Re: faraday cages (II)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ray von Postel by way of Terry Fritz <twftesla-at-uswest-dot-net>" <vonpostel-at-prodigy-dot-net>

Dave:

I agree with every comment you made, but there still remains the need to
determine the extent of the problem in terms
of the amplituede and frequencies of the energy output by the typical Tesla
coil.

Tesla list wrote:

Snip...

> > Perhaps some one on the list has the information on the r.f. spectrum
> > generated by Tesla coils and will share it with us.
>
>         Varies with the coil.  Gnerally under 1MHz, usually under
>         200 KHz.  Harmonics above, of course, these usually drop
>         rapidly in power.

I assume you are talking about the "fundamental" frequency of the coil
recognizing that the output occupies a fairly broad band.  What IS the power
of the 10th harmonic at a distance of 20 feet from a coil running 10 KW input?
That rehtorical question is asked to emphasise the point I am trying to
make that there is room for a lot of productive experimentation and study.

>

>         Filters (as discussed in other threads) can ne designed
>         & understood.  The usual filter for this sort of application
>         is more or less (or should be) 'wideband', intended to block
>         a range of freqs.
>

Would you use low pass filters having cut off frequencies for the power
lines of around
100 cps and for telephone lines close to 5000 hz.?

>
>         A second key (which can take some learning) is that a filter just
>         plunked into a wire can be easily useless.  threat signls, even
>         if highly blocked by the filter, may well simply radiate around
>         it.  An analagy is water, in stream, flowing around a rock.
>         A filter is best emplyed as 'part' of the wall.  Build the
>         filter into the wall (or build the wall around the filter,
>         either way.
>
>         For lowish freqs, the radiate around problem may not be severe,
>         but (as power levels go up) It can be bothersome.  Put another way:
>         a line may (will) pick up   from a radiated field, or a connected
>         source (eg a spark gap, etc..) conduct 'noise' along.  The filter
>         might FULLY block the conducted portion , the radiated portion
>         _radiates_ around, recouples and is conducted again.  This
>         effect is more severe at higher freqs than typical coiling.  At
>         coiling powers tho, it can be bothersome.
>
>         Ferrites can be attractive, for simplicity, however, they
>         have limits.  Many are optimized for VHF.  If working with
>         found materials, it may not match the problem at hand.  Hnad
>         crafted air, or 'iron', cored coils can be more effective/
>         predictable.  The losses are not objectionable, since the objective
>         of the filter IS loss....
>

Agreed, the comment on a screen room for VHF was not to suggest a model.  The
intent was point out that there was no need to "re-invent the wheel" .
Frankly, I
am lazy, and don't have an incentive, to do the necessary library research.

>
>
>         We built a quie useful one (for work) out of copper screen.
>         Took a while to debug the users...

>
>        Shield room, etc are well documented.
>         Coild feqs and effects are not THAT different than those involved
>         in (N) EMP work.  (OK A BIT smaller).  There is much in the open
>         literature.  It requires exact construction (make sure edges
>         are jointed, based on the 'threat' freq, filter the lines, with
>         care as to placement.C

Considering that you can have excellent electrostiic shielding with very
poor electromagnetic shielding. is it axiomatic that Electro Magnetic Pulse
precautions are always effective for electrostatic shielding?  If so, then
might it be well to concentrate on the EMP side of things?

Would you agree that not much is known about the nature of the output
of the Tesla coil and that construction is largely an art?  My contention
is that
you really don't know something until you can write an equation for it!
The art is
what makes coiling fun.

Ray