Faraday Cage testing question

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Slugg
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Faraday Cage testing question

Post by Slugg » Tue Mar 29, 2016 9:55 pm

Results:
I placed an AM/FM radio in the box and it goes to static. Cell phones ring and 2 way radios still work with the box closed.

Test:
I took the metal box and placed a small cardboard box inside. I would then place the various electronics on the cardboard box to prevent it from touching the metal box. Once the metal box is closed I would test the function of the electronics. I then open the box to be sure the electronic did not fall down during testing. I am sure the electronics are not touching the metal box in any way.

Build plan:
I have an old metal tool box that opens on a hinge similar to a shoe box. I painted the outside of the box due to surface rust while it was closed to not interrupt the lid and base contact. I then plan to tape cardboard on the inside of the box to prevent electronics from coming in contact with the metal.

Is this Faraday Cage working properly? If not, can I fix it or should I start from scratch. I can provide pictures in needed.
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Maast
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Re: Faraday Cage testing question

Post by Maast » Wed Mar 30, 2016 4:21 pm

Sorry, it's not going to work for you. If you can get any signal of any kind in your shielded container it's a bad cage.

Faraday shielding is directly related to the conductivity of the lining material, and steel is a really crappy conductor so it's just not going to work well - unless your steel is VERY thick.

Now if you were to line the box inside and out with thick aluminum foil and have no more than a half a mm gap between anything then that'd work.

As your own test, you can wrap a phone in aluminum foil and try to call it.
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Maeklos
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Re: Faraday Cage testing question

Post by Maeklos » Wed Mar 30, 2016 7:57 pm

Seen the Mythbusters use copper mesh a few times to make closet-sized Faraday cages for testing radio equipment before. Aluminum foil may be a cheaper alternative as noted above, but copper mesh is usually pretty durable. It'll also be the most conductive material commonly available, too.

Or, use an old microwave oven. Microwaves are Faraday cages. You can test this by popping your cell phone into your microwave and trying to call it. If it rings, get a new microwave. And maybe get your sperm count tested. Just to be sure.
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Re: Faraday Cage testing question

Post by CrossCut » Fri Apr 01, 2016 9:57 am

There are multiple variables in your test: the output power of the transmitters (the am/fm stations, cell tower, other 2-way radio), the distance from the transmitter, the receive sensitivity of the radio in the cage, and the different frequencies involved. If the am/fm radio receives a station outside the cage but gets only static when inside, then everything else being equal, the cage is providing some amount of signal attenuation. The problem is without the proper test equipment you have no way of knowing how much attenuation it provides, or whether that amount would be sufficient protection for your "X" device from the effects of a E1 EM pulse of "Y" strength and at "Z" distance away. It's not like every electronic device (and components within them) is going to have the same failure threshold either.

Might check out Arthur Bradley's youtube vid on his tests with an aluminum garbage can, IIRC he got 80dB of attenuation with the garbage can when using aluminum tape to seal around the lid. He believes that should be a sufficient amount of shielding in most cases. Too lazy to look it up, but 80dB is something like 99.9999% attenuation. The futurescience site has a lot of information on EMP and cages too.

Short answer might be to put your toolbox inside a cardboard box, and put that in a garbage can with the lid sealed with aluminum tape. At least the protection provided by the garbage can is a semi-known value, and one that a guy with a PhD in EE believes is enough. Better too much than not enough.

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Re: Faraday Cage testing question

Post by Rege » Thu Apr 07, 2016 8:59 am

Hello.

My 2¢ from when I worked in a Faraday cage.

It was at a navy shortwave transmitter site.

The total rf power was over one half million watts!

We had a 12' x 12' room lined with copper screen so we could test and repair broken transmitter bits without interference.

Anyway, the clue that am radio (.5-1.7 mhz) was blocked but cell phones (1850-1990mhz) came thru a box with a lid.

Also the aluminum can with Aluminum tape stopped all frequencys - note this tape is special tape with rf conducting adhesive.

What I think is happening with the box is the gap between the box and lid is acting like a "slot antenna". Everybody is familiar with a regular whip antenna. Here us a neat thing, a 1/2 wavelength slot in a sheet of metal works exactly as a 1/2 wavelength wire in air!!!

The can/tape combo eliminates the slot antenna effect.

Hope this helps.

Rege

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