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DIY Faraday Cage

Building Your Own Faraday Cage

EMP protection for electronic devices
Friday, October 3, 2014, 2:58 PM

A Faraday cage is an enclosed space with an outer layer that conducts electricity. The physical shape of the Faraday cage does not matter: it can be spherical, cylindrical, or a box. Either the cage itself can be made of a conductive material, or the cage can be built of a non-conductive material such as wood and then covered in a conductive material.

The conductive material can be as simple as several layers of aluminum foil, which makes constructing your own Faraday cage a fairly simple and inexpensive affair.

What are Faraday Cages Used For?

Faraday cages are designed to guard whatever is inside of it from excessive levels of static and non-static electricity. This can be accomplished either by reflecting incoming electric fields, absorbing incoming fields, or creating opposing electrical fields.

The Faraday cage can help to protect whatever electrical equipment is contained within it from the kind of electromagnetic pulse (EMP), it’s a good practice to keep your emergency electronics such as radios and GPS devices stored in a Faraday cage so they are not incapacitated in the event of an EMP.

How Does a Faraday Cage Work?

Incoming fields are canceled when the free electrons in the conductive material on the Faraday cage instantaneously realign themselves and block the incident electric field.

For this to work, the cage has to be made from a conductive material; otherwise, the free electrons are not sufficiently mobile to realign themselves. The layer of conductive material can itself be quite thin. This is thanks to the “skin effect,” which is a term that describes the inclination of electrical currents to move mainly on the outer layer of a conductor. Provided that the conductive layer is more than the skin depth of the material, the electrical shielding of the Faraday cage will be outstanding because there will be very high levels of absorption loss.

The skin depth is a function of the material the conductor is made of and the frequency of the incoming wave. Typically, wrapping your Faraday cage in several layers of heavy-duty aluminum foil will give you the needed skin depth to protect your electronics from high-frequency radiated fields like the kind generated by a EMP.

Building a Faraday Cage

The material you use for your Faraday cage does not have much influence on how effective the cage will be at protecting your electronics from high-frequency fields. Virtually any metal has the necessary conductivity to allow free electrons to realign and cancel out incoming electric fields.

Certain metals, are more conductive than others, which gives them a reduced skin depth – for example, at 200 MHz, silver has a skin depth of less than five microns, as compared to aluminum, which has a skin depth of 24 microns at the same frequency. But on a macro scale, that difference is negligible, which is why you can use heavy-duty aluminum foil, instead of far more expensive materials.

Your Faraday cage can have small holes in it, provided they are not too large with respect to the wavelength of the incoming electromagnetic wave. This is why you can also use fine aluminum mesh to build a larger Faraday cage. For example, a 1 GHz wave has a wavelength of 0.3 meters in space.

Generally with these kinds of mesh cages, the cage door is typically the part that causes the most leakage, but this can be fixed by taping the seams with conductive tape.

You can also use existing metal containers as Faraday cages, including metal ammunition boxes, metal garbage bins, anti-static bags, and even unused microwave ovens. Each of these has its own level of effectiveness: the main concern is that gaps and seams are minimized to reduce leakage.

You do not have to ground your Faraday cage in order to protect the electronics contained within, although doing so will help to keep the cage from becoming charged and possibly re-radiating charge, which could be dangerous if you touch it.

Large Faraday Cages

If you want to build a larger “shield room,” as engineers refer to rooms that are essentially large Faraday cages for storing electronics, you can do so by covering the inside of a small room or closet with several layers of heavy-duty aluminum foil.

Overlap all of the seams and tape them with regular cellophane tape. Cover all outlets, light switches, and other conductive breaches with aluminum foil, and do not plug anything into any outlets. Once the floor is covered in foil, place a piece of plywood over it so you do not damage it by walking on it. Such a room can store all of your emergency electronics and protect them from incoming high-frequency radiated fields.

Have You Made a Faraday Cage?

Have you made a faraday cage or room? Have you ever had to use it? How did it turn out?

~ Clayton Krebs


Clayton Krebs is a preparedness consultant and team member of The Ready Store.  He writes informative articles and information for the ReadyBlog, the Ready Store's blog and educational section pertaining to topics of the economy, resiliency, and preparedness issues. 

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9 Comments

Bytesmiths's picture
Bytesmiths
Status: Silver Member (Offline)
Joined: Apr 28 2008
Posts: 201
Been there, done that...

