Why might you want to build a Faraday Cage? By the way, Faraday Cages are also called Faraday shields, RF cages, or EMF cages. One common reason to build a Faraday Cage is for a variety of testing purposes. There was one in the engineering department at the company where I used to work. Big enough to walk inside, it had a work bench in there… All sorts of industry use them.
Another common reason among the prepper-preparedness community… For ‘just in case’. To protect select electronic devices in the event of an EMP (electromagnetic pulse).
Though hopefully very unlikely, several nations have EMP weapons that could be used in War. So, logically, one might take some precautions in case such insanity is ever unleashed.
Although there will be much bigger concerns at first, following an EMP event (to say the least!), there are some devices that would come in handy afterwards. Just to name a few, things like handheld 2-way radios. Ham radio equipment. A portable battery operated shortwave radio. There are lots of practical things that would be useful here. But that’s not the purpose of this post (I’ve got other articles on that topic).
What is a Faraday Cage?
It is an enclosure formed by conductive material, or by a mesh of such material.
The enclosure blocks external electric fields. It does this by channeling electricity along and around, but not through, the conductive material. Thus, there’s a constant voltage on all exterior sides of the enclosure. Since the difference in voltage is the measure of electrical potential, no current flows through the space.
A Faraday Cage operates because an external electrical field causes the electric charges within the cage’s conducting material to be distributed such that they cancel the field’s effect in the cage’s interior.
Faraday cages are named after the scientist Michael Faraday, who invented them in 1836.
Do Faraday Cages Have To Be Grounded?
Contrary to what many believe, the Faraday Cage does not have to be grounded to do its job. If you have trouble with that statement, do some research (web-search) and you will discover the physics. The Faraday cage keeps the charge (EMP) on the outside (during that fraction of a microsecond till its gone). While a grounded Faraday cage will simply bleed the charge to ground during that same fractional microsecond. In either case, the electronics inside are protected (to the extent of the construction properties of the cage itself).
Note: Faraday cages are intentionally grounded when the intended use is to operate an electronic device inside (e.g. a Faraday enclosed room) while the electronic device itself requires a proper ground (and proper isolation techniques from the incoming power source).
How-to Make a Faraday Cage
Now that we know what a Faraday Cage is, can you build one yourself? Yes, it’s easy.
I’m going to keep it simple rather than getting into formulas for design, dB attenuation properties, etc.. Because the concept is simple.
You’re looking for (or building) a electrically conductive enclosure, all the way around. The most common thing that preppers use are ordinary galvanized metal trash cans with its metal lid. The only thing you have to do is not let any of the devices inside touch the metal in the can (read the next paragraph). You need an insulator. You can accomplish this a number of ways. Maybe you put everything in a cloth sack. Or heavy duty trash bag. Put devices in Ziploc bags… Or, more common, you line the interior of the trash can (and the bottom) with cardboard. That’s it. Done.
Do You Need To Insulate The Inside Of A Faraday Cage?
“If you had a Faraday Cage made of a perfect conductor that is also perfectly sealed, then you don’t need any inner non-conductive lining to insulate it from the items inside.” Why? because technically, the outer skin layer of the Faraday Cage exterior will block all of the outside signal from getting to the inner surface of the container. However, that’s with a perfect situation. Your DIY Faraday Shield will not be perfect. So, yes, you should provide some sort of isolation between the interior and your devices, just in case.
What I’ve just describe is a minimalist design. A well sealed galvanized trash can will provide about 50 dB of attenuation. Will that be enough protection? Maybe. It depends. Your location (distance) relative to the EMP source. The field strength of the EMP. There’s more to it, but the aforementioned two factors are quite important, for example.
By comparison, the U.S. military requires a minimum EMP hardening of 80 dB in the plane wave field from 10 MHz to 1.5 GHz in accordance with MIL-STD-125-1 and -2.
dB is a tricky scale. It’s logarithmic. The difference between the metal trash can Faraday Cage and the U.S. military spec is 30 dB. That’s 1,000 times more attenuation.
Get More dB Attenuation With Better Seals
So if you want more EMP protection than the approximate 50 dB as noted above, you can bump it up a notch…
It’s all about the conductivity, better electrical seal on gaps, overlap… In other words a perfectly tight conductive enclosure will provide more dB attenuation than otherwise.
A new galvanized trash can versus an older one which may have a poor fitting cover (dings and dents).
Here’s an idea… To enable a better ‘electrical’ lid fit, cut some aluminum window screen about 6 inches wide and as long as the circumference of the can. Fold the metal screen in half, length wise, and then place it around (and fold over) the lip of the garbage can. The lid should then fit snugly over the screen as you place it on the can. This ensures better electrical conductivity around the circumference of the lid.
Conductive Foil Tape
The window screen method might be a bit too tight fitting, depending on the fit of your specific can and lid. So instead, you might consider sealing over the lip (as described above) with a conductive adhesive tape. It will crinkle a bit as you work around the perimeter, but the point is to simply get a more snug fit with the lid, and better electrical conductivity at the lid-to-can seal.
Or, you might simply seal around the outside of the lid itself, once it’s put on the trash can. Although you will have to cut it to regain access.
Here’s an example of copper foil tape. You could also use this kind of conductive tape in the construction of your own Faraday Cage / box. Such as sealing the corners with the conductive walls (depending on your materials), etc..
They make the following in 1″, 2″, or 3″ widths.
Copper Foil Tape with Conductive Adhesive
(view on amzn)
Any conductive enclosure can act as a Faraday Cage. Food metal-to-metal contact at all seams is imperative.
Other common ideas for a do-it-yourself Faraday Cage include cookie tins, popcorn tins, ammo cans (beware of rubber gaskets interfering with seals), cardboard box exterior lined with Heavy Duty aluminum foil (watch for tearing), foil-wrapped Ziploc bag with device inside… You get the idea?
Here’s a common question:
Is a Microwave Oven a good Faraday Cage for EMP?
Why? Although a microwave oven is a very good Faraday Cage for the Microwave Oven frequency (2.45 GHz), it is NOT good at attenuating other frequencies within the fairly broadband spectrum of an EMP. Not enough…