the coupling
to the earth ground.  without a counterpoise, the "other plate" for the top
load capacitance is the ground, the walls, anything conductive that happens
to be near the TC (like you), and the RF current flows through it
(generally a "bad thing", when the path happens to be your feet, or the AC
green wire ground, the garage door opener wires, etc.)

> 
> This is not the best at all, but does work. Unfortunately, there is a
> significant voltage at the bottom of the coil. A very significant voltage.
> Set up a coil using a large counterpoise (the bigger the better), and you can
> draw bright hot sparks off of this "ground" with a real grounded wire.

If you can do this, then the counterpoise isn't big enough, because it
means that some of the RF energy from the top load is coupling to things
other than the counterpoise.  It has to be significantly bigger than the
top load.  It SHOULD be connected to safety ground also, but that
connection shouldn't carry any significant current (it is a safety ground).


> 
> If you were to get close to this (VERY easy to do.......its four foot
> square!) you'd get a very hard zap. You would discharge the capacitance of
> the counterpoise through your body. Use nothing more than a solid
> earth-ground while you're operating for the public. In your workshop (with a
> friend, not by yourself) using a counterpoise is perfectly acceptable.
> 
> Of course, if you already have an earth ground, there is absolutely no reason
> to use a counterpoise.
> 
> (btw I have run small coils grounded to the AC ground (building ground) at
> hamfest's before. This has never hurt anything, but it is definately not a
> good idea)
> 
> Later,
> 
> Justin Hays
> KC5PNP
> G-3 #1150
> Email: pyrotrons-at-aol-dot-com
> Website: www.hvguy-dot-com