EMP Methods and Preps – Part IV

For the other three posts in this series, see….

This article deals with an important preparatory item necessary to the survival of electronic equipment and spares that you have decided you can’t do without. That item is commonly known as the “Faraday Cage” (Wiki Article) . For purposes of mitigating the effects of a grid-killing EMP strike, if prefer to call this an EMP Cage.

Before I get into some of the things I wish to share with you, I want to address one of the proposed designs that a cursory search will turn up. The chicken wire /mesh /screen “cage”. Each of these designs have a couple of things in common. First, they are variations on Mr. Faraday’s actual bird-cage style design, which nominally had the purpose of redirecting charges from generators around the outer dimensions of the enclosure. This protected the items and life forms inside from shock. Very well. It even reduces Rf signals to some extent. Even better. But it is the second common aspect that convinced me to stay well away from such useless projects. Slot antennas and wave propagation. The energy from a nuclear EMP device ranges across a very wide spectrum, is only marginally predictable, and has not seen action against today’s electronics. Because of this, I can not advocate the use of anything other than a sealed metal enclosure for maximum protection. No openings and no pass-through conduits – nothing but a wall of metal facing the pulse from whatever direction it wishes to approach. Near the middle of this article, holes in a cage are discussed.

Building mesh “cages” can actually take quite a bit of time and energy. Starting off with an existing metal enclosure is a very efficient way, depending, of creating a sealed container. I say “depending” because some require more preparation than others. What I have done here is collect a few YouTube videos produced by various individuals with differing backgrounds. I will present them, and add comments below each window. There is something to be learned from each of them. When that short exchange is complete, I will offer some closing thoughts.

The Trash Can EMP Cage

I would not rely on the concentricty of the can’s rim and its mating surface inside the lip of the lid. If you watch closely, you will see that both surfaces are not truly round or very close in shape. The “tightness of the can’s id is created by the inner lip, which has an interference fit with the can. They work against each other. The design of such cans only seeks to keep the lid from blowing off in the wind, or coming off if the can is tipped over.

The Wooden Box and Foil Method (Author Alert)

Aluminum foil is very easily torn in just about any situation where it is being moved or manipulated. Even picking up that box the wrong way could cause one if its sharp edges to penetrate the foil if the foil slides on the surface of the wood. He might do better to spray the box with 3M Contact Adhesive, of the type used in art projects, to give the foil some grip, and actually force the wood and foil to work together. Placing a heavy container inside a thin and mobile foil cover is asking for trouble.

He makes an excellent point regarding the metal tape. Aluminum tape, of the type used in metal ductwork, has metal on one side, and glue on the other. There is no metal-to metal contact. Under the right conditions, that very small separation might act as a passage. Conductive tape is a must when seeking to secure metal components together in a truly conductive way.

He shows how to make a super cage. His premise is that a 100dB reduction in signal is sufficient to ward off EMP energy. The relation between Rf signal strength and the radiated energy of a pulse is necessarily ironclad. Many cage builders state that blocking strong and broad signals successfully is better than not. True. It is. But it should not be the final say. Again, a sealed container is what we seek.

Another Garbage Can Cage

Again, aluminum tape has a glue side. He could have used foil and then the tape to secure its position it. Alum tape will hold its shape fairly well, and if the application is a bit sloppy over the top of the foil (not the edges), the creases will actually add resistance to deformation.  As for the roll of foil, I am a fan of the rolled-tube foil gasket. Using a sheet of foil longer than the circumference of the needed gasket, roll it into a tube of about five or six thicknesses, about 1 1/2″ in diameter. Gently press it over the lip of the can, forming it into a loose cap. When the lid is pressed down on top of it, the foil will compress, doing more to match the irregularities of the two mating parts than the parts themselves. He could then stuff rolls into the gap between lid edge and can body as a secondary measure.

Good vid. You’ll notice he questions the  Rf signals testing methodology.

The Paint Can Cage

CommsPrepper made this vid. Make sure any epoxy or coating that may exist on the can is removed. Remember, we want metal-to-metal contact. Otherwise, I expect this cage to have a 100% compression seal.

