Building a Faraday's cage ?

[John Larkin]

** Exactly.




** Crapology.

It takes a good deal of copper, several mm roughly as I recall, to
produce enough eddy-current effects to cut a 60 Hz magnetic field in
half. Skin depth in copper is about 8 mm at 60 Hz, so that sort of
brackets the attenuation numbers. 1 oz copperclad FR4 starts being a
useful shield at some 10's of KHz.

John

 

[Rich Grise]

You're wasting your time !

I do know a guy who built a huge Faraday cage in Mark Knopler's studio in London
btw.

In my day they were called "screen rooms". We had one at Motorola SPD
that was 10' X 10' X 10', 2" X 6" studs, wire mesh on both surfaces.
[/QUOTE]

In the USAF, in avionics, they were routine. At one base where I worked,
the whole shop was a screen room, (which they called 'shield room') to the
extent that they used incandescent lights and the air conditioning was
woefully inadequate. The wall paneling, ceiling, and floor were sheet
metal.

It was pretty quiet (electrically), however.

Cheers!
Rich

 

[Rich Grise]

Thanks for the definition. In this real world situation, you
have an imperfect cage, and you have things inside coupled
to the outside.

Well, the only thing is, he's wrong[1]. A "Faraday" cage does _not_ shield
you from magnetic fields - electrostatic, yes, if it's properly grounded,
and _electromagnetic_ (not magnetic), more commonly known as RF radiation.

And if you use window screen, it's transparent above the wavelengths on
the order of the screen mesh size or so.

Cheers!
Rich

1. Although it could have been a translation error.

 

[Rich Grise]

A Hoffmann 14-gauge formed-and-welded steel electrical enclosure is a
pretty substantial hunk of metal, and won't dependably attenuate a 60
Hz magnetic field by 2:1, a mere 6 dB. And the bigger the box, the
worse the shielding.

Just make the box out of CoNetic:
http://www.google.com/search?&q=conetic

Cheers!
Rich

 

[John Larkin]

Just make the box out of CoNetic:
http://www.google.com/search?&q=conetic

Not easily. These alloys are fragile (bending wrecks the
permeability), seams will leak magnetically, and you still need an
absurd wall thickness/box size ratio to get good shielding. And they
cost a fortune.

John

 

[Phil Allison]

"John Larkin"
"Phil Allison"

It takes a good deal of copper, several mm roughly as I recall, to
produce enough eddy-current effects to cut a 60 Hz magnetic field in
half. Skin depth in copper is about 8 mm at 60 Hz, so that sort of
brackets the attenuation numbers. 1 oz copperclad FR4 starts being a
useful shield at some 10's of KHz.

John

** Kindly calculate the attenuation figure at 50Hz for a box the size of a
toilet cubicle made from aluminium or copper fly screen.

That is what the game is here.




........ Phil

 

[Eeyore]

"John Larkin"
"Phil Allison"

** Kindly calculate the attenuation figure at 50Hz for a box the size of a
toilet cubicle made from aluminium or copper fly screen.

That is what the game is here.

So true !

Copper foil makes a decent 'belly band' for transformers though. It's only
recently I had occasion to get into that side of things.

Graham

 

[Phil Allison]

"Eeyore"

So true !

Copper foil makes a decent 'belly band' for transformers though. It's only
recently I had occasion to get into that side of things.

** Wot - fit a copper flux shorting strap around an R-core ??

Least it can be done, I suppose.

Unlike with a toroidal.







......... Phil

 

[Eeyore]

"Eeyore"


** Wot - fit a copper flux shorting strap around an R-core ??

Actually I was thinking of one on a ferrite 'E-E' core I did a while back.
Reminded me of some I've seen wrapped round E-I 50Hz transformers though -
typically those out of Asia.

Least it can be done, I suppose.

Very true. Haven't ever needed it but that's a good point.

Unlike with a toroidal.

Hmmm... Quite.

Graham

 

[John Larkin]

"John Larkin"
"Phil Allison"


** Kindly calculate the attenuation figure at 50Hz for a box the size of a
toilet cubicle made from aluminium or copper fly screen.

