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18 Comments

  1. I know that there is a lot of bad information on the ‘net when it comes to EMP effects and mitigations. What is not clear to me, is why an ordinary “used” ammo can is not an adequate protection against radio frequencies, all by itself? At one point I tried calling a cell phone inside a can and was able to ring it until I sealed the lid… then silence. To my way of thinking that achieved the desired goal, even without taking the extra steps of improving the conductive properties of the gasket. If someone has done more sophisticated testing, I would be interested in seeing that before spending yet more money on a fancy gasket.

      1. Not quite every point. If you can’t get at least a quarter of a wavelength into the conductor, no significant energy will be absorbed. That’s why I’m unconcerned about the gasket — we’re talking about less than a centimeter.

        Also, remember that radio waves are line of sight, so to get around the gasket, the signal has to be a short enough wavelength to reflect off the can, reflect again off the inside of the lid, bounce back and forth there (losing energy on every reflection), make the 180 at the lid, and then reflect back through the slit greated by gasket into the case, and then, after all that, find a conductor to dump its energy into in the contents.

        That’s why I’m not worried about the gasket. And all of this assumes that the lid is not electrically bound to the case, in which case skin effect is going to prevent the bouncing, and we don’t even get that far.

  2. New metal paint cans can be had at various hardware stores. Once the sealant is cleaned off where the lid meets the can I believe that these will make a servicable faraday cage

  4. I used a metal trash can and instead of metal tape to seal the lid, I used thin strips of aluminum foil doubled over the top edge of the can itself and kept adding foil until radio signals and cell signals would not reach inside….Then I marked the lid and side of the can with a marker to orient the lid and can the same each time the lid was removed and replaced….easy to get into…however you may have to add layers of foil from time to time if you are in and out of the can very much…as with most methods we probably will not know if they are effective unless we have an EMP which I hope never happens…

  5. I would like to see some test results. For example: https://www.youtube.com/watch?v=uYWhTMmv6bs

    Personally what I’ve done is used some of those Christmas Cookie tins (large metal box, metal lid, no paint on the inside edges) to store some radio equipment, USB drives for backing up data, and more.

    Is this any better or worse? I don’t think any of us really know.

  6. If the lid overlaps the container edge it will shield out radio waves. Case in point: My I-phone fits nicely inside a Bass-pro gift card tin box. The tin box is enameled steel, inside and out. The phone does not respond when called, even though I did not put any tape or foil around the edge. Same for a galvanized steel trash can. If the lid fits tight all the way around, put a battery operated FM radio in it (tuned in, and loud) and close the lid. If it’s protecting,
    the radio will fall silent.

  7. One of the things this article seems to be missing is the concept of wavelength. Radio waves are relatively long waves compared to light. I’m not sure what wavelengths we’re trying to block here — to get around the lip of the can, the waves would have to reflect back and forth between the lid and the can and then around the tiny gap created by the compressed rubber gasket.

    So what are we talking about here? Sub millimeter? Let’s say it’s .2mm. We’re talking terahertz frequencies at that point, and you’re just not going to have the ability to build up much of a pulse at that frequency. This is trying to solve a problem that doesn’t exist. Teraherz radiation is absorbed by things as diffuse as clouds and fog, so by the time it goes through atmosphere, then your roof, then your ceiling, and whatever else is in the way (closet door, for example) it’s going to be so weak it never had a chance to start with.

    More likely, if there is an improvement, it is entirely from electrically joining the lid and the can. The thing is, you can do the same thing with a jumper wire soldering them together, or a few pieces of conductive tape. (You don’t even have to seal around the whole case, just a spot or two to connect them.

  8. ALL VERY interesting, but the devices being used in these “tests” are trying to detect a signal measured in pico-volts. A 50KV to 100KV signal is a whole nuther animal. A popsicle stick will insulate against a 120 volt AC power line, but would you trust it handling a 100,000 volt line? It is, after all, electricity. My sources suggest a cage thickness of .100″ of steel. Thinner gauges will certainly help but we won’t know of sure what the threshold of failure is until That Day. 12 gauge thickness is .109″. The utility of most of the items we are trying to protect will probably not matter much. Radios, cell phones, all worthless when their supporting infrastructure dies. Smart phones can hold files containing information, but their value as a communicator is lost. The many comments on the folded lids are solid. Even radios placed in file cabinets were protected during simulator tests, despite the generous gaps in the drawers. Small devices are inherently more robust….they have very little antenna footprint, and are not connected to the grid. i-Phones are shown to work after exposure to threat levels of insult with NO cage around them. Depends on the architecture of the device. It’s far cheaper to design in surge suppression than to retro-fit protection.

