Paging physicists: looking for answers about a Faraday cage

[AnonEmous]

The past week I have been trying to find concrete answers about a Faraday cage's ability to protect a radio and other small electronic devices from lightning, solar flares, an EMP pulse, or some other electrical interference. Any definitive information, preferably sourced, is appreciated. My goal is to cut through half-guesses and estimates about Faraday cages from a lot of Internet information and get to the ground truth for what they can and cannot do.

1) what makes a good, inexpensive, and simple cage or shield
2) how to test the cage in a practical but thorough manner
3) what such a simple cage can protect against
4) whether the cage needs to be grounded to be effective (and how to do this in an apartment setting)

This post is organized by the above 4 points. Much of what I have found has been contradictory. Probably one of the best-written pieces I have read can be found here…http://www.futurescience.com/emp/emp-protection.html. Also, http://suburbansurvivalist.wordpress.com/2010/02/07/faraday-cages-to-protect-electronics/ offers a good overview, but also runs into contradictory information.

1) What makes for a good, inexpensive Faraday cage…

The winner for simplicity, size, and expense seems to be the galvanized trash can with cardboard placed inside to act as an insulator from the can’s side. A smaller version of this which I have tested myself (more below) is an empty paint can (bought from Home Depot for about $4.00).

The simplest method seems to be to wrap any electrical device in multiple layers of aluminum foil with an insulator in between such as a plastic bag.

Another alternative is a used (or working) microwave, since it is designed to contain specific microwave radiation frequency. That brings us to the second point.

2) How can a Faraday cage be tested simply and effectively?

Most of what I have found recommends testing a cage with a radio or cell phone.

The sealed paint can blocked my cell phone and radio, but I do not know what other frequencies could be getting through. When I called the cell phone, the static from the radio changed, so some type and level of signal appeared to be getting through.

Aluminum foil wrapped entirely around the cell phone and radio (with an insulator in between) appeared to block out the VHF and the cell phone signals.

The cell phone in the microwave easily received several calls. (edit... cut some information here that turned out to be incorrect).

One source (unfortunately I cannot find it now) recommended testing with the highest frequency (largest GHZ number) electronic device possible. If a metal trash or paint can protects against a high frequency, would it automatically protect against all wavelengths up to that point? Does anyone know what the frequency/ frequencies or energy levels of lightning or solar flares can be? I am probably not using the right terminology, but would like to know how these potential threats overlap.

3) What can a cage protect against?

So I know my well-sealed paint can blocks VHF and cell phone frequencies (about 1900 MHZ), but how would it work against a nearby lightning strike or EMP? According to http://www.futurescience.com/emp/emp-protection.html, “apparently DC-like currents are the problem“ from solar flares. Lightning can damage electronics through close but indirect contact.....

KnightoftheRoc noted...http://www.zombiehunters.org/forum/viewtopic.php?f=6&t=79282&p=1744905&hilit=#p1744905

Last night, (edit: this was April 2011) while I was at work, lightning struck a tree on our property line. The dog run was attached to this tree, vinyl coated 1/8" braided steel cable, about 50' long. Fully half the dog's run is gone, and I mean GONE. sections of blackened, EMPTY rubber coating litter the yard, the cable vaporized right out of it. Thankfully, the dog was NOT outside at the time- all I could find of the dog lead that attaches to her collar was the pulley that rode the run cable, made of the same stuff.

The resulting magnetic wave from the strike wiped out most of the electronics in the house, including, but not limited to (we're still assessing damages) a laptop computer, cable modem (replaced it today at the cable co.) two of three phone extensions, three televisions

4) To what extent should a Faraday Cage be grounded?

After everything I have read, it seems an effective cage does not need to be grounded to offer protection. But if one was inclined to ground a cage, are there any recommendations about how to do this in an apartment setting with no direct access to the ground?

