Author Topic: Question - Faraday Cage  (Read 50182 times)

Offline “Mark”

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Re: Question - Faraday Cage
« Reply #30 on: July 03, 2009, 03:23:53 PM »
A poster on another thread cited a formula which seemed to indicate electromagnetism could not penetrate an opening <5.3 inches.  Anyone know if this is accurate?

For frequencies lower than 2.4 GHz (the microwave frequency that resonates with water). Any frequency higher than 2.4 GHz would pass right through (such as visible light or gamma rays).

Offline IRKCOD

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Re: Question - Faraday Cage
« Reply #31 on: July 03, 2009, 04:55:31 PM »
Have acquired an old front opening double door freezer.
 
Had been thinking about stripping out the metallic shelves, replacing them with wooden ones and covering the internal walls with cardboard. Then earthing 'box'.
Will use to store CB radios, back-up computer equipment and hard drives.

Any thoughts? 

Would I need to apply an external aluminum foil coating?

Fred_47460

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Re: Question - Faraday Cage
« Reply #32 on: July 03, 2009, 05:13:17 PM »
Have acquired an old front opening double door freezer.
 
Had been thinking about stripping out the metallic shelves, replacing them with wooden ones and covering the internal walls with cardboard. Then earthing 'box'.
Will use to store CB radios, back-up computer equipment and hard drives.

Any thoughts? 

Would I need to apply an external aluminum foil coating?

No, I would think the shell of the freezer would suffice. Have you given any consideration on how you will power the equipment you protect from EMP?? I'd throw in a AM/FM/Shortwave radio as well....you are gonna want news from the outside world!!

Offline IRKCOD

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Re: Question - Faraday Cage
« Reply #33 on: July 03, 2009, 05:53:07 PM »
Fred_47460

Thanks for that.   Have a wind-up AM/FM/Shortwave radio to go in as well as a small solar array for recharging deep cycle batteries along with an inverter.

Offline ozarked

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Re: Question - Faraday Cage
« Reply #34 on: July 03, 2009, 08:06:09 PM »
I've been working on an EMP primer, but it is difficult to do it justice and maintain accuracy without resorting to a lot of "its the way it is because I said so."

I think an EMP primer would be a tremendous contribution to the Prepping Community, and look forward to it.  I do hope, however, you are able to bring it down to our level and translate the science and technical data into terms we all can understand and use.  I was a worker bee in the USAF, so "its the way it is because I said so" is perfectly OK with me.  I'm used to it.


Offline ozarked

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Re: Question - Faraday Cage
« Reply #35 on: July 03, 2009, 08:28:16 PM »
For frequencies lower than 2.4 GHz (the microwave frequency that resonates with water). Any frequency higher than 2.4 GHz would pass right through (such as visible light or gamma rays).

OK then, what is the frequency of an EMP?  If 2.4 GHz or less, then it should be unable to penetrate an opening of about 25 square inches or less.  Do I understand correctly?

Offline shadowalker_returns

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Re: Question - Faraday Cage
« Reply #36 on: July 06, 2009, 08:03:03 AM »
Does anyone know of good reference source on building your own Faraday Cage?


Every forum I've ever joined has a thread on EMP and most have incorrect or incomplete data or rely on 2nd or 3rd hand hearsay. Below is a partial post and re-edited version of a mini article I wrote for another forum. I hope it helps clarify what EMP is and relive some of the anxiety people seem to feel whenever this subject comes up.

Begin Post:
Hi folks, at last I can add some info to the forum that may be a partial payback for all the good stuff I've leached off the forum for myself and my family's use. Some background I'm an Electronics Engineer by trade. I also build EMI/RFI shielded rooms as part of my living. I've written two dissertations and a factory instruction manual on shielding techniques and have performed many experiments using Wimshurst generators, Tesla coils, Van de Graf generators, as well as modern very high voltage generators such as those used to power x-ray and CT scanners and the cross-field amplifiers of high powered (megawatts) radars.

First some good news: EMP (for the individual) is not a Godzilla sized problem . Its at most a cockroach sized problem. A problem that you can get your head around and deal with rather easily. So don't worry but if you must worry then worry about as much as you worry about cockroaches.

