Building a Faraday Cage: Is 2-Layer Aluminum Screen Enough?

[Mogan]

I am building a walk in size faraday cage. In order to make it re-positionable, I am creating 6 wooden frames (top, bottom and sides) covered in 2 or 3 layers of aluminum screen from a hardware store. I am using at least 2 layers of screen, because that is how many layers it takes to cause my cell phone to lose it's signal. I am wrapping and stapling the screen over the edges of the frames and then assembling the frames with the screen-covered edges pressed together. My goal is to shield the inside of the cage from cell phone tower and wifi related radiation. I am educating myself about electricity and RF shielding, but I am still very new this. I will buy a Cornet RF meter before the project is complete - unless I learn of a more appropriate meter for this project.

My questions are:
1) Is the conductivity of the screen likely to be interrupted where there are two pieces pressed together (rather than using one continuous piece of screen)?

2) If I layer 2 or more pieces of screen together and some radiation makes it through the gaps in the first layer, bit hits the second layer, will I be "trapping" that energy inside the cage.

I hope my questions don't sound ridiculous. Thanks,

 

[sophiecentaur]

Welding or soldering could help a lot, if you can manage it. A professional screened room I used, had phosphor bronze spring fingers along all the joins.
I seem to remember that a λ/4 gap between inner and outer boxes can be very effective in increasing the screening effect. This is achievable (near enough for Jazz) if the bandwidth you are operating over is not too great. The door seal can be made like a microwave oven, with λ/4 slots running round the gap (look it up) to make an open circuit gap look like a short circuit.
Those last few dB are very hard to win!

 

[meBigGuy]

I've never seen joined screens that were not a high quality clamping joint with no possibility of leakage (always a metal backing at the joints). Even then it can leak after its been moved.

From http://www.ets-lindgren.com/iSeries-71. Lots of construction images via google.

 

[marcusl]

Aluminum is a poor choice because it is naturally covered in a non-conductive oxide. It is unlikely that you will get good electrical contact at your overlapping regions. Switch to copper screen and clamp or solder the seams. You'll only need a single layer.

 

[Baluncore]

Avoid aluminium.
Good screened rooms have two conductive walls that are electrically independent.

It is important that lines of contact are electrically bonded. Where a gap of some length exists in the bond, there is in effect, a slot antenna which is a dipole of that length. The slot will couple RF efficiently between the inside and the outside of the screen. You do not want that.
There will be big problems with the door seal if you cannot eliminate the slot antenna effect. You may need both spring fingers and RF gasket.

You might consider using brass fly screen mesh for ventilators. It can be soldered easily.
The cheapest surface will probably be thin galvanised or tin plated iron. It can be soldered along all the seams.

 

[Mogan]

Thanks guys! I've been away for a while. I see a couple new replies since I last checked.Very helpful!

 

[berkeman]

I am building a walk in size faraday cage.

My goal is to shield the inside of the cage from cell phone tower and wifi related radiation

Can you say what the purpose of this Faraday Cage is? What are you planning on doing inside the cage that requires RF attenuation of outside signals? Will you be using AC Mains power inside the cage? If so, you will need to add filtering to the AC Mains feedthroughs, and that filtering will need to work up at cell phone frequencies...