[whisk.e.rebellion]

After everything I have read, it seems an effective cage does not need to be grounded to offer protection. But if one was inclined to ground a cage, are there any recommendations about how to do this in an apartment setting with no direct access to the ground?

I'm pretty sure you're going to want to ground it. You're going to want a return path for all that current.

Is your apartment so old that it only has 2-wire Romex? If not, you can always just run a wire from your cage to the ground receptacle on your outlets. It's ugly, but it works.

[KJ4VOV]

Grounding is a two-edged sword though. Yes, it will bleed current and RF from the cage, but it can also lead current to the cage, depending on how you make the ground. Ideally, you want the grounds as short, straight and heavy as possible (stranded cable is better than solid for this, and braid is better than stranded) and you want them to go directly to a good earth ground. Grounding to house wiring is probably your worst choice if you're trying to protect against EMP, because that same power cable binging current into the house is going to conduct the effects of an EMP induced current into the house from the outside lines. Since the ground conductor in Romex is centered between the two current conductors it's also going to carry a spike.

Being in an apartment above ground level complicates the whole thing quite a bit, and there really aren't any truly surefire solutions to the problem.

My suggestion would be, if at all possible, use a plastic container to hold the items inside a metal container (military ammo container works well) and bury that container beneath 12 to 18" of soil for the best protection. Yeah, it'll be a hassle to get at the items, but you weren't going to be using them inside a Faraday cage anyway, were you?

[Bunsen]

Answers from a physicist follow:

I'll start with the grounding question, because that's the easiest to answer: Doesn't help a bit. All that matters is that the metal container is conductive and doesn't have gaps (ammo cans are bad at isolating from UHF on up because that rubber gasket only leaves the lid connected to the body at the ends, and that allows radiation in if the wavelength is short enough). If you're concerned about direct lightning hits, then having the can connected to ground could attract lightning to it, especially if it's much above ground level. I'd leave it ungrounded.

The paint can sounds like a great solution for anything small enough to fit inside. Since it's designed to make an airtight seal, you know you have metal-to-metal contact all the way around the edge of the lid. Trash cans aren't very good on that point -- the lid probably only touches the can at a few points, leaving long (also thin, but it's the long part that matters) gaps, so RF can get inside. They, like the ammo cans, will still protect well from low frequencies (such as indirect lightning effects). Insulation on the inside isn't really necessary (the whole point of the Faraday cage is that currents only flow on the outside surface), but I suppose it can't hurt.

Radios are a fine way to test isolation. It's better if you can control the signal strengths involved and have some basis for comparisons, so you can get an idea of how much attenuation the can provides. The point about testing at high frequencies is valid -- in all but a very very few cases, low frequencies will always be better-isolated than high frequencies. The microwave, I think, is one of those few counterexamples -- it's a resonant cavity tuned to one specific frequency (2.45 GHz), and the edges of the door are positioned at natural zero-current areas for that frequency. At any other frequency, the fact that the door isn't electrically connected to the body around most of the edge allows some RF through.

As for what frequencies matter for what sort of threats, it's time for numbers. Starting with the lowest, and therefore easiest for a Faraday cage to handle:

