faraday cages (II)

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

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




> Again, yes. All leads going through the shielding have to be filtered.

	Just so.

> Again, this is frequency dependent because you have to design, build
> and/or purchase filters. You have to know at least the frequencies
> involved.  Filters are probably the least expensive of your problems.

	They can be critical.  All the bonded shield cage in the world
	can be defeated if the filtering is not right.

> 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.

	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.

	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.... 
   
> The construction of a shielded room, once you know what you are shielding
> against, can be expensive.  My recollection, which probably is wrong,
>is that the Andrews Co. who specialized in broadcast, FM, and TV
> antennas systems also built shield rooms.  I am reasonably sure that the
> "Proceeding of the Institute of Radio Engineers", or  Proc. of
>Electrical Engineers" will have construction details for shielded room for
> various purposes.

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

> The magazine, "Electronics" published by McGraw-Hill is an other possible
> source. There is a book which has selected articles from "Electronics".
> Vin Zeluff and John Markus, "Electronics Manual for Radio
> Engineers", 1st. Ed. 1949, McGraw-Hill Book Co. Inc. See page 430-432
> "Construction Shielded Room in VHF Field".  While this deals primarily
> with the VHF band who is to say how much energy your experiments with
> Tesla coils might generate.

	Relatively little, at VHF.  Unless one sets out to.
	And VHF capable rooms are more expensive/elaborate than those
	suitable for, say, sub 1 MHz...

> A possible problem, which is largely dependent on power level and the
> proximity of transistorized equipment, such as meters, scopes, and
> computers is that of magnetic pulses.  Faraday screens are ineffective
> for this.  It is even more difficult to shield against.  It may or
> may not be a problem.

	One of the advantages of using sheet steel (gavalizied, or chicken
	wire, or whatever) is that these, as ferrous, provide SOME level
	of mag shielding.  The mechanism is different, but if ferrous metals
	are used, sat least some shileding is available.  Off hand, at
	coiling freqs, a repectable amount of shielding is possible (its in
	the books...).  At Really low (audio to power line) it gets
	harder.

> Hopefully someone has done some research on this.  Yes, I know that
>Hertzian waves have both an electrostatic and electromagnetic
> component.  Faraday screens work for the electrostatics but not the
>magnetics.

	If made of 'steel' there is some mag shielding, by different
	effects.  Not much, but better than none.

> Summary:  I don't think enough is known about the problem.  Therefore,
> until you can state the exact problem no one can give you anything but
>a "try it and see it worked for me" answer.
	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.