Re: Double Throw Spark Gap (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 15 Oct 2007 11:19:41 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
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?  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.
> 
<snip>

The topload and secondary self-capacitance form one "plate" of a 
capacitor that "rings" with the secondary inductor. One might ask where 
the plate of this capacitor resides. It's every other conductive body 
around the coil, including the primary winding. If your coils is sitting 
in an empty field, the conductive soil near the coil becomes the other 
plate, so connecting the base of the secondary to an earth ground makes 
sense.

However, if you instead place a relatively large conductive plate (or 
counterpoise) underneath the coil and connect it to the base of the 
coil, you create a well-defined, low loss, lump of capacitance that the 
secondary inductance can efficiently work with - a virtual ground. Now 
ideally, the plate should be infinitely long in all directions, but it 
can be shown (from antenna theory) that using a counterpoise with a 
diameter that's only 2-3 times the total height of the secondary will 
work almost as well. Practically , the counterpoise is usually connected 
to earth ground as well since otherwise it can develop potentially 
dangerous RF voltages that may cause a shock or an arcing hazard.

Many commercial multi-story buildings use a steel supports in the floors 
and ceiling, and often use steel "2 x 4's" in the walls. Although this 
metallic "cage" is not a complete Faraday shield, it does create an 
excellent capacitor plate for your coil IF you can gain easy access to 
it. The building's electrical mains are (eventually) connected to the 
buildings steelwork and to earth ground, but not necessarily physically 
close to where you're running your coil. And, you are MUCH better off 
connecting your ground to a nearby steel beam than running an 
independent connection to a remote earth ground.

By using a counterpoise, you directly "collect" a good portion of the TC 
secondary's total displacement current. By also grounding the 
counterpoise to the building's ground (ideally directly to a nearby 
steel beam) you can efficiently gather the remaining displacement 
current o the ceiling and walls. Connecting the counterpoise to mains 
ground can superimpose significant RF current into the mains earth 
ground IF it is not electrically connected to nearby steelwork. 
Fortunately, this is usually not a problem when the wires are run within 
grounded conduit and the conduit is electrically attached to the nearby 
steel infrastructure.

Bert
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