EMP... What is it? Its an Electro-Magnetic Pulse induced by the rapid ionization of the outer atmosphere by some form of energy (large nuclear bomb, solar flare, etc.). This is slightly misleading as the real dangers involved are caused by the induced voltages and current flow of ELECTROSTATIC discharges from the event. I emphasized the word Electrostatic on purpose because most folks don't know that when dealing with EMP your dealing with an Electrostatic problem more so than a simple electrical problem. Ohm's Law still applies but weirdly for those not familiar with extremely high voltages and plasmas. The slipperiness of electrons at such high energy potentials makes them difficult to control or even understand and this is why there is so much confusion (and worry) about EMP and its ability to affect (destroy) electrically powered equipment. It is true.. Your candle lantern or oil lamp will not be effected by EMP. I however will be using modern electric lighting before, during and after the event.

Electricity is safe for most of us because at the relatively low voltages we use most non-metallic things do not conduct (or support electron current flow), they are insulators. Dry air is a pretty good insulator. This is why you don't get shocked when you walk by a wall outlet. At the extremely high voltages (and frequency) that are commonly dealt with in electrostatics that is no longer true. Air will not stop current flow. Tesla proved quite effectively that power could be transfered through 'empty' space at these extremely high voltages and frequencies. The energy of an EMP pulse is transfered in the same manner as Tesla transferred energy with his Tesla coil (and by the way a good ground is not necessary to initiate a high voltage discharge, just a difference in potential, its not only slippery its down right sneaky).

Why is an EMP so dangerous to electrical devices? Its all a matter of scale. Its a cockroach sized problem but as any restaurant owner will tell you a lot of cockroaches will put you out of business. You can walk by an operating Tesla coil putting out 100s of thousands of volts and not get fried because the current flow is minuscule, minute, i.e. very, very small. An EMP can produce current flows 100s or 1000s of times greater. Also remember that we are dealing with electrostatics where anything 'submerged' in air can have a very high voltage static charge. The EMP induces a very high voltage charge on everything exposed to it. If the thing with this high induced charge should become attached to anything with a lower charge (or potential) then the energy stored in the high induced charge will be conducted (have a current flow) through and be dissipated by the thing until the high induced charge reaches the same potential of the thing with the lower charge. EMP can generate thousands upon thousands of volts in potential (charge). The earth (or ground) is taken as zero volts potential (charge). Now you can use Ohm's law to get an idea of the energy and power levels were dealing with. Ohm's law E=IR, I=E/R, P=EI, E is Voltage, I is Current, R is resistance and P is Power. Lets make it easy E=10,000Volts, R=10Ohms. Current then is (I=E/R) 1000amps! WOW a 1000amps!!? Why have we not all keeled over dead?! While the voltages appear high and the induced currents appear high, the actual resistances are usually much higher and the duration of the pulse is very short like nanosecond short. This short duration limits the total energy and your body absorbs and dissipates the energy as heat. Electrical devices do pretty much the same thing. The old style gear (circa 1930-1960s) used heavy wires. metal, woods and glass the can absorb and dissipate large amounts of heat. Because of that those older devices are relatively immune to EMP effects beyond initial signal disruption. They will tend to recover or be recoverable. Modern micro-miniature electronic devices are a completely different critter. They are almost entirely based on a thing called a PN junction. This junction is an area within the materials of a component that is very very thin. Not measured in hundredths or thousandths of an inch but in the number of molecules thick... These multi-molecule thick junctions lack the mass to dissipate large influxes of high potential (voltage) energy. When the high voltage static electricity generated by an EMP device discharges through one of these extremely thin junctions its like firing a Deathstar Laser Beam through them. These junctions unable to dissipate the heat behave as all other thing that are subjected to way to much heat... They liquefy and then vaporize. Leaving you with one dead device. By the way two other terms for 'High Potential Electro-Static Discharge' are Spark and Lightening Bolt. The situation is made worse by the fact that electrical energy can be collected and directed even EMP and even when you don't want it to. Any large, long wire or exposed conductive surface can act as a collector of electrical energy. The antenna on you radio is a collector of electrical energy, as are fence wires, as are phone and power lines from the utility companies (great big multi-mile long collectors of electrical energy)... EMP is electrical energy. Are you getting the picture? By the way HERF guns work almost exactly like EMP.

Although the above is way oversimplified, I hope it clarifies what an EMP is and how it damages equipment.

What to do? Well we know the following:
1) for EMP to damage your equipment it must somehow discharge its energy through the equipment. Prevent this discharge and no damage occurs.
2) EMP is collected and focused by long wires and exposed metal surfaces. Disconnect your devices from long wires and exposed metal surfaces and you reduce the amount of energy available for the discharge.
3) EMP is electrical energy. Electrical energy always takes the path of least resistance. Provide it a lower resistance path around your equipment and it will discharge through that path and not through your equipment.