• Solar flares and resulting geomagnetic storms: Hundredths of a Hz. Complete non-issue for anyone but the guys running the power grid and pipelines. Small risk of surges on the power lines themselves from transformer failures, but absolutely no RF risk (i.e. if it's not plugged in, it doesn't give a damn).
• Lightning: Mostly below 1 MHz. That means wavelengths of hundreds of meters, so anything that more or less surrounds your electronics will protect from the electromagnetic waves (i.e. the indirect effects that extend hundreds of meters from the strike). Direct hits from the strike itself are nearly impossible to protect against, since a lightning strike can easily blow a hole through something like an ammo can. But those follow conductors, so don't store your Faraday-protected electronics next to that wire running to the old TV antenna on your chimney. For testing in this frequency range, try an AM radio tuned to the strongest station you can find.
• Nuclear EMP: Worst below 100 MHz, but significant up to several hundred MHz. Wavelengths as short as several inches. This is where things become demanding. Gaps of several inches in length may allow RF to penetrate into a Faraday cage. Making sure the lid contacts the body around its whole circumference, or at least every inch or two, is important. To test isolation for this sort of thing, try at least UHF (FRS/GMRS radios operate around 460-470 MHz, which is a good example).
• Non-nuclear EMP bomb: Up to several GHz, perhaps tens of GHz. Wavelengths down below an inch. Damn hard to shield against, but short-ranged and, in my opinion, not likely to be seen unless you're on the wrong end of a serious attack from a high-tech power. If you're still concerned about it, then look to absolutely, completely seal your Faraday cage. Consider soldering the lid on to that paint can. Testing at cellphone/wifi frequencies would be a start, but threats could go well beyond that frequency range. There just isn't much consumer hardware that uses frequencies this high.

[KJ4VOV]

Bunsen, thank you for jumping in on this thread and providing such excellent information. I, for one, appreciate having someone with better knowledge and experience point out where I've erred and give me a little educational update in the bargain.

[whisk.e.rebellion]

Agreed. Thanks, Bunsen. Also, that you for mentioning wavelengths

[AnonEmous]

Bunsen: agree with the other comments. Your write up to my questions is probably the most straightforward explanation I have found. I need to read over your contribution again and may have a few questions, but essentially you have provided great context. I figured solar flares could be a problem for electronics, but it sounds like the power grid would fail before my emergency radio would.

Thanks again for including the math and a simple to understand approach.

[AwPhuch]

Find the biggest nastiest OLDEST POS microwave...cut off the prongs that feed it power (so you don't "nuke" your electronics), leave the ground

Instant Faraday Cage, if it can reduce 800-1000 watts of RF radiation enough to COOK YOU...it should be able to attenuate any EMP or other high power radio magnetic wave strong enough to fry even the most finicky microcircuit

[whisk.e.rebellion]

Find the biggest nastiest OLDEST POS microwave...cut off the prongs that feed it power (so you don't "nuke" your electronics), leave the ground

Instant Faraday Cage, if it can reduce 800-1000 watts of RF radiation enough to COOK YOU...it should be able to attenuate any EMP or other high power radio magnetic wave strong enough to fry even the most finicky microcircuit

The shielding on microwave ovens only stops microwave radiation. And, frankly, most microwave ovens operate around 2.45Ghz which is why their operation interrupt 802.11b/g signals as well as 2.4GHz cordless phones and not 900Mhz or 5GHz WiFi signals. So, a couple of takeways from that are:

1) If your microwave oven interrupts your WiFi, its shielding is really shitty and would make a really shitty Faraday cage since it can't even keep its own signal from bleeding out

2) If you plan on using a microwave oven as a Faraday cage, you must be planning on shielding against a very small range of frequencies

[crypto]

The cell phone in the microwave easily received several calls because it operates on a higher frequency that the microwave.

So I know my well-sealed paint can blocks VHF and cell phone frequencies (about 1900 MHZ),

Speaking of contradictory internet information, you appear to have some of your own.

The vast majority of microwave ovens operate around 2400MHz. High frequency than cell phones (unless we're talking about 4G WiMax phones, but I dont think we are).

The reason your microwave oven doesnt attenuate 100% of the signal is almost certainly because the door doesnt have a positive continuous bond against the surrounding chassis of the oven. If you had copper tape or a RF fence of some sort between them, no signal would get in or out.

[crypto]

The shielding on microwave ovens only stops microwave radiation.

the solid chassis will stop pretty much all signal, the mesh door will stop 2.4MHz and lower frequencies. Enhancing the bond between the door and chassis will attenuate it to nil.

[whisk.e.rebellion]

The shielding on microwave ovens only stops microwave radiation.

the solid chassis will stop pretty much all signal, the mesh door will stop 2.4MHz and lower frequencies. Enhancing the bond between the door and chassis will attenuate it to nil.

