Faraday Cage galvanized trash can

Faraday Cage For The Prepper, Just in case…

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).

Screen Material

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?

Answer: No.

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…

[ Read: Nuclear EMP Components E1, E2, E3, and what they mean… ]

[ Read: Will EMP Destroy Electrical Devices NOT Plugged Into The Grid? ]

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19 Comments

  1. Ken, I have 2 large cans and 2 small cans, all sealed with metal tape. Been thinking about using a job tool box to make next one…first to put a ham radio set up with handhelds and also to protect scoped rifles. Any thoughts or ideas ?? Jim

  2. Jim S,
    a job box would make an excellent Faraday cage, just tape up the seams with some foil backed tape.
    good luck!

    1. A lot of the new freezers are primarily plastic, so I guess it would depend. One of the old, old refrigerators maybe, if you sealed it up good.

  3. I wonder if that silver colored metallic tape used for ductwork will do the job of insulating properly?

  4. Is it true old microwaves could be used as a faraday cage? A friend told me they bought two of the old kinds. Keep their phones in there at night.

    1. Nope. Part of the problem is mentioned above. The other part is that the “cage” isn’t complete. It has gaps; turntable, ventilation, etc. When I tore one apart I was amazed at how many gaps there were.

      1. Lauren,
        dw forgot about some sweet potatoes in the microwave for a few days. when i discovered them there were about a hundred fruit flies in there with them. if a fruit fly can get in there so can a electrical pulse : )

      2. Believe it or not a microwave oven is not a good faraday cage because the holes in the door that allow you to see whats in there are sized for that microwave frequency wavelength. All other RF frequencies will pass through the door with ease.
        A large (new) garbage can with a tight lid, inside put a slightly smaller plastic (kitchen) pale to insulate from the sides. Even go the next step for the smaller items by placing them in metal canisters with tight tops inside the above layers.
        Maybe pressure wash some paint cans or such. (too much thinking out loud here, lol)

  5. Bogan,
    it’s what i use. i hope it works! people have tested this with electronic testers and it works for that, but i have not heard of anyone testing them with actual nuclear bombs other than the US and Soviet tests in the mid 50’s .
    i think they make a copper tape as well. just make sure that your metal surfaces are clean, clean, clean down to the metal before taping it up. lots of info on youtube.
    just my 2 cts.
    good luck

  6. When I built my shop, I tied the rewire and rebar in the floor to the steel frame. The steel siding is grounded to the frame and the steel doors are ground strapped to the frame. The windows have wire mesh screwed to the frame. I don’t know how much protection it provided, but cell phones won’t work inside with the doors closed nor will AM or FM radios.

    1. I just had a weird (probably incorrect) thought. Could solar panels be protected by surrounding them with wire mesh screening? I know the sunlight collected would be limited.

      1. Lauren,
        I’ve read the opinion of many who suggest that generally speaking, solar panels are not terribly vulnerable. And I tend to conquer to an extent. Their biggest vulnerability are their blocking diodes (typically located behind where the positive and negative leads are located. Having those spares may be prudent. I have also added fast-response surge protectors all over my solar system. And, grounding the panel frames themselves into the earth (ground rods).

  7. Would it help increase the dB Attenuation by nesting several galvanized trash cans inside each other of different sizes (with tight fitting lids)? Maybe even finishing it off with a cookie tin inside the smallest trashcan? Hmmm, now you have my tiny little brain thinking!

    1. Johnny Blaze,
      I had looked into that. Evidently, it won’t help all that much more (compared with a well-designed Faraday Cage enclosure to begin with). However, from the perspective of do-it-yourself (potential problems thereof), a nested arrangement would help ensure that “if” there’s a ‘leak’ in the first containment, then it’s good insurance to have a second level nest. So, perhaps one might consider the nesting arrangement for extra high value devices? For example, lets say you’ve shelled out the big bucks for quality night vision… a massive force multiplier… I wouldn’t want to lose that!

      Thanks for the comment.

  8. Can an EMP be directed by enclosing the source inside a Faraday cage open on one end?

    1. Markkens,
      Yes, sort of. Rifles that produce an electromagnetic pulse are fielded by government agencies to take down drones. Very small scale compared an EMP from a nuclear weapon, though.
      I believe there could be several countries fielding directional emp satellites , as it is old tech. Look up Star Wars Initative by the US govt.

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