With few exceptions protection from EMP utilizes one or or a combination of the above principles.

By using all the above you can begin evaluating any piece of gear for probability of vulnerability to EMP (NOTE: EMP is Very High Potential Electrostatic energy there are no guarantees only probabilities, when I say this stuff is slippery I mean siliconized Teflon on ice slippery and Nixon sneaky).

Too make an already long story short(er). Is the thing conductive or does it use conductive parts or electricity for its operation? If answer is no to all then device is immune to EMP. If conductive or electrically active, can all components and sub-components dissipate 1 kilowatt or more of heat? If yes then Extremely resistant to EMP unless connected to long wire runs or large exposed conductive surfaces or antennas. Does the device contain any modern micro-miniaturized electronic components? If yes steps may need to be taken.

One quick way to tell if a component is sensitive to EMP is how it was packaged. If it came in packaging that says 'electrostatic sensitive component' or anything similar or if it was packed in silver or pink mylar bags/wraps, then EMP will be a problem for that device unless steps are taken. All modern consumer electronics are at some level of risk. The type of device, its packaging and use will determine exactly how much at risk it is.

Microwave ovens as Faraday cages? Not without some modifications. Microwave ovens, specifically the cooking chamber are in fact a type of Faraday cage. The cooking chamber is designed to contain and circulate the Microwave RF energy produced by the magnetron of the oven and prevent spurious emissions of RF energy. Here's the problem microwave ovens have a lot of penetrations in them as well as power cords and a big leaky door. They also were not designed with electrostatic protection in mind. The power cord will act as an EMP collector and any leaks in the shield would allow a significant static charge to build up within the chamber. If precautions are take an old microwave oven can be converted into an EMP resistant Faraday cage... But so can any metal box. The absolute best affordable Faraday cages are constructed of solid copper boxes. Copper is one of the best conductors of electrical energy. Technically to provide the absolute best protection that solid copper box would then be connected to an earth ground through a large surface area copper conductor. The total resistance from any point on the copper shield to earth ground should be less that 2 Ohms. The Earth ground to Earth resistance should also be less than 2 Ohms and less than the resistance between the copper shield and Earth ground. The large surface area conductor is required because at these types of frequencies most of the power conducts along the surface of the conductor and not through the center. Flat strip or braid is usually considered the best type of conductor to attach between the copper shield and Earth ground. In practical terms for small boxes (say less than 1/2 cubic feet in volume), The earth ground is not so critical. The charge built up on the box will discharge or dissipate very quickly and with little effect on its non-combustible surroundings, similar to when you touch a doorknob or drag your feet on the carpet and touch your (clothes horse) buddy (who makes fun of all us married with small children types who end up coming to work with spit-up, oatmeal or other remnants of breakfast with toddler on their shirts) while he's drinking his favorite fruit punch flavored Gatorade just after he changed into his new white shirt, just before the big budget meeting with the Board of Directors  (I'm not evil just a little unwell  ;))
End Post:

To build a Faraday Cage for your gear you must understand the difference between ordinary radio waves and EMP. Most of what you KNOW is wrong. EMP is about electro-statics not electro-magnetics. The rumor running around that only steel or shielding with ferrous component will stop EMP damage is wrong (they are focusing on the magnetic component of the wave). The equivalencing of EMP to light or cosmic energy is wrong. The need for conductive shielding is true. Using aluminium foil for shielding is only partially true. Ordinary household foil will not work. You'll need heavy duty aluminium foil at a minimum and that is iffy. The Items in your cage should be well insulated from the cage walls (cardboard is NOT recommended as paper is conductive when wet and most paper is hygroscopic) and you need a conductive path to ground that is less than 2 ohms with a conductor of sufficient size to handle the rapid heating that will occur (think lightening protection). It is imperative that there be no holes or gaps in the cage. If water can enter the cage EMP can enter the cage. Don't bother with mesh walls unless you really KNOW electro-magnetics and electrostatics. Make your cage solid metal walls. Lead is not necessary or even desired. Copper is the best (3oz sheet is probably the most cost effective copper based shielding material for the ordinary guy/gal). Aluminium and steel can also be used (I'd choose 22ga or thicker). Ammo cans can be modified into good cages if you can get a good metal to metal seal on the lid and properly ground the can. I'm doing some experiments with modding the Christmas popcorn tin cans into EMP resistant containers and I've modded some 30 gallon open head steal drums into very effective containers (tested with a electrostatic generators to about 300,000 volts)

Regards all,
Shadowalker
« Last Edit: July 06, 2009, 08:08:45 AM by shadowalker »

Offline Tinker

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Re: Question - Faraday Cage
« Reply #37 on: July 30, 2009, 07:43:34 PM »
+1 Great info Shadowalker.