"If you want to build a larger “shield room,”... you can do so by covering the inside of a small room or closet with several layers of heavy-duty aluminum foil."

Seems like a lot of work. I'll stick with my tinfoil hat, thank you. :-)

 

Arthur Robey's picture
Arthur Robey
Status: Diamond Member (Offline)
Joined: Feb 4 2010
Posts: 3936
Good reminder, thanks.

I will have to make a cage for all the electronics on my yacht . The source of the EMP will most likely be lightning.

I would hate to lose the lot.

jonesb.mta's picture
jonesb.mta
Status: Silver Member (Offline)
Joined: Jun 11 2008
Posts: 126
Microwave

A microwave oven is a Faraday cage and there are old ones everywhere. We keep a backup hard drive in an old microwave.

rmurfster's picture
rmurfster
Status: Bronze Member (Offline)
Joined: Apr 17 2008
Posts: 84
A microwave is not a good

A microwave is not a good Faraday cage. A simple test is to place a battery operated radio into your cage that is playing. When you put it in a Faraday cage, it should block the reception. You will find that a microwave doesn't block it, but a properly built Faraday cage will.

Bytesmiths's picture
Bytesmiths
Status: Silver Member (Offline)
Joined: Apr 28 2008
Posts: 201
Protection against WHAT?

I'm a bit confused about the interest in Faraday Cages. What do we need them for, exactly?

I worked professionally with such things in the 80s, and as I recall, professional units costing tens of thousands of dollars weren't all that good. I seem to recall the "shake-n-bake" one we used for testing military gear provided 30 to 40 db of isolation.

Given that a nuclear EMP might generate 40 or 50 dbW of power, you're still going to have watts to tens of watts getting into your milliwatt-sensitive electronics.

As for lightening protection, the very best thing you can do is disconnect the device, at the device. No amount of shielding will protect it if it still has wires running outside the cage! And if you do disconnect it at the device, no amount of lightening short of a direct strike is going to induce much current into it.

Keep in mind that, for current to flow, you need a circuit. Electromagnetic force can cause voltage to appear across opposite ends of a conductor, but unless it is connected to something, current won't flow. Disconnect your long conductors going to your device, and little EMF will be induced.

My experience as a professional RF Engineer is about 30 years old, so perhaps things (outside of basic physics) have changed since then. (As a licensed ham radio operator, I do still keep in touch with some RF engineering stuff.)

So I'm a bit skeptical, and I'd like to learn more about why people think a cage is a useful thing.

(Incidentally, getting into ham radio can be a valuable thing for "doomers" and "preppers." You'll learn a ton about electronics; you'll learn emergency communication protocols, and you'll meet a lot of people with a lot of knowledge about electronics and radio.)

Arthur Robey's picture
Arthur Robey
Status: Diamond Member (Offline)
Joined: Feb 4 2010
Posts: 3936
Thanks Bytesmiths.

Lightning generated EMP has been a worry for me. Think of the Aluminum mast on a flat 2d field of salt water.

I do know that electronics have been wiped out by lightning on yachts. The dΦ/dt must be enormous near the current path. I am wondering if I should shield the copper earthing conductor with steel, with a drain on the lower end to the Keel?

Bytesmiths's picture
Bytesmiths
Status: Silver Member (Offline)
Joined: Apr 28 2008
Posts: 201
I've only had a little marine

I've only had a little marine experience. That mast certainly can raise some induced potential!

I don't think shielding the ground will do any good, as they're going to be at the same potential.

Unplugging the power and antenna AT THE DEVICE should keep it safe, though, and is certainly easier than building a cage!

Arthur Kile's picture
Arthur Kile
Status: Member (Offline)
Joined: Nov 20 2014
Posts: 1
Garbage Bin

I have a metal garbage bin in my metal buildings detroit. I use it to store some of the electronics. It works fine for me. The idea of a shield room seems interesting. I'll try building one.

Boomer41's picture
Boomer41
Status: Silver Member (Offline)
Joined: Nov 30 2008
Posts: 132
Metal Boxes

Any metal box with a tightly fitting lid will make a good faraday shield. I have in mind cookie tins

This will do the job http://www.amazon.com/Danish-Butter-Cookies-4-Pound/dp/B00196P9PA/ref=pd_sim_gro_5?ie=UTF8&refRID=0VV52X52G1X6DYQKNC66  although you will have to eat the cookies first.

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