testing a Faraday cage

Discussion in 'Off Grid Living' started by thermax, Oct 28, 2011.

  1. thermax

    thermax Monkey+

    I built a small test faraday cage this afternoon, about 12X12 on a plywood board. What I noticed was that the radio I placed in it did in fact loose its signal. I also noticed that when the radio was moved near the wire it picked up signal again. I'd slide it over to the middle and it would loose signal. It appears that if the object being protected is to close to the wire it may not be protected at all. Is there any rule of thumb as to how big to build a cage and how far away the electric piece needs to be away from the wire? Example, a 1 square foot short wave radio needs to be centered in a 36X36 cage. Just because a radio signal is blocked, how does that compare to several thousand volts from a EMP. Thanks
  2. dragonfly

    dragonfly Monkey+++

    It's NOT the volts...it's the assorted and varied wavelength's!
    The EMP is as fast as light, 186,000 miles per second. As long as the radio inside is NOT touching any part of the metal screening, it will be safe.
    The screen MUST have NO openings any larger than 1mm in any direction.
    I used aluminum window screen, then for safety, I re-covered it again.
    Mine has wood shelves and the radios are 3 inches in any direction from coming into contact. They also set on rubber or silicone standoffs.
  3. -06

    -06 Monkey+++

    Have my old "Ham" rig, CB base unit, and some hand helds in a 3' long sheet metal box. Everything is isolated from the box and from each other. Will know one of these days if it works.
  4. thermax

    thermax Monkey+

    That’s the reason you used a couple of layers to help make the holes in the screen smaller ? Are you at all worried about the lacquer coating on the screen? I'm also considering these bags as well 24 x 30" Reclosable Static Shielding Bags S-7654 - Uline I checked one of my generators and found that I can remove the face plate and harnesses and put all the electronics in put them in one of these bags. Then reinstall them later.

    I just found this, you can purchase rolls of anti static materials, I wonder how this would work? http://www.uline.com/Product/Detail/S-6272/Static-Shielding-Bags/36-x-200-yds-Static-Shielding-Rolls
  5. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    All Radio and Pulse Energies conform to the Inverse Square Law of Physics. if you double the distance, the signal is reduced by the Square. Since the EMP Pulse is postulated to be in the Megavolts/Meter, for every mile you put between you and it reduces the Pulse Energy by the Inverse Square. Same is true for inside the Faraday Cage. The bigger the Cage the farther you are from the shield, and the more Isolation, you get. This is just Basic Physics
  6. STANGF150

    STANGF150 Knowledge Seeker

    Aluminum works? I thought it had to be Copper!!!
  7. ghrit

    ghrit Bad company Administrator Founding Member

    One thing to bear in mind is the shielding effect from screening size. Wide openings will handle longer wavelengths easily, but shorter ones will waltz right in and dance on your mini circuits. Ask me what the wavelengths are in a given EMP, and I fail open. But I do know that solid, rather than screen, will shield all wavelengths. At least one military installation solidly lined the rooms where the sensitive gear was installed. (This is from well over 20 years ago, so I'm not up to date; but that was the reasoning behind the solid sheet liners.)

    Note also that some radars back then also used wave guides to steer the active pulses thru a conduit from the emitters to the air. Meaning, in essence, that pulses need not travel in a straight line, so if there are gaps in the shield, leaks WILL happen, and if there is a reflecting surface (like rebar in a concrete floor that behaves like a bedspring antenna) pulses can come at you from unexpected directions. I'm not enough technically astute to tell you how big a gap can be, but gaps happen around doors without rather specialized door seals. The inverse square law applies from the leak to the sensitive stuff, so if a tiny bit gets in, your gear could be far enough away to attenuate the pulse to inconsequential levels. Or not.

    Grounding the shield has been hashed and rehashed, and I have no opinion on need. But I do know that the installations in question were grounded from hell to breakfast, multiple paths and multiple connections, intended to handle HUGE induced voltage spikes. At that time, research indicated more is better.

    Do I know what I'm talking about? The observations are accurate, the theory is fuzzy here and there.
  8. Illini Warrior

    Illini Warrior Illini Warrior

    Conductive Metal

    can be steel also ..... just needs to be able to conduct electricity/charge
  9. tulianr

    tulianr Don Quixote de la Monkey

    I had the opportunity to observe a National Security Agency operations building being constructed from beginning to end (I was escorting building contractors in my off hours within the security area). The main concern for the NSA was keeping the electromagnetic radiation in, rather than out, but the principle should be the same.

    Once the walls and roof was in, all interior surfaces were sprayed with a metallic substance, which was a goldish-copper color, but I was told that it was zinc. It seems that a similar product would provide some flexibility in the construction of faraday cages, such as the product at this link:
    Caswell Inc. - Copper Conductive Paint and Sealer -as long as the coated surface was grounded. It would allow you to construct the "cage" out of almost any material, and just coat the interior.