I am thinking a US Surplus aluminum medical chests will make a nice cage for me. Without a gasket the lid is still very tight and it is roomy enough to hold a lot of electronics.

What would you recommend for cheap insulation for the inside? Wool? Cloth? Paint? The insides are already painted, but I worry and chipping.

Offline donaldj

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Re: Question - Faraday Cage
« Reply #38 on: July 31, 2009, 09:26:39 AM »
I have a few radios and my inverters I'd like to store in a shielded manner. They will be stored in the basement.

What do you think about a couple ESD bags (the metal faraday bags, not the pink plastic bags) for shielding? I was thinking 2 bags per device, and all the devices stored in a metal ammo can. This would all sit on my steel wire shelf in the basement.  None of it would be grounded.

Any thoughts on what kind of attenuation this setup would provide to the contents?

Offline Pathfinder

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Re: Question - Faraday Cage
« Reply #39 on: July 31, 2009, 12:01:39 PM »
With all of the research that has already gone on with EMP (e.g. supposedly AF One is "hardened against EMP, so someone is doing something), why is there so little information out there as to EMP reality? Where are the test results? Has all of it been marked SECRET for some reason?

Finding that research could well eliminate the questions on garbage cans, metal sheds, ammo cans and the like. And make the EMP Primer a lot easier to write.

Offline shadowalker_returns

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Re: Question - Faraday Cage
« Reply #40 on: August 06, 2009, 08:08:43 PM »
+1 Great info Shadowalker.

I am thinking a US Surplus aluminum medical chests will make a nice cage for me. Without a gasket the lid is still very tight and it is roomy enough to hold a lot of electronics.

What would you recommend for cheap insulation for the inside? Wool? Cloth? Paint? The insides are already painted, but I worry and chipping.

Most of the large medical chests I've seen have rivets on the latches not welds. Electrostatics is slippery stuff. if the rivets don't fill the hole completely then your hosed. Also putting all you electronics in one cage means if that cage is compromised you loose everything. I never put all my eggs in one basket. Its easier to shield smaller boxes. Boxes the size of footlockers and the MASH sized medical chests also increase the SURFACE AREA which collects more of the electrostatic energy. You want as little charge as possible on the case of your box. IMO its easier for amateurs to shield boot/shoe-box size containers than larger containers. In this case smaller is actually better as its easier to ensure the continuity of your shielding material. One of the things most people don't know about aluminum is that it can behave more like ceramics than metal when cast or stamped, meaning it can have microscopic cracks that are virtually transparent to EMP energy. There are real reasons that copper and steel are the predominant materials used by the pros to build Faraday cages, shielded rooms and electrostatic shielding for sensitive electronics. I really don't recommend aluminum for critical shielding, though others will say its ok. I don't build aluminum rooms for my large RF shielding projects either, though some people do. Copper, Steel or Brass if you really want your electronics don't bother with anything else.

Regards,
Shadowalker

Offline shadowalker_returns

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Re: Question - Faraday Cage
« Reply #41 on: August 06, 2009, 08:14:42 PM »
I have a few radios and my inverters I'd like to store in a shielded manner. They will be stored in the basement.

What do you think about a couple ESD bags (the metal faraday bags, not the pink plastic bags) for shielding? I was thinking 2 bags per device, and all the devices stored in a metal ammo can. This would all sit on my steel wire shelf in the basement.  None of it would be grounded.

Any thoughts on what kind of attenuation this setup would provide to the contents?

Read my previous post in this topic (the large post). ESD bags by themselves won't work. The kind of attenuation? none. Read my large post to understand how EMP works to damage your equipment. Then rethink your options.
Regards,
Shadowalker

Offline shadowalker_returns

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Re: Question - Faraday Cage
« Reply #42 on: August 06, 2009, 08:35:42 PM »
With all of the research that has already gone on with EMP (e.g. supposedly AF One is "hardened against EMP, so someone is doing something), why is there so little information out there as to EMP reality? Where are the test results? Has all of it been marked SECRET for some reason?

