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Old 11th Jan 2005, 3:55 pm   #1
mjizycky
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Default Faraday cages in workshops

I've got this brand new shed which I'm kitting out for eventual use as a workshop - for now, it'll be a shed but I've decided to insulate it and put up white hardboard interior walls to make it more light inside when it attains its eventual role.

When I install the insulation, the idea is that it goes like this:

outside wall->vapour barrier->insulation->retainer->hardboard.

The thought occurred that I could just use nylon fruit nets to keep the insulation in place, then I considered that if I used chicken wire I could earth it and use it as a Faraday cage - which, considering that most MW alignment points coincide with powerful "locals" round here might not be a bad idea. The only down-side as I can see is the need to extend the wire to the floor, ceiling and hope that too much RF doesn't stray in via the window/door - or am I wrong here? Perhaps someone with more RF knowledge than me could advise on the matter - or even if it's worth bothering with?
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Old 11th Jan 2005, 4:50 pm   #2
Ed_Dinning
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Default Re: Faraday cages in workshops

Hi Mike, I have seen Faraday cages constructed for EMC pre-testing work. They seemed to work best when there were 2 layers of chicken wire seperated by about 2"-3". A floor and roof covering also seemed to be required as well. It would also be possible to have a detachable panel to clip over the window, or use wire armoured glass.

Ed
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Old 11th Jan 2005, 5:17 pm   #3
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Default Re: Faraday cages in workshops

Chicken wire comes in several sizes, presumably depending on the size of your chickens I've seen 2", 1" and 0.5" mesh.

Mike, you live in the countryside so I guess you can easily get the stuff from a farmers supply place which is likely to be a lot cheaper than DIY stores. You really do need to cover all 6 sides of the shed with continuous mesh. Including the door and window. Smaller mesh will give better screening and 2 layers of mesh will be better still though I'm not sure what benefit is gained fom separating the 2 layers as Ed suggested.
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Old 11th Jan 2005, 7:38 pm   #4
Paul Stenning
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Default Re: Faraday cages in workshops

I remember a demonstration on an EMC training course I attended a few years ago. They had the spectrum analyser in the shielded room tuned into a radio station (probably R4 on LW), and connected to the usual radiated emissions antenna. With the door completely shut there was no reception. With the door open with less than a 1" gap, there was good reception.

This shows that you need the faraday cage to completely enclose the area. Any gaps and you are probably wasting your time bothering at all. Certainly with window-sized gaps plenty of radio signals will get in.

Could you use that glass with 1cm square wire mesh in it? I'm not sure how you could make reliable contact with the mesh, but it would look a lot tidier than a piece of chicken wire mesh pinned over the window, and would also have security advantages.
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Old 11th Jan 2005, 7:48 pm   #5
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Default Re: Faraday cages in workshops

Quote:
Could you use that glass with 1cm square wire mesh in it? I'm not sure how you could make reliable contact with the mesh, but it would look a lot tidier than a piece of chicken wire mesh pinned over the window, and would also have security advantages.
What I really need is that glass covered with a gold film, like they used to use on some railway locos (Class 47 IIRC) for demisting purposes!

Thanks for all the contributions so far - keep 'em coming!
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Old 14th Jan 2005, 8:26 pm   #6
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Default Re: Faraday cages in workshops

I had a Faraday cage installed in a workshop some years ago. The test after it was constructed was to place a fairly high powered transmitter inside and make sure that nothing was received on the outside. The testers' party trick was to put a 5 note in the door seal and with the door closed, a signal was received out side.
Incidentally the room had two 300mm square vents for air conditioning and these were of a honey comb construction, each segment being about 6mm across.
The walls, floor and ceiling were a sandwich of two sheets of steel with particle board in between. It was ok to screw things to the wall as long as you did not bridge the gap.
 
Old 15th Jan 2005, 2:30 pm   #7
pmmunro
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Default Re: Faraday cages in workshops

Mike,

It's very possible that you know a lot more about building insulation than I do and the material of the outer wall may have a considerable bearing on the matter, but normal practice in buildings which have masonry walls and dry-lined interiors is to put the vapour barrier(s) as close to the inside surface as possible.

This is because most of the water vapour in a house is generated by the occupants and their activities. If the vapour permeates the inner wall and insulation, the thermal gradient and the dew point are likely to meet just where it is least desireable - where supporting wooden studs are fixed to the masonry, thus inducing rot in the perishible fabric. When using polystyrene insulated plasterboard, for example, it is recommended that a latex paint is applied to the interior surface and there is either a polythene or metallic foil between the plasterboard and the polystyrene. If further insulation is to be placed between the studs which support the plasterboard, another vapour barrier should separate the polystyrene from the studs and their insulation.

There is possibly a risk that putting in two vapour barriers, one each side of the insulation could trap and moisture which enters inadvertently.

If the walls of your shed are timber, I would consider treating the inside surfaces against rot, if this has not already been done, before fitting any insulation.

For more information, see the Mitchell's Building series of books and publications by British Gypsum and the Building Research Establishment.
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Old 17th Jan 2005, 2:37 pm   #8
mjizycky
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Default Re: Faraday cages in workshops

Quote:
Originally Posted by pmmunro
Mike,

It's very possible that you know a lot more about building insulation than I do and the material of the outer wall may have a considerable bearing on the matter, but normal practice in buildings which have masonry walls and dry-lined interiors is to put the vapour barrier(s) as close to the inside surface as possible.
On the contrary, I know very little about building insulation and your post has been most illuminating and helpful. It's a timber-walled shed which has already been treated against rot - I just added a good dose of woodworm fluid as well. I'll probably give it another coat of anti-rot treatment as well to be on the safe side.

Many thanks for the tips and advice, they're greatly appreciated.

Mike.
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