  9. It is, after all, electricity.

    That’s just it. It’s not electricity — it’s EM radiation. It doesn’t work like electricity — it works like as much like light (which is also EM).

    You aren’t trying to ground it. You aren’t trying to insulate against it. You’re trying to use skin effect to have the EM pass around the cage.

    We’re doing things like electrically joining the pieces so that the EM sees it as one piece. You’re absolutely right about the filing cabinets, because the overlapping metal sections (where the drawer fits into the casket) create radio traps that reflect the signal back out rather than allowing it in.

    It’s a real esoteric subject that I really only wrapped my head around once I started making antennas (which is the opposite task, trying to absorb the EM rather than shuck it) and I can’t come up with a good, plain english analogy. The best I’ve come up with is that you are making a boat to ride out the EM storm, not trying to build a wall to stop it.

    1. Everything on the grid will be fried because a magnetic field passing through a long wire is a huge antenna. Antennas convert magnetic radiation into electricity.

      The objective is to not allow it to become electricity…

      If you have two 2-way radios in an effective Faraday cage, you can have comms, post event. If you have shortwave comms, you can communicate with parts of the globe which were not hit.

      So, if I have a used ammo can with reasonably good electrical contact between the lid and the base — does that construe an adequate Faraday cage? What other tests can we use to determine the effectiveness … beyond the basic cell phone ring test?

      1. What other tests can we use to determine the effectiveness … beyond the basic cell phone ring test?

        Like everything in testing, the devil is in the spec. What are we testing? Wavelengths shorter than the box itself we can discount, because if you can’t get a full length into it, it can’t dump any energy. SO right off the bat, we can ignore any wavelength longer than 40cm. That puts us at about 800mhz. The trouble then is getting the antenna right. Once you are into terahertz lengths, it doesn’t matter, because we can’t even measure frequencies that high digitally (we have to do tricks like dividing the freq, so we’re just adding more complexity.)

        So we’re talking really about gigahertz freqs. The only thing most people have that operates at that level is cell phones and wifi. That suggests that the cell phone ring (about 900mhz depending on your carrier) and wifi (2.5-5 ghz depending on your router) are our best tests. Put your phone on Pandora on wifi, and stick in the box. If the music stops at the end of the song, and you can’t ring your phone, that’s probably the best test you can manage. If the energy is low enough that it’s in the noise, it’s low enough to not wreck your electronics.

  10. It is useful to note that a nuke provides a FULL spectrum of EMR from the shortest to longest of wavelength at very large magnitudes for a very short time. I believe that covering my SW radio and Laptop and Hard Drive in bubble wrap pouch and then Aluminum Foil and then again bubble wrap pouch and then again in Aluminum Foil to be place within the metal box of whatever design will provide the best protection for my necessary backup electronics. I keep my backup electronics in a Job Box which has been altered to seal it up and lined with cardboard. I back up once a week and consider all my desktop and day to day toys to be sacrificial- after all, what do I know about the timing of a mushrooms appearance.

  11. Because it takes so little extra, I double up the protection hoping to have nearly 100% success. The large pop corn tins sealed with the aluminum tape are my favorite. The electronics are contained in a thick insulation material, and are wrapped in aluminum foil with the seams taped with aluminum tape, and is protected from the can with another thick layer of insulating material. Desiccant is used inside the packaging containing the radios.

  13. Once we are hit by an effective man made EMP and our country is foundering (by N Korea?), there will other enemies (Russia, China?) who will want to keep us from recovering. Therefore there will be follow up EMP strikes. These will probably be weeks and months later. What then? It will be risky to have any equipment out of it’s protection.

  14. Openings and gaskets are an important aspect of having an enclosure that will shield against EMI and the shielding effectiveness (SE) is a function of frequency.

    1 simple 1 page reference on openings:
    http://www.omegashielding.com/info/tech-data-minimizing-the-effects-of-openings

    And an 88 page reference for the phd electrical engineers:
    http://www.emcfastpass.com/wp-content/uploads/2017/04/Engineering-Aspects-of-Electromagnetic-Shielding.pdf

    And if you a glutton for punishment, a 200 page reference on using gaskets for shielding.
    http://www.robertmckeown.com/pdf/emi2.pdf

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