Finding that research could well eliminate the questions on garbage cans, metal sheds, ammo cans and the like. And make the EMP Primer a lot easier to write.

RF Shielding is one of my specialties. I've tracked developments for some years. Over that time I collected a large amount of contacts and research data including damn near every reputable site on the web that concerned itself with RF shielding, EMP and HERF. About 5 years ago many of the sites started redacting their EMP and HERF data. About 2 years ago every government site I had including all the DARPA sites I visited regularly that had EMP and HERF data disappeared and I mean like fell off the face of the earth. I've been informed that do to recent advances that make EMP/HERF devices rather easy to build that data has been removed and the shielding data was removed to make it harder for a third world adversary to shield themselves from the effects of a device should they choose to build and deploy one. Now I don't know if that's true. My source is still plugged in to DARPA but he also keeps a 3oz copper foil hat on his person and has certain other proclivities that seem to plague the super intelligent... All I can say for sure is that all the sites I used to trust for advanced research in this field are gone, with no parking pages and the URLs don't exist. Thats never happened to me before.. well except for the homemade cruise missile debacle... but that's another story.
Regards,
Shadowalker

Offline dudekrtr

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Re: Question - Faraday Cage
« Reply #43 on: August 06, 2009, 08:49:47 PM »
Back up a few posts, a question I also have: would a defunct refrigerator serve the purpose?

BTW-great post Shadowalker!

Offline JPH

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Re: Question - Faraday Cage
« Reply #44 on: August 07, 2009, 09:23:52 AM »
Ok so let us say someone wanted to take a 20’ x 8’ x 8’6” shipping container, (these are the ones that look to be ½ length of the normal full size shipping containers), and turn it in to a faraday cage at a BOL…

In response to the “if water can get in then EMP can get in” statements above I have the following questions and thoughts…
  • If the shipping container was redundantly grounded with standard grounding materials like copper rod and flat copper braid and heavy solder at all connections would that be enough or would the door seams still be an issue?
  • Would lining the building with rubberized coatings like that used on pools or maybe even the do it yourself truck bed liners (depending on their true makeup) in addition to building PVC pipe shelving for everything to sit on be enough insulation?
  • If the answer to question #1 and #2 is no, then would one have to line the shipping container with copper sheeting, soldering ALL joints, (i.e. a time capsule style of storage to only be opened in case of EMP) and would building PVC shelves inside the copper box be enough insulation? If so then that’s a lot of money just in the copper alone!!!!

If a cost effective faraday cage can be made then it would be easy to store a Honda 2000i generator, a few solar panels, some deep cycle batteries, a small set of handheld radios, a complete HF long-range communications setup, all the components of a local 2m or 6m repeater, a few laptops, massive amounts of backed up data on external hard drives, car parts, entertainment products, and even a mini frig or freezer. However if the cost is through the roof then the list of items to be stored gets real short, real fast.

Thanks for your time and help, experts in this field at few and far between.

Offline LdMorgan

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Re: Question - Faraday Cage
« Reply #45 on: August 07, 2009, 11:35:26 AM »
My understanding is that grounding Faraday cages is definitely best, but not mandatory.

If ungrounded, a Faraday cage will absorb the incoming energy and become electrically charged--like with static electricity, and also warm up slightly.

A charged cage could shock the snot out of someone who touched it right after a pulse, just like a capacitor, but the cage would soon lose it's static charge by ion interaction with the air.

You can probably make a pretty good EMP-safe from a 55-gallon steel drum with a 30-gallon drum inside. Pack the space in between with any kind of cheap metal scrap. Aluminum filings from a trailer factory, for example. Make a metal-filled pillow to sit on top of the smaller drum as an EMP lid.

Your primary metal box should be filled with smaller metal boxes: Ammo cans inside filing cabinets, for example, with the storage item itself wrapped in foil. The more layers the better.

Theoretically, you should be able to use salt water as an EMP barrier since it is conductive and wouldn't have any holes in it. It might take a little creative engineering but you could sink watertight packages in a 55 gallon drum full of saltwater. Even a weighted 30-gallon drum, as long as it was reliably watertight.

If you have a swimming pool or a lake handy, you could use that. Even fresh water will have a shielding effect, and a few feet of fresh water should equal a few inches of salt water.

If you are using a water shield, you can go with plugged PVC for reliable watertightness, no problem-o.

