Re: [TCML] Re: Sculpture with Tesla coils

["Dex Dexter" <dexterlabs@xxxxxxxxxxx>]

Wire mesh grounded cage wouldn't eliminate 
EMI completely for sure.
More important is the elimination (reduction)
of E-fields.Magnetic field,especially that of
twin coil systems,dies away more quickly with
the distance than E-field anyway.
True,I haven't done analysis of all the details.
Can you point to links or references with 
corresponding measurements we could see?

Dex  

--- jimlux@xxxxxxxxxxxxx wrote:


Methinks you haven't actually done any analysis of Faraday cages around 
Tesla coils or measured the EM fields at least with respect to 
regulatory compliance.

As it happens, the cage will prevent a spark from going past the cage 
(which is the usual function),so you can pretty much prevent someone 
from getting a shock or hit by an arc,  but it doesn't do a very good 
job suppressing EM interference.

Let's see.. you're operating at 100-300 kHz, so the wavelength is "very 
long" compared to the distances of interest.  This means it's most 
likely a "near field" problem.  Near field, from an EM situation, means 
you need to analyze both magnetic and electric fields, separately.  The 
cage might do a find job controlling the electric field, but most cages 
are basically transparent to the magnetic fields, which, of course, hold 
just as much energy as the electric field.

Now let's talk about higher frequency interference (e.g. many MHz).. 
most of the cages I've seen for tesla coils are made of chicken wire or 
similar woven material.  This isn't particularly good as a shield, 
because the strands go between the inside and outside of the cage.  If 
you did it of welded mesh, then you're probably better off.

Now let's talk about "death to near by electronics" by induced 
transients from the sparks.  Here, the propagation is mostly by the 
magnetic field in the circuit that goes topload, spark, ground, 
secondary coil... forming a loop with a perimeter of several meters. 
Unless you happen to have a shield that matches up with the magnetic 
field from the current in that loop, most of the transient field will 
happily couple to a nearby victim loop.  This problem is most notorious 
with a Marx generator (really big di/dt, because the erected impedance 
is fairly low, unlike a TC, which has a big ol' honking inductor in the 
secondary circuit), but still an issue with a TC of any decent size.


Bottom line:   a cage prevents shocks and direct arc contact, but not 
much for EMI, unless very carefully designed.



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