    Am I smoking crack, or does this seem reasonable?
  10. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    No, You are correct... It really doesn't matter what the Thickness is, as long as it does NOT have any holes in the Conductive Material, either before, or made by the Electronic Pulse. So a solid layer of Conductive Paint is as good as a Screen, or Solid Wall.
  11. dragonfly

    dragonfly Monkey+++

    I chose aluminum for several reasons...1) cost, 2) weight, 3) availability!
    Copper, brass, silver, even gold will work, but I can't afford any of those!
    Then you have the proper way to ground also . One of the most important things to deal with. (Not connected to water pipes or any solidly constructed bldg, or large metallic concentrations). EMP charges have a habit of being "soaked up" or "concentrated" by large or dense metal structures.
    One of the "IFFY" things, is all dependent on the way the EMP is turned loose. IF it's a on the ground detonation, that's a mild thing ( providing you are distant!) If it's up in the ionosphere, it's a hard thing to call as it may last for several hours bouncing around up there. My cages are basically made of 2 x 2's with the aluminum screen stretched over them and stapled in tight. The door is covered on all sides (wrapped) so when it closes, there are no holes other than the screen itelf. I doubled the screen as it was imperative to secure the cages. Insurance! I used peg board for the shelving inside and additional 2 x 2's for shelf braces. All the hardware is brass. Mine are 2' x 2' x 3' and 2' x 2' x 4' You can even go one further and place a layer or 2 of aluminum foil over the cages, which serves 2 purposes:1) makes sure there are NO openings to allow the EMP waves inside, and 2) keeps any dust and dirt out ( which can easily transfer electrical charges)
  12. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    You can buy Zink primer from automotive paint suppliers. It is usually copper/green in color. Expensive and not something for the average guy to spray, but it is for water resisting auto bodies before primer. High tech and pricey
    dragonfly likes this.
  13. ghrit

    ghrit Bad company Administrator Founding Member

    Frankly, I'm suspicious of thin film coatings for this purpose. I don't think they can carry much current in the event of a large burst. For anti static, might work very well.
  14. Falcon15

    Falcon15 Falco Peregrinus

    Nuclear EMP is actually an electromagnetic multi-pulse. The EMP is usually described in terms of 3 components. The E1 pulse is a very fast pulse that generates very high voltages. E1 is the component that destroys computers and communications equipment and is too fast for ordinary lightning protectors. The E2 component of the pulse is the easiest to protect against, and has similarities in strength and timing to the electrical pulses produced by lightning. The E3 pulse is very different from the E1 and E2 pulses from an EMP. The E3 component of the pulse is a very slow pulse, lasting tens to hundreds of seconds, that is caused by the nuclear detonation heaving the earth's magnetic field out of the way, followed by the restoration of the magnetic field to its natural place. The E3 component has similarities to a geomagnetic storm caused by a very severe solar flare.
    dragonfly likes this.
  15. thermax

    thermax Monkey+

    I drilled a hole in the concrete in my garage and drove in an 8' ground rod to connect to the Faraday box I hope to build tomorrow. I didn't want to use any of the houses grounds. Now I'm a little concerned about the re-bar being an amplifier being close to the new ground rod. I'll post some images of the job when I'm done. Thanks guys
    Falcon15 and dragonfly like this.
  16. dragonfly

    dragonfly Monkey+++

    Thye are also now using an "L" shaped rod made of 2 of the copper rods. The first si 8 footlong, with another 4 foot piece that may be "brazed" in an "L" or even a "T" shape. It is then buried 4 foot or deeper. The connecting wire 6 gauge solid copper is recommend, is also brazed to the grounding assy.
    One note about any "tests"...you can only work witht the radio bands we know about and are in common use today., EG: VHF/UHF/AM/FM/CB/Ham and cellular signals. Those created via EMP are far more varied in frequencies. Some we probably have never even heard of. When doing "tests" use any and all types of receivers you can and then mix them by transmitting or trying to place a multiple of receivers in the same "cage". You get some interesting results....There's a piece here in the threads about EMP and faraday cages...
  17. ghrit

    ghrit Bad company Administrator Founding Member

    Therm, it isn't an amplifier, it behaves as a reflector. Generally, rebar spacing is wide enough that only very long wave pulses would be so affected. The rebar itself should be grounded already which helps the situation. You are right to use a separate ground for the test box. If the bottom of your box is made of the same stuff as the top and sides, you'll be as good to go as you are going to get.
    ColtCarbine and dragonfly like this.
  18. thermax

    thermax Monkey+

    Must the cage block all signals mentioned above to prove its built right?
  19. thermax

    thermax Monkey+

    instead of having a door I'm going to be setting the box on a bed of screen 4 layers thick. The box will cover my generator and rest on a 4 layer thick screen. The generator weights 300 lbs and I figured setting the box on top of it will work better than trying to get it out of a door. I'll simply lift the screen box off the top so I can run the generator and put it back on when I'm done. I've going to use tie downs to hold it in place with pressure so that the wire screen on the box will make good contact with the screen on the base.
  20. dragonfly

    dragonfly Monkey+++

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