From reading "Lights Out" I'd say the first thing you would want to put in your EMP-Free Box would be any medical appliance you need, then the electronics needed to keep your car running, and after that communications gear, calculators, and the like.

On the dry side, Christmas tins could be used as a Faraday cage. They come in graduated sizes, so you could nest them like Russian dolls. Cheap and easy.


Offline LdMorgan

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Re: Question - Faraday Cage
« Reply #46 on: August 08, 2009, 10:05:28 AM »
(Duh!! I missed that first super-good post!)

Offline JPH

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Re: Question - Faraday Cage
« Reply #47 on: August 08, 2009, 11:29:29 AM »
(Duh!! I missed that first super-good post!)

? I don't get it????

Offline LdMorgan

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Re: Question - Faraday Cage
« Reply #48 on: August 08, 2009, 10:09:10 PM »
I was referring to reply #36 by Shadowalker, posted on July 06/09.

He definitely knows his stuff.

Offline Cave Dweller

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Re: Question - Faraday Cage
« Reply #49 on: August 21, 2009, 10:28:24 PM »
I have a few radios and my inverters I'd like to store in a shielded manner. They will be stored in the basement.

What do you think about a couple ESD bags (the metal faraday bags, not the pink plastic bags) for shielding? I was thinking 2 bags per device, and all the devices stored in a metal ammo can. This would all sit on my steel wire shelf in the basement.  None of it would be grounded.

Any thoughts on what kind of attenuation this setup would provide to the contents?
Good idea.
Ammo cans have a rubber seal under the rim.
Maybe if you pull that out and replace it with wire.


Offline LdMorgan

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Re: Question - Faraday Cage
« Reply #50 on: August 22, 2009, 05:03:37 AM »
Just a stray thought. I bet an EMP would raise merry hell with electric detonators.

I better move my blasting caps out of the kid's room.

Offline Orionblade

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Re: Question - Faraday Cage
« Reply #51 on: October 13, 2009, 03:03:05 AM »
I worked in a pulsed power lab for a while, and the most common thing we did for shielding was a combination of impedance matched instrument cable (teeeeeensy tiny coaxial cable) and either zinc or nickel spray paint inside our enclosures. Our devices were NOT grounded.

To better understand the problem, you can think of all electrical components as having three parts describing their electrical function:

Resistance/Conductivity (how hard it is to jam DC current through the device)

Capacitance (looks like an open circuit to DC, conductivity depends on AC frequencies, essentially how well the device stores electric charge for a given voltage)

Inductance (how hard it is to CHANGE the current flowing through the device - reluctance/reactance)

In the pulsed power applications we were working with, we typically generated Mega-, Giga-, and occasionally large fractional to small integer Terawatt pulses. For scale, a Kilowatt is somewhere on the order of how much power your hair dryer sucks down (in joules per second), while a Terawatt is about what two or three nuclear power plants crank out.

Your average rifle bullet is a good analogy for pulsed power. A typical .30 caliber bullet has a power transfer on target of about 1500 to about 2000 joules, assuming the bullet completely stops inside the target. This is enough energy to flash boil a teaspoon of water, but your coffee cup would barely notice. When you account for the transit time of the bullet from the surface of the target to the time the bullet/pieces stop within the target, you're talking about 6 inches, so at 2500-ish feet per second, you can guestimate at about 50 microseconds. Since POWER is ENERGY divided by TIME, you wind up with a pulse power in the hundreds of Megawatts. Since it only lasts a few microseconds, you can't power your house with it - after all, it's only enough energy to boil a teaspoon of water, but the power level reflects how quickly the energy has been deposited into the taget, and quite obviously it's enough energy, delivered quickly enough, that it breaks the chemical bonds between molecules which make up even the toughest deer meat, long before that kinetic/thermal energy can be dissipated through the whole pile of steaks, ribs, shanks, briskets and barbecue.... er... tissues.

The same sort of thing is going on with your electronics. A previous post had it right with regards to valve state (Tubes) compared to solid state (transistors/chips). A bigass pulse of electricity knocks a ton of electrons around for a short time. A chunk of wire sitting in vacuum can dissipate all sorts of joules in virtually no seconds, and also has a relatively low resistance compared to its capacitance and inductance (relative to an intersecting EMP). A chip, however, is tiny itsy bitsy little components that have relatively high resistances compared to the inductances and capacitances that the components have at EMP pulse-lengths and effective frequencies.