Touche. I was referring to the mesh on the door. But, even if you put copper tape up to seal the door, wouldn't radiation with a wavelength < 125mm still bleed through the mesh?

ETA: Or should I be worried about lower frequency waves?

[AnonEmous]

The cell phone in the microwave easily received several calls because it operates on a higher frequency that the microwave.

So I know my well-sealed paint can blocks VHF and cell phone frequencies (about 1900 MHZ),

Speaking of contradictory internet information, you appear to have some of your own.

Yes, correct, I should have written I assessed the cell phone in the microwave easily received several calls because it operates on a higher frequency. And that was incorrect. Good catch.

[crypto]

The shielding on microwave ovens only stops microwave radiation.

the solid chassis will stop pretty much all signal, the mesh door will stop 2.4MHz and lower frequencies. Enhancing the bond between the door and chassis will attenuate it to nil.

Touche. I was referring to the mesh on the door. But, even if you put copper tape up to seal the door, wouldn't radiation with a wavelength < 125mm still bleed through the mesh?

ETA: Or should I be worried about lower frequency waves?

Higher frequency waves will undoubtedly get through, yes.

FYI, when I mention copper fencing, what Im talking about are collectively called "RF gaskets". Youve definitely seen them before, most likely in pre-built computers by large computer companies who have to obtain FCC Part 15 certification for consumer electronics. Some are copper strips with springy fingers on them, some are foam tubing wrapped in metal mesh.


http://www.tech-etch.com/shield/ is a good place to look at them and check out specs.

[NoAm]

Great Thread and Information, Thank you!
Faraday Cages are on our prepping 'to do list' for 2012, so this info has come in handy for our research just in time!

[whisk.e.rebellion]

FYI, when I mention copper fencing, what Im talking about are collectively called "RF gaskets". Youve definitely seen them before, most likely in pre-built computers by large computer companies who have to obtain FCC Part 15 certification for consumer electronics. Some are copper strips with springy fingers on them, some are foam tubing wrapped in metal mesh.

Yeah, we've got a bunch of stuff like that floating around work. It's a godsend for getting FCC 15 on shitty designs

[bae]

Wavelength is important, as is math

Here's a "Faraday cage" for very very sensitive gear:





And here's the far-less fancy cage used for much of the rest of the merely-sensitive gear:



Notice the door has a metal gasket around it.

And the wire mesh continues inside the walls/doors/windows:



Here's the view out the merely-sensitive area's window, for a hint where this is

[Bunsen]

Touche. I was referring to the mesh on the door. But, even if you put copper tape up to seal the door, wouldn't radiation with a wavelength < 125mm still bleed through the mesh?

Not really. The mesh I've always seen has holes only a couple millimeters across, which should do a good job of blocking wavelengths longer than about a centimeter. That all depends on what you consider good enough, though, since the loss of shielding at higher frequencies is gradual.

Don't forget that sealing the door gap against other wavelengths with RF gasket would also require scraping the paint off the mating surfaces.

I strongly suspect, but have not verified, that frequencies below 2.45 GHz would also make it in and out of a normal microwave pretty easily. Since the door does not make electrical contact around its edge, the only way I can imagine that it would achieve good shielding would be to depend on the standing waves in the cavity having nulls at the edge of the door. That wouldn't apply at most other frequencies, so it would be rather poor from VHF on up.

bae: Are those reel-to-reel tapes I see in the fourth picture? At that point, you'd think it would be cheaper to take the very small technological step back to banging rocks together and forget the shielding.

eta: Kickass view out the window, though. Looks like an interesting place.

[williaty]

bae, you just made my wife jealous! She worked a summer at NRAO Greenbank but getting to work a the VLA is much cooler.

[KnightoftheRoc]

I heard my name used....

