Re: Double Throw Spark Gap (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 15 Oct 2007 11:48:46 -0700
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Double Throw Spark Gap (fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Sun, 14 Oct 2007 17:28:25 -0500
> From: Crispy <crispy@xxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: Double Throw Spark Gap (fwd)
> 
> Thanks for the advice on the counterpoise - I'll try that.  I'm
> wondering why the problems of feeding RF into the building's ground
> disappear when a counterpoise is used ... or is this true at all?

The idea is to give the RF currents from the topload a low impedance way 
to get back to the bottom of the secondary.  They could either 
capacitively couple to the environment, including any appliance or AC 
power wiring, and then, by some circuitous route, make their way to the 
green wire you used to ground your TC, perhaps through the parasitic 
capacitance of the neon transformer's windings to case.

OR, they can capacitively couple to a nice flat low inductance mesh 
under the coil, and return by a short piece of wire to the bottom of the 
coil.

Basically, you have a circuit where you have a HV source with RF on it, 
that is capacitively coupled to lots of things that eventually wind up 
back a the coil.  What you want to do is make sure that the "desired" 
path is a LOT lower impedance than all the other paths, so the current 
flows in the desired path.

The other aspect is the magnetic field from the sparks. This is what 
couples to victim devices in the vicinity, even if they're not connected 
to anything else.  Essentially what you have is a loosely coupled 
transformer, with one turn formed by coil:topload:spark:grounded target: 
coil, and the other turn formed by some loop in your victim device.  If 
you contain the current flow in the sparks to flowing just in the area 
of the coil, that tends to reduce the field that is farther away.  The 
field drops off quite quickly (1/r^2 to 1/r^3, depending on geometry)



   I may
> also try to completely bury a real ground and have some mechanism where
> can attach and detach the grounding wire to this underground
> construction even where the construction itself isn't visible.

Don't even bother.  The grounding circuit for an indoors operation will 
be so long that's it's not worth worrying about.  More likely that your 
ground wire will form the counterpoise, than any actual current will 
flow in the "real" ground.

OTOH, you SHOULD make sure that there is some safety ground (third 
prong, green wire) connection for things that *you* can touch, if they 
are conductors.  (I assume none of the spectators come within touching 
range of anything)


And, you can connect your RF ground/counterpoise to the green wire 
ground if you like, although, I'd favor (my opinion only, others differ) 
putting some sort of RF choke in it.  That way, if some line frequency 
wire happens to touch the groundplane, you won't get shocks (e.g. if one 
of the HV leads from the primary shorts to the counterpoise, say by the 
conductivity of a stray spark).  But this is a 50/60 Hz thing, not a 
100kHz thing.