Cage in a Cage, with Sealed Penetrations

Double can design. Cage in a cage. Great for extremely high powered localized EMP strikes, such as experienced when a nearby device is detonated in the air over a target. The region near an aerial detonation of a nuclear warhead can experience a strong local EMP burst that might overpower even a sealed cage. I frankly don’t understand how that can happen, but several trusted individuals have stated it is a concern. I think the best reason for going with a nested approach is to reduce the effects of a screwup in the sealing of the first cage.

He needs a lid seal. A gap can potentially create a slot antenna which can transmit energy to the interior of the can, which then becomes CHARGED. Bad times ahead for that individual…

Doomsday Prepper, the Faraday Fail

What not to do. Doomsday Preppers getting ready to be surprised on Doomsday. No seal in that lid. At all. A locked box won’t keep out pulse energy. A SEALED box will. The painted edges and the lack of a tight fit should have stood out to you. Once again, Doomsday Preppers isn’t really interested in instructing – just showcasing and profiting.

Some Thoughts

The rivets that anchor handles to can bodies and lids are not always tight. Penetrations of the shell are not a good thing. They can be sealed in a few ways.  In one video, we saw how they were removed entirely, and the openings covered inside and out with foil that was taped in place. I like this method. The only thing I would alter is the type of tape. Toss the plastic tape and use conductive tape, or even aluminum ductwork tape. Just be sure the foil patch is in excellent contact with the metal around the area in question. Scour it with light sandpaper (120 grit or so) to enhance the contact.

Another way to deal with rivets is to solder them closed. A torch and some plumber’s solder can be used to fill the holes. It will try to run through and out, so an aluminum backing might be needed if the gaps are large.

As for sealing that lid. You might want to consider a good “permanent” seal. Imagine a can inverted, with the components safely inside. The lid’s lip is facing upwards, and the can body sits inside. In this position, you can fill the gap with solder, and create a cheap weld that creates a 100% seal. A small butane torch would be needed to destroy the seal when retrieval is desried, but this is easily done with the cage on its side. The melting solder will run down and out like water.

Inside the Cage

Your electronic devices are subject to corrosion due to moisture. Every cage needs something inside it to protect devices from moisture. If the cage experiences wide temperature changes, moisture will condense on interior items as the temperature rises. This is because heat liberates moisture from the air, and the cooler items, yet to rise in temperature themselves, will allow surface condensation. Metallic surfaces do this best. Unfortunately, this will cause issues inside electronics. Cages with faulty seals may draw in and trap moisture during weather changes.

To avoid this, seal the cage with desiccant included. Toss in an O2 absorber for good measure. It will eliminate oxygen and work to prevent rust and corrosion.

Cardboard will absorb moisture. It may not do it quickly enough to protect devices stored bare. If you use cardboard as a insulator, be sure to not rely on it alone for protection. Every item in the cage needs to be wrapped in a non-conductive material that will not sweat. Cloth works. Paper is a good wrapper. Once covered with an absorbent, wrap it in foil, an ESD bag or some other non-porous wrap. We do not want the absorbent wrap to come into contact with the cardboard liner.

Some people use rubber padding as the cage liner. This is good from more than one standpoint. It won’t deteriorate like cardboard will in the presence of moisture. It has impact cushioning properties. It can be built up in thickness very quickly and conforms easily to a round container. One bad thing about it is that moisture will stay on its surface, ready to wick into anything that comes along. If you use a rubber liner, be sure to include desiccant! also. Line the lower padding with a towel or rag.

Ammo Can Cages

Ammunition cans incorporate a rubber gasket seal in the lid. It creates a nice tight air seal, but also a perfectly spaced gap between the metal lid and body. Slot antenna. Energy leak.

If you want to convert a can, remove the gasket and sand both mating surfaces to bare metal. Create a metallic crush gasket and work it into the old gasket flange, or lay it over the lip of the can body. Be sure the gasket is thick enough to require a good amount of force to close the thing, but not so much that the lid deforms. Metal-to-metal!

Small Cans

Smalls cans used to sell cookies, baked goodies and such generally have a much better lid to body fit than trash cans. They are small, and the lacquer coating can be removed fairly easily. They are good for small items such as thumb drives, automotive computers, phones, PDAs and small radios.


Don’t. Grounding is necessary to eliminate a charge that is created when the generator is inside the cage. Charges from the outside do not penetrate a sealed box. But a melted or disfigured box can be compromised. The E3 component of a pulse can travel through the earth to its target. Grounding your cage simply provides it with a path to your equipment. Don’t do it.


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