An interesting, possibly pertinent, point about my own little EMP event- as I think back on it now, not one thing with a GROUNDED electrical cord was damaged, anything that WAS damaged, had only a 2 prong plug on it. I would normally figure "voltage spike" instead of EMP, except that some of the damaged items were not connected to the house wiring, like the cordless phone handset. Oddly, my fiancee's cell phone was connected to her charger (a transformer type with a 2 prong plug), and that was fine. also, despite the amount of wire involved in the motor windings, no motors were effected (fridge, freezer, boiler, washer, dryer, etc.). Make of this what you will, I've read and been told so many versions of what works vs. EMP that it makes my head spin anymore. I've just come to the conclusion that paper records are a good thing.

[NoAm]

I was taking a little trip through youtube, in search of making faraday cages.
What do guys think about this?

If this idea is plausible, I was thinking of getting some of the computer bags (motherboard, etc. larger ones) to individually place items in, inside of the can.
Tell me what you think?

[williaty]

The guy does a bunch of things that totally blow his credibility in that video. Two of the worst:

1) Using FM radio to test to see if the can is leaky. FM radio waves are over 3m long. The can could easily seal with no gaps large enough to be "visible" to an FM broadcast radio wave yet still be leaky as hell at higher frequencies.

2) Shortening the antenna and then getting your hand near it doesn't mean anything like what he thinks it does. When the antenna is extended, it's resonant at the frequencies used for FM broadcast. When he collapses it, it gets too short to resonate at the correct frequencies, therefore the reception sucks. When he brings his hand near to the antenna, the antenna becomes capacitively coupled to his body, changing the resonant frequency of the antenna enough that it begins to be efficient enough to actually receive and demodulate the signal.

[Bunsen]

The guy does a bunch of things that totally blow his credibility in that video. Two of the worst:
...
2) Shortening the antenna and then getting your hand near it doesn't mean anything like what he thinks it does. When the antenna is extended, it's resonant at the frequencies used for FM broadcast. When he collapses it, it gets too short to resonate at the correct frequencies, therefore the reception sucks. When he brings his hand near to the antenna, the antenna becomes capacitively coupled to his body, changing the resonant frequency of the antenna enough that it begins to be efficient enough to actually receive and demodulate the signal.

There's more to it than just compensating for the de-tuned antenna. His arm is acting like the center conductor of a transmission line, and is introducing a lot more RF into the bottom of the can than would be there without it. If you've got a big metal can around, you can try it yourself using a radio that doesn't need its antenna extended. It should even work on AM. But your point about FM broadcast being a bad test is entirely valid, if you're looking for protection from nuclear or engineered EMP threats. The can would do a pretty good job against indirect lightning effects, though.

In my opinion, the "I don't know WTF I'm talking about" moment was when he mentioned solar flares (twice!). I thought up a nice little demonstration of how completely, utterly insignificant solar storms are for household electronics: The ground-level effect of a CME impact (which is the only spaceweather event that has any effect down here) is a slight change in the geomagnetic field. That changing field does induce currents in conductors, but (math warning!) the rate of change is on the order of nanoteslas per minute (maybe up to a few microtesla/min for really big geomagnetic storms). The Earth's magnetic field is on the order of 50 microtesla, and rotating an object in a field has the exact same effect as changing the field around a stationary object. So just by turning something around faster than once a minute, you can test the effect of a magnetic field change hundreds of times larger than the worst solar storm in history.

[williaty]

There's more to it than just compensating for the de-tuned antenna. His arm is acting like the center conductor of a transmission line, and is introducing a lot more RF into the bottom of the can than would be there without it. If you've got a big metal can around, you can try it yourself using a radio that doesn't need its antenna extended. It should even work on AM.

Married to a physicist and majored in it myself, so you can actually give me a good answer or link me to one (aka no need for the obligatory car analogy )...

Why is the arm acting like a transmission line? I'm probably missing something obvious, but I don't see why arm would do a better job than air.