Many chips are protected from ESD by waht amounts to a tiny little lightning arrestor. At some point a threshold voltage is exceeded, and an incoming voltage spike is shunted to ground through this disposable junction. The problem with EMP's as compared to a wool-socks-on-linoleum accident is that the pulse can be much taller than the ESD's pulse, and when translated through the menagerie of reactances and reluctances and resistances within the circuit, it's spread out a bit in the time domain, and can wreak havok after that little fusible link has burned itself to a crisp, essentially turning the whole circuit into a charging capacitor that then breaks down and shunts to ground, frying everything on the little doped silicon, gallium or germanium die.

Since we know how known regions of pulses (frequency ranges) work with known capacitance and inductance values, we can impose shielding that takes care of all of these problems.

A good place to start with not-so-sensitive items is to place RF chokes and shunts on your power supply. It's not so critical to put it between the wall and the back of an appliance, but definitely on the low voltage side of things, anywhere it's regulated, so that a pulse doesn't follow the electrical cord in. This only realyl works if the device is plugged in, or at least grounded.

Higher frequency/shorter duration pulses will fry even the most basic filters/chokes/shunts, so you must then turn to shielding.

If you decide to swap out single wires for coaxial cable, keep in mind that only one end of the shielding gets grounded. Otherwise the shielding becomes an antenna, and you might as well toss your expensive equipment in the river. Our enclosures used a floating ground. That is, instead of tying the ground to earth, we tied all the grounds together for a particular experiment, along with the experimenter, but we didn't care if you could watch spanish language re-runs of Golden Girls between earth ground and the enclosure on the Oscilloscope. Neurosurgery suites do this so that any stray charge built up on surgical instruments, surgeons, or patients, is maintained until after surgery, since even picovolts can cause traumatic brain injury when incident upon bare-to-the scalpel neurons.

In short, the best policy is to get a hardened device, but if you can crack the case, and swap out any external leads with coaxial cable (if it's a radio, then run coax out and relocate yourself a removable antenna) and a little impedance matching network - this is a fairly simple procedure with nothing more than carbon composition resistors, if you pick up a book on RF circuit design. Coat the inside of the case with nickel spray paint from Mouser, NewarkInOne, or any other big electronics supply house, and then spray some lacuqer on top of that to prevent shorts. Metal cases without significant breaches can be left as-is, as long as power cables don't ground to the case. Again, this will provide a direct path into the enclosure for any incoming EMP.

I suggest that any coaxial cables coming out of a device be grounded to the device shell, and left to float, while the center conductor carries your desired signal. For a standalone faraday cage, a large cardboard box lined with aluminum foil is adequate, but ideally you want something with higher conductivity, approaching that of silver.

You'll have to look up expected pulse widths for an EMP, and check out the skin depth for that frequency in the material you've chosen, since at high frequencies even "conductors" have a measurable dielectric constant and slef-inductance which change their in-circuit behavior from the expected DC response curves.

You don't need to go getting into hundreds of pounds of lead to shield your stuff - after all, aircraft with conductive skins can get struck by lightning and the passengers don't even know it, since they're flying faraday cages!

I'll see if I can find a skin depth chart, and a pulse width for common EMP sources and post them soon. I'm finally getting sleepy!

Take care,
Orion

Offline Orionblade

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Re: Question - Faraday Cage
« Reply #52 on: October 15, 2009, 11:59:57 PM »
Just a quick update: looks like the typical pulse width for an EMP (nuclear or otherwise man-made) is about 10-100 nanoseconds, yielding 100MHz down to about 10MHz.

This is a bit different than just turning on your radio at 100Mhz, however, since you're dealing with a sharp rising and falling edge. This is a 100MHz square wave, coming in a single or a short train of pulses, and carrying tremendous amounts of energy compressed into highly powerful pulses.

I'm doing some more research into the skin depth issue, but in my work with high voltage pulsed power supplies, the rise time was more critical than the actual pulse width. In order to make a proper assesment of the required thickness of the conductor and density i'd have to find the expected rise time. I may simply reverse engineer a pulse generator I saw during my research. Two coils of wire, fed with power, and loaded with high explosive inside the inner coil. One time use, but it provided powerful pulses for research purposes. I may even have a data sheet lying around.

I'll update as I get more info, but likely nothing worth losing sleep over until after the weekend.

