Saturday, February 23, 2013

A Primer on Faraday Cages

There is a great deal of confusion about Faraday cages. Not only about how to build them, but also what they actually protect against. In this article, I will answer a few basic questions and perhaps debunk a few myths. 
What is a Faraday cage?  
A Faraday cage (a.k.a. Faraday shield) is a sealed enclosure that has an electrically conductive outer layer. It can be in the shape of a box, cylinder, sphere, or any other closed shapeThe enclosure itself can be conductive, or it can be made of a non-conductive material (such as cardboard or wood) and then wrapped in a conductive material (such as aluminum foil). 
Figure 1: Building a homemade Faraday cage: (a) gather tape, box, and aluminum foil, 
(b) cover box and lid completely with foil, (c) line box with cardboard and store items, and 
(d) close Faraday cage
What does it do? 
The cage shields the contents from both electrostatic fields (i.e., fields that don’t change over time) and non-electrostatic fields (i.e., fields that do change over time). It is particularly useful for protecting against an electromagnetic pulse that may be the result of a nuclear detonation in the atmosphere (a.k.a. EMP attacks). Despite rumors to the contrary, a Faraday cage is not necessary to protect against solar coronal mass emissions because the frequency content of such disturbances is at much lower frequencies—they don’t couple enough energy into small-scale electronics. Solar emissions do however disrupt radio transmissions, damage satellites, and like an EMP attack, can potentially destroy the electrical power grid. 
How does the cage work? 
The free carriers in the conductive material rapidly realign themselves to oppose the incident electric field. If the cage is made from something non-conductive, the free carriers are not mobile enough to realign and cancel the incident field.  
How thick should the conducting layer be? 
The conductive layer can be very thin because of something known as the skin effect. That term describes the tendency of current to flow primarily on the skin of a conductor. As long as the conducting layer is greater than the skin depth, it will provide optimal shielding. The skin depth is a function of the frequency of the wave and the conductor material. As an example, consider that for a frequency of 200 MHz, the skin depth of aluminum is only about 21 microns. Therefore, wrapping a box in a couple of layers of heavy duty aluminum foil (typically about 24 microns thick) provides the necessary conductor thickness to protect against high-frequency radiated fields.  
Does it matter what type of conductor is used? 
Not much. The conductivity of nearly any metal is good enough to allow the carriers to easily realign to cancel external fields. For example, if silver (the best conductor) were used in place of aluminum, the skin depth at 200 MHz would be reduced to about 4.5 microns. Of course, the high cost of silver would prevent using it for such a purpose. 
Can a Faraday cage have holes? 
Yes, as long as the holes are small with respect to the wavelength of the incident electromagnetic wave. For example, a one GHz wave has a wavelength of 0.3 meters in free space. As long as the holes are significantly smaller than that dimension (i.e., a few millimeters), they won’t let in much of the incident wave. This is why fine conductive mesh can also be used for making a Faraday cage. In practice, the lid or door usually causes the most leakage. Taping the seam with conductive tape greatly reduces this leakage. 
Can you use existing conductive enclosures? 
Yes, there are many conductive enclosures that can be used, including ammo cans, metal garbage cans, anti-static bags, and even old microwave ovens. Each has its own level of effectiveness as covered in the book, Disaster Preparedness for EMP Attacks and Solar Storms. 
Does the cage have to be grounded? 
There is a great deal of confusion regarding grounding of a Faraday cage. Grounding of the cage (i.e., connecting it to some Earth-referenced source of charge) has little effect on the field levels seen inside the box. Grounding primarily helps to keep the cage from becoming charged and perhaps re-radiating. 

Written by Dr. Arthur Bradley, author of the Handbook to Practical Disaster Preparedness for the Family, the Prepper’s Instruction Manual, and Disaster Preparedness for EMP Attacks and Solar Storms. To sign up for his free “Practical Prepper” newsletter, send an email to newsletter@disasterpreparer.com. 

