Author Topic: Question - Faraday Cage  (Read 50168 times)

Offline pokeshell

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Re: Question - Faraday Cage
« Reply #60 on: January 15, 2012, 11:59:10 PM »
Here is a simple test to see if your "cage" is EMP resistant (I hesitate to say "proof" since I've not tested it with an actual EMP ;) ): But anyway, simply put your cell phone in it, with the ring on the loudest it will go. Then close the cage up and call your cell phone.  If you can hear your cell phone ring, the your cage (or at least the item) is NOT EMP resistant.

I just did the cage test using a microwave and an ammo box as cages, testing each separately.  I placed my cell phone on top of a small cardboard box inside the "cage" and closed it up.  Alas, when I called my cell phone I could still hear it ring inside the cage.  Then I wrapped my cell phone loosely, but completely,  in aluminum foil, again placed the phone on top of the cardboard box inside the cage but where so the foil did not come in contact with the cage walls, and closed it up...and when I called my cell phone this time, it did NOT ring...even when the calling phone was just inches away.  Success!   

Note: Wrapping the cell phone in the foil, but WITHOUT placing on top of the cardboard box inside the cage, does NOT prevent the cell phone from ringing.

Can we see pictures of the hat you made after your testing? ;D

Offline LibertyBelle

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Re: Question - Faraday Cage
« Reply #61 on: January 16, 2012, 11:32:23 AM »
Can we see pictures of the hat you made after your testing? ;D

 Sure...here you go! :rofl:

Offline idelphic

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Re: Question - Faraday Cage
« Reply #62 on: January 16, 2012, 12:43:00 PM »
One thing to keep in mind, is that EMP can and will generate surface energy - the same as static.

To keep the surface energy from discharging into your device, you need to ground it.

Offline pokeshell

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Re: Question - Faraday Cage
« Reply #63 on: March 10, 2012, 03:27:46 PM »
...even when the calling phone was just inches away.  Success!   


Inches away from what, the phone tower?  LOL

Offline LibertyBelle

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Re: Question - Faraday Cage
« Reply #64 on: March 14, 2012, 07:00:43 PM »
Inches away from what, the phone tower?  LOL

Nope, but my FIL has two cell phone towers on his property, so I'm going to take my little cage and my cell phones right below the towers and see if I get the same results. :)

Offline Perfesser

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Re: Question - Faraday Cage
« Reply #65 on: March 15, 2012, 10:32:04 AM »
I found this page on EMP and cars.
http://www.futurescience.com/emp/vehicles.html

and EMP Personal Protection Page.

http://www.futurescience.com/emp/emp-protection.html

Offline Alan Georges

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Re: Question - Faraday Cage
« Reply #66 on: March 15, 2012, 06:21:17 PM »
I found this page on EMP and cars.
http://www.futurescience.com/emp/vehicles.html

and EMP Personal Protection Page.

http://www.futurescience.com/emp/emp-protection.html

Thanks, Perfesser.  He has some good, concrete advice on the topic.  I just gave it a quick read, and will be bookmarking and printing it to re-read right after I post this.  Something that cleared up a lot of my questions was the part about Faraday cage grounding:
Quote
For shielding small items like radios and other electronics equipment, use the nested faraday cage system of alternating foil (or screen) and plastic, and don't bother with the ground connection (unless you plan to physically bury your equipment).  EMP grounding gets very tricky, and the ordinary rules for grounding do not apply.
It seems like a majority of my prepper friends have taken the stance "If it's not grounded it's pointless, so I'm not going to bother at all."  That always flew in the face of the couple of college courses I had on E&M, but I also know that at very high field levels weird stuff can happen.  Having a shielded box or two for a backup drive and a radio just seems like a classic case of 80% of the benefit for 20% of the cost, and that's the way I go.

The only question I have is that, if the skin depth in aluminum is about 1/1000 mm at frequencies corresponding to E1 rise times, do you even need to double-cage things?  I guess having belt and suspenders never hurts anything besides my wallet.

