Faraday Cages....

["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>

Tesla list wrote:
 
> Original poster: "Gary Johnson by way of Terry Fritz
<twftesla-at-uswest-dot-net>" ><gjohnson-at-ksu.edu>

> >Doesn't a Faraday cage only work if it's a COMPLETELY ENCLOSED room?

	That way, it is 'guaranteed' to work.  Lesser solutions may
	be acceptable, depending on conditions/applications.  Caution
	(as has been noted) is in order.

>> I don't mean airtight (but there is a specific maximum size hole for
>>a given frequency protection limit)?

	Some have been cited, which seem plasuible.  The one i used to
	use/hear was:
		No opening with a side longer than 1/10 pf a wavelength.
	Thus, a cage for VHF work has a very differnt door than one for
	Tesla coiling.

	As also noted, its the single longest dimension of an opening
	with 'matters'.  Thus, a metal door, with gasketing, but
	NOT gasketed along one side (or with failed gasketing) is the
	same as a six foot hole.  And no door, but a gap of 6 feet
	between fasteners, is the same.
	(6 feet is likely fine for the usual coiling freqs (100KHz =/-)
	'tis useless at VHF....)
 
> Christopher:
 
> You have been getting some good advice on this issue. Just thought I would
> add my experience. I built a large ag-type building (a Morton Building) to
> do my Tesla coil stuff in.  It has roof and outside covering of a steel
> sheet metal. I built a room in the corner of this building that I could heat
> and cool. It is also covered sides and top with steel sheet metal. Think of
> a steel box inside a steel box. Inside the room I have a double wall copper
> screen room, 8 by 10 ft by 8 ft high. (The local EE Department was throwing
> it away.) My computer and scope are inside the copper screen room while the
> Tesla coil is out in the large bay.  Screen room power comes through a 5 kVA
> transformer. It is not specifically designed as an isolation transformer.

	This can work, depending on details.  One problem with a transformer
	NOT designed as an isolation is stray capacitive coupling from one
	side to the other.  In the Coiling case, this can lead to stray
	where it is not expected, nor wanted.

	A tranny desinged for isolation of noise, etc, will have a
	grounded screen (and or other tricks) internally, between
	primary & secondary.

	The other key (as, i believe has been mentioned) is how
	wires/penetrations are handled.  Stray RF WILL ride in/out
	on ANY wiring, unless it is handled appropriately going thru
	the screen.

	Background:
	Years ago, i did EMI/FCC Compliance engineering.
	Included building/debugging screen rooms & watching engineers
	learn (the hard way...) some of the above.
	
> am not doing medical research or building an surgery facility for which a
> true isolation transformer becomes more important. I just want my scope to
> survive whatever the Tesla coil is doing 15 ft away. The main function of
> the transformer is to separate the local Tesla coil ground from the utility
> ground. The screen room and the equipment inside are all connected to the
> local ground.  I disconnect the phone line from the computer when the coil
> is running. I don't know how much of this is necessary, but I have not lost
> any electronic equipment due to electric or magnetic pulses from the coil,
> to my knowledge.

> In specific answer to your question, I use a transistor radio to check the
> effectiveness of the screen room. The AM radio receives a full range of
> stations inside the airconditioned room but outside the screen room.

	An excellent technique.

> Steel sheet metal on four walls and ceiling has little effect on reception.

	How are the edges handled?
	Pro screen rooms are routinely built of galvanized sheet steel.

> Maybe the signal is coming up through the concrete floor?

	Possible,  I get fuzzy on that.  we had full on conductive floors.
	Fer one thing, one would want the screen walls bonded to the
	floors.  (eg bury the sheet metal in eath, or however couple
	it to concrete...)

> But as I carry the radio into the screen room, by the time I am 3 ft
>inside, it is dead!

	Yep.  Its them longish AM Wavelengths.  (That's why auto AM
	fades under a bridge:  wave lengths are too long to 'fit' in.

> This is with the heavy copper door still wide open. So I never bother to
> shut the door when the coil is operating.

 	If interested, try the same with an FM station.  +/- the
	door edge being bonded, that may demonsate the use of the door.

> Anyhow, based on my one data point, I would be concerned about chicken wire
> or any other simple, cheap, quick and dirty scheme using steel sheet metal
> being of substantial help.

	I should expect it to work fine (for coiling) IF the edges and
	penetrations are well handled. (filtered at/in the screen 
	wall boundary.).  (I've a lovely 1893 article thereon, which i can
	NOT find..
	
	best
	dwp