6 comments:

Rob Hanus said...

Unfortunately, a shoe box wrapped in aluminum foil isn't going to protect your electronic gear from EMP.

This is very easy to prove: simply put an AM radio in your box and you can still hear the radio. Once you wrap that AM radio in enough foil to block out the signal, try it again at the base of a 50,000 watt commercial AM antenna. You'll see that a poorly-wrapped box with a single layer of foil isn't enough.

To find out more, check out:
EMP Preparedness

ConwayBob said...

Rob has a point. Tin foil has limitations, especially if it is ungrounded. Also the description of it being a fully sealed enclosure is not a perfect fit either. Metal mesh screen like chicken wire offers effective RF isolation, depending on the frequency. If the wavelength of the unwanted signal is wider than the gaps in the screen, it will be effectively blocked.
What also isn't mentioned is that for any Faraday Cage to do it's job, whatever you are intending to protect must always remain inside the cage & remain unused. You can't anticipate a EMP Pulse, so whatever you are trying to protect can never be outside the cage.

byah said...

Disappointing that right here on this very page that was supposed to clear things up for good, we already have conflicting information; Rob's comment, and the article. I see this all the time, someone says they have the correct information and you think "Finally, now I have real answers", then someone else responds claiming that they instead have the answers. I think it's time a panel is put together consisting of qualified experts to finally put this to rest. I am tired of mixed messages and hopping between theories when someone says they have the real answers.

On another note, here's something these articles never mention: It's great and all that your electronics are protected by a faraday cage. Trouble is, how will you ever know when it's safe to take them out? Should people have cheap "sacrificial" electronics in separate cages to take and out see if they get fried at random intervals?

ConwayBob said...

Too often we all seek simple answers to something that cannot be simplified, and the theories surrounding Electromagnetic Pulses and it's effects are just too broad to be boiled down to a simple definition. You make a EMP whenever you turn on a light switch. All electric motors make a EMP when they function. You touch a doorknob and draw a static spark... that's a EMP Pulse. So do we protect our electronics for that? In some instances, yes we do, in others the protections are already engineered in the design. So it all works out that Faraday Cages serve a necessary function, and in others it's a not so important. If you really feel the need to protect against a EMP just build a Cage, test it to be sure it works, and buy two of everything... one to use & one to store away for a EMP day.

Photonfix said...

I would urge you to check out this page: http://www.futurescience.com/emp.html

and go here: skip about 25 minutes into the video. These might help. There are big differences between electrostatic shielding vs RF shielding.
http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/lecture-5-electrostatic-shielding-faraday-cage/

Rob Hanus said...

I agree with ConwayBob. On the topic of EMP, you will find disagreement even among the experts, as the science and data is either really old or classified.

However, you can apply what we know about EM radiation (light, radio, etc.) and make some valid assumptions. As I often say on my podcast, do your own research so you know what information you can trust.

As for byah's question on how long to keep your items in a Faraday container, I actually do answer that in the Kindle book I have on Amazon about EMP Preparedness (http://amzn.to/PMq52B). Basically, you either need to either wait long enough to make sure that there will be no more EMP attacks. Or, as you said, have a throw-away set of gear.

EMP attacks are sudden and are over within minutes. Whatever is going to be damaged will be damaged relatively soon. However, there can be more than one attack, spaced out over days or even weeks.

Another issue with EMP information is that the same info is being spread around. No one is checking these sources. For example, it's a common myth that a MYlar bag or a microwave oven make good Faraday cages. This assumption is based on a test that is done with cell phones. You place a cell phone in either of these and it loses signal. Thus, the assumption is made that these containers must be good Faraday cages. They are not, for reasons I outlined above.

There are many more aspects to EMP that a prepper needs to consider. I know we preppers like to have clean and simple solutions, but sometimes there isn't one. I wanted better answers to EMP protection, so I did a lot of testing, and made that information available on my website and my book.

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