BTW, there's more in first draft form at the same site: http://www.futurescience.com/emp/emp-grounding.html

Offline Orionblade

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Re: Question - Faraday Cage
« Reply #67 on: March 24, 2012, 03:42:50 PM »
Thanks, Perfesser.  He has some good, concrete advice on the topic.  I just gave it a quick read, and will be bookmarking and printing it to re-read right after I post this.  Something that cleared up a lot of my questions was the part about Faraday cage grounding:It seems like a majority of my prepper friends have taken the stance "If it's not grounded it's pointless, so I'm not going to bother at all."  That always flew in the face of the couple of college courses I had on E&M, but I also know that at very high field levels weird stuff can happen.  Having a shielded box or two for a backup drive and a radio just seems like a classic case of 80% of the benefit for 20% of the cost, and that's the way I go.

The only question I have is that, if the skin depth in aluminum is about 1/1000 mm at frequencies corresponding to E1 rise times, do you even need to double-cage things?  I guess having belt and suspenders never hurts anything besides my wallet.

BTW, there's more in first draft form at the same site: http://www.futurescience.com/emp/emp-grounding.html

There is absolutely nothing wrong with not grounding the faraday cage - it just allows the outside of the cage to float at whatever potential is induced, which should dissipate as the pulse passes - there's no charge transfer to or from the cage, so it should be neutral before and after the pulse.

Also, keep in mind that most implementations of coaxial cable only ground the shield at one end - grounding at both ends turns the shield into an antenna, dumping noise into your ground plane.

Separating two layers of shielding with an insulator effectively creates a capacitor, which may serve to increase the effective "input impedance" of the whole assembly. Couldn't hurt.


Offline Dainty

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Re: Question - Faraday Cage
« Reply #68 on: March 25, 2012, 03:56:09 PM »
I have an accidental faraday cage. :o

It's an airstream trailer. The fact that the interior walls and doors are enameled steel sheeting may or may not contribute.

It isn't grounded, as far as I know.

Just outside the door is full cell phone reception. Just inside the door you sometimes get a little recetion. Everywhere else there's nothing, unless you count sending a text and periodically hitting "retry" over and over again until, hours later, it finally gets through. I have tried basically every signal booster out there. I've switched phone companies.  I've changed phones. No difference. Anyone with a cell phone walks in here, they've got themselves a nice paperweight.

Although, I should mention 911 calls still get through so some sort of signal must still be penetrating.

In case that info is helpful to anyone...

Offline Orionblade

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Re: Question - Faraday Cage
« Reply #69 on: March 25, 2012, 04:50:18 PM »
911 calls are required to be made from any cellphone, activated or not, on any carrier - so if the cell phone has a dual mode transciever (analog/CDMA/etc.) then you're still going to be able to get a wee bit of signal leakage, whereas a digital signal will deteriorate rapidly.

I would suggest adding a grounding lug /strap between the door itself and the trailer structure, since gaskets and plastic bushings would isolate the door itself. You may also want to add window coverings grounded to the structure. I would suggest a stiff fabric like canvas which can be stitched up into something like a roman shade, with magnets in the hem all the way around, and fine copper or aluminum window screen wire either on one side, or between two plies of the fabric - just make sure you provide a pass-through so the magnets can clamp a contact point (extra piece of wire, or a "tab" of the screen) between the magnet and the wall of the trailer. Cell phone frequencies are rather high, and could be comparable to the width or height of your window, allowing for significant leakage.

In some cases, apertures in otherwise closed, conductive boxes can act as antennae themselves, as is seen on some satellite waveguide antennae constructed from what amounts to a space blanket formed into a corrugated, inflatable panel with apertures etched into the aluminum or gold coating, providing both a resonant cavity, and an impedance-matched aperture.

Offline savannah kay

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Re: Question - Faraday Cage
« Reply #70 on: March 28, 2012, 07:49:35 PM »
I don't know anything about how to build a Faraday cage, except some ideas from the internet.
They mentioned simple ones using aluminum foil or mylar.
I put my cell phone inside a mylar bag sold at the Dollar Tree to keep food hot or cold.
When I called my cell phone it went straight to voice mail. I had to turn the phone off then on to get a signal.
I tried this a couple of times with the same result.
Do you think the mylar bag worked?

Offline Alan Georges

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Re: Question - Faraday Cage
« Reply #71 on: March 28, 2012, 08:21:40 PM »
Do you think the mylar bag worked?

I'd say so.  If it can stop a cell signal, that's way better than nothing.  If you doubled-up with an external box (say with a backup hard drive, etc. in there too), you're probably safe from about anything you'd care to survive.