Cheers,
Orion

Offline dudekrtr

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Re: Question - Faraday Cage
« Reply #53 on: October 16, 2009, 06:23:47 AM »
It seems to me we are missing the most important part of this. The protective cages won't do us much good if we don't know when to put the equipment into them. What would be the best way to protect equipment (specifically computers) that are in use almost all the time? I understand the value of wrapping the equipment up and stashing it away, but how much time will we have to actually do that? The only ones I see that working for are those out in the sticks a good distance away from any possible attack to begin with.

Anyone think through any scenarios like this? I come up short when trying to make it all work in my humble cranium.

Offline donaldj

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Re: Question - Faraday Cage
« Reply #54 on: October 16, 2009, 06:35:01 AM »
You'll likely have no warning. The EMP travels at light speed, so by the time you feel a shockwave, or heard the rumblings on an explosion, the EMP will have come and gone.

If you have gear you want to protect, it should be stored in a shielded manner all the time. If you need the gear day-to-day, buy extras to store, or don't include that gear as part of your prep methodology.

D

Offline Orionblade

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Re: Question - Faraday Cage
« Reply #55 on: October 16, 2009, 08:55:05 AM »
I totally see your point.

I'm thinking I'll have a little faraday cage in my BOB - there's a "GPS" pocket in the bag that fits very nicely around my PDA, and the bluetooth GPS reciever, and a little data wallet with some SD and USB cards in it.

If we can find out what thickness shielding you need, and what density, then it should be rather academic to either shield things like your glove box so you can keep things there, or to pop open a plastic case, spray it with conductive paint, attach a ground lug, and put it back together. (assuming you give yourself a lead for your antenna if it's a cell phone or walkie talkie...

That would be fun.

*imagines - "I don't have any bars!"*

Later,
Orion


Offline atherts

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Re: Question - Faraday Cage
« Reply #56 on: June 13, 2011, 04:01:57 PM »
I didn't see this information anywhere in the discussion. I ran across this on a blog.
Basically it says that a microwave oven is a Faraday Cage in that it can keep microwaves in, and could conceiveably keep emp out.
An old microwave from a garage sale could work. If nothing else, put that inside your DIY Faraday cage for extra protection/insurance.

Microwaves, Faraday Cages, and EMP Protection
http://www.utahpreppers.com/2009/02/microwaves-faraday-cages-and-emp-protection/

Offline pokeshell

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Re: Question - Faraday Cage
« Reply #57 on: October 27, 2011, 01:17:42 PM »
This is a good topic and I hope more people add to it.
The microwave is a good idea. Not sure it works but it makes sense. I would assume it would work.
About once a month or so I back up my hard drives on a separate external hard drive that hooks up with a USB. Then I put that HD in my fire proof box. But if an old microwave would work to prevent EMP damage I would use that.

Little late to the game here, but cheap fireproof safes will not protect your drive from the heat. Most drives will die from 10 minutes at 150 degrees. Most cheaper fireproof safes keep paper from burning. Paper burns at 451, well past destruction of a hard drive (I know you can send it off and get data back for $15,000).

Best bet is to take a copy of your data on a drive once a month(more if needed) or so, and leave it at work. Just the drive goes. The cables stay at home, and throw a copy of http://www.truecrypt.org/ for free, to keep people from easily accessing your documents.

Offline COG_K

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Re: Question - Faraday Cage
« Reply #58 on: December 24, 2011, 10:11:57 PM »
What about using the gold lined paint cans that Jack has talked about in the past?

http://www.thecarycompany.com/containers/paintcans/goldlined.html

Offline LibertyBelle

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Re: Question - Faraday Cage
« Reply #59 on: December 26, 2011, 07:37:19 PM »
Here is a simple test to see if your "cage" is EMP resistant (I hesitate to say "proof" since I've not tested it with an actual EMP ;) ): But anyway, simply put your cell phone in it, with the ring on the loudest it will go. Then close the cage up and call your cell phone.  If you can hear your cell phone ring, the your cage (or at least the item) is NOT EMP resistant.

I just did the cage test using a microwave and an ammo box as cages, testing each separately.  I placed my cell phone on top of a small cardboard box inside the "cage" and closed it up.  Alas, when I called my cell phone I could still hear it ring inside the cage.  Then I wrapped my cell phone loosely, but completely,  in aluminum foil, again placed the phone on top of the cardboard box inside the cage but where so the foil did not come in contact with the cage walls, and closed it up...and when I called my cell phone this time, it did NOT ring...even when the calling phone was just inches away.  Success!   

Note: Wrapping the cell phone in the foil, but WITHOUT placing on top of the cardboard box inside the cage, does NOT prevent the cell phone from ringing.