That is what the game is here.

The use of "screen" is a new stipulation. Of course, screen isn't very
conductive, so the eddy currents will be far lower than for a solid
sheet of metal.

Still, the magnetic attenuation isn't "** exactly" "ZERO" as someone
has recently claimed.

John

 

[Phil Allison]

"John Larkin"

The use of "screen" is a new stipulation.

** No it is NOT !!!!!

Read the OP's first post.

" I'm planning to build a 2m x 2m Faraday's cage/booth to protect active
electric guitar pickups from external electrostatic and electromagnetic
noise 20hz-96khz "


What the HECK did YOU imagine such a booth is like ??

Of course, screen isn't very
conductive, so the eddy currents will be far lower than for a solid
sheet of metal.

** So low as to be insignificant at 50 Hz.

Still, the magnetic attenuation isn't "** exactly" "ZERO" as someone
has recently claimed.

** Wot a pedant !

Maybe you can say how to make a usable guitarist's booth with 150 dB
attenuation as was claims by that Rene idiot.


" For the lower frequencies, iron would be the choice.
I'd expect an attenuation in the order of 150dB
or better. "




....... Phil

 

[John Larkin]

"John Larkin"



** No it is NOT !!!!!

Read the OP's first post.

" I'm planning to build a 2m x 2m Faraday's cage/booth to protect active
electric guitar pickups from external electrostatic and electromagnetic
noise 20hz-96khz "


What the HECK did YOU imagine such a booth is like ??

Well, we did discuss using copper sheet and aluminum foil. And I did
suggest that sub-mm layers wouldn't attenuate 60 Hz fields much. Even
foil would be usefully effective against h-fields at his high end, if
he indeed had any floating around.

** So low as to be insignificant at 50 Hz.




** Wot a pedant !

Actually, I'm an engineer, and I like to understand (and predict, when
it comes down to it) numbers no matter their magnitude. I suppose that
when you say...

"There is ZERO attenuation of low frequency magnetic induction."

and clarify that with

"** Exactly"

then I'm being a pedant to assume you actually meant it.

You didn't actually mean it?


John

 

[John Larkin]

Very true. Haven't ever needed it but that's a good point.



Hmmm... Quite.

Why not?

John

 

[Eeyore]

Why not?

Not very practical to apply one due to mechanical issues.

Graham

 

[Phil Allison]

"John Larkin"

Well, we did discuss using copper sheet and aluminum foil.

** Try answering the damn question - asshole.

See the word "cage " in the heading ??????

Mean anything to you ?

Actually, I'm an engineer,

** And just like so many of them, a useless, PITA autistic pedant.





....... Phil

 

[John Larkin]

"John Larkin"


** Try answering the damn question - asshole.

See the word "cage " in the heading ??????

Mean anything to you ?

It means pretty much this to me...

http://en.wikipedia.org/wiki/Faraday_cage

"Faraday stated that the charge on a charged conductor resided only on
its exterior, and had no influence on anything enclosed within it. To
demonstrate this fact he built a room coated with metal foil,..."

So the original Faraday "cage" was in fact made of metal foil.

Does that answer the damn question?

John

 

[Eeyore]

Does that answer the damn question?

Whatever - it'll be like a red rag to a bull as far as Phil's concerned you
mischeivious person !

Graham

 

[Phil Allison]

"John Larkin"

It means pretty much this to me ...

* What crapology !!!

The heading and the OP's first post describe his intended application VERY
clearly.


You were a born pedantic, autistic IDIOT - Larkin.

When you die is the only time that will change.





....... Phil

 

[John Larkin]

* What crapology !!!

The heading and the OP's first post describe his intended application VERY
clearly.


You were a born pedantic, autistic IDIOT - Larkin.

Gosh Phil, sometimes you're right, and sometimes you're wrong, but
you're always angry.

When you die is the only time that will change.

But I'm having fun while I'm alive. You should try it.

John

 

[Phil Allison]

"John Larkin"


You were a born pedantic, autistic IDIOT - Larkin.

When you die is the only time that will change.

Hope for the sake of humanity it is real soon.







....... Phil