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Re: Faraday screen-do these specs sound OK?
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> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Faraday screen-do these specs sound OK?
> Date: Monday, April 05, 1999 6:53 AM
>
> Original Poster: Doug Brunner <dabrunner-at-earthlink-dot-net>
>
> I'm building a Faraday screen for my new coil, since it'll be used both
> in a theater and a residential area, two places NOT to have stray
> high-power RF. I just wanted to make sure that its design is one that
> would be effective. How does this sound:
>
> The material of the screen is 1" hexagonal chicken wire, built on a
> frame of PVC or wood (I haven't decided yet, probably wood). It forms a
> 6' wide by 6' deep by 8' tall rectangular prism around the coil's
> filter/choke circuit, primary tank, secondary, and toroid. The high
> voltage supply could also go inside, if it would be necessary. The
> chicken wire would be connected together at the edges, and hooked via a
> high-voltage insulated wire (to keep standing waves from arcing to
> nearby objects--tell me if this is unnecessary), about 60' long, to a
> dedicated ground outside.
Some comments (in no particular order)
1) Wood is find as a structural material, however, consider what happens if
a streamer hits the wood. Is fire a posibility?.. What about using
thin-wall EMT (electrical conduit) for framing. Cheap, a wide variety of
couplings are available (or you can just hammer it flat and drill a hole
for a bolt), can't burn.
2) The thing to avoid is wires penetrating the wall of the cage. This would
particularly apply to your HV tank supply wires. EXCEPT, if the filter is
really good. We're not talking about a filter to keep the RF out of the NST
so you don't fry it, but a filter for EMI screening, which is a much
tougher to make (many more dB of isolation required). You might consider
putting your AC control gear (variac, relays, metering) outside (where you
can reach it) and putting the transformer inside. Then, you can use a
conventional EMI filter for AC lines to bring the power inside. Same for
spark gap motor control, if you have one. Do some surplus scrounging, and
you can find high current filters suitable for this purpose for under $20.
3) The cage becomes the RF ground, and no external ground is required (or,
at least it shouldn't make any difference if you do have one). Hook the RF
grounding points of the TC to the cage (inside the cage!) and there
shouldn't be any standing waves on the ground wire. If there are, you've
got leakage, and you need to fix it. You can just ground the cage to the
"third wire" ground in your AC power system, because it is just for safety,
not for RF.
4) Your coil will tune much different. The frequency will drop somewhat,
because the top load C is now bigger (it is working against a capacitor
plate that is much closer than before).
Good luck, and let us know how it works.