[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Faraday cage

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>

OK.  I have worked in a several pro built cages and debugged
one 'expedient, but good' one.  Much good advice so far.

1) For usual Tesla frequencies and nearby harmonics 2" or
so mesh (hex or square, makes no diff) is fine.  Arguably
BETTER than window screen as the joints between wires are
sealed/welded/bonded, which the window screen isn't.  In
practice, either will work.

2) Following on with the above:  leakage is proportional to
the SINGLE longest dimension of any opening in the screen
wall, measured in wavelengths.  As a rule of thumb, openings
should be under 1/10 wavelength.  Tesla Coil fundamentals
and harmonics are in the hundreds of KHz or low MHz.
Wavelengths of 100Meters and longer.
(On paper: this means the gasketed door is overkill.  I'd do
so anyway...  8)>>)

3) One thing i have NOT seen mentioned:

For best effect, ALL wires entering/leaving MUST be filtered
and the filter MUST BE 'in' the shield wall.

Many installations may be running set up other ways, if it
works, fine.  But.  The right way is as above.  Why?

RFI/EMI can/will RADIATE (or otherwise couple) to ALL wires.
If a filter is 'in mid air' the emi/rfi can/will radiate around
the filter.  Putting the filter 'in' the cage wall evades this.
(This means, in practice, putting the filter in a shield can,
a doghouse, bonded as well as possible to the shield wall.

4) Properly done, a shield FLOOR is included, bonded to walls.

5) Grounding to an external ground, for shielding, is not
necessary.  Easy to do, tho, arguably safer.

6) I can't think of a way for Al mesh to ignite, but its possible,
i guess.  (fine Al WILL burn, as will fine steel, etc.  But:
there is lots of 'area' to keep things cool.  Another advantage
to 'chickenwire' (2" hex), Hardware Cloth (1/2 inch sq) or the
bigger 2"ish square meshes.

7) I would not expect effects OUTSIDE from any bolt hitting the
INSIDE.  If so, I'd look for another problem: unbonded joint,
etc.  The physics says the effects s/b inside.
	In Boston, at the Boston Museum of Science, the have a
	smallish Tesla coil (I dunno the details) AND a LARGE
	van de Graaf.  say 30 feet high. 10 foot toroid.  This
	is a machine retired from driving a linear accelerator
	at MIT.

	Part of the demo is (was?) for the operator, in a
	largish birdcage (Faraday Cage) is to lean against
	the inside with Serious Bolts (20 ft?) arcing to the
	OUTSIDE, directly adjacent.  Now this DC, not
	strictly comparable, but indicative.

	(The operator cage, for interest, is IIR, 1" bar
	(tube) spaced about 12" apart, maybe 20 ft high, looking,
	literally like a birdcage...)