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Ask Slashdot: Wooden Chasis and EMF 196

Red Leader asks: "Hi. I'm writing in the hope that some electrical people will be able to help me out with the nitty gritty aspects of shielding a computer case. You see, I'm building myself a computer case out of wood. I have already built one, but it was merely a wood case (replacing the plastic) which overlayed a standard metal chassis. Well, that was too heavy, and I also want to use my own layout for the 'guts' of the box. So aside from heat, noise and grounding (which i've pretty much figured out) - I'm worried about the electromagnetic interference aspect of this new machine. It's most likely going to be a dual Celeron based Ultra-Wide SCSI2 system - which I think (!?!) will generate quite a large electromagnetic field. I've been thinking of shielding the case by lining it with this with copper fabric or something similar - but don't know how it sizes up to a solid metal case. Any help would be greatly appreciated - and I'll be sure to post pics of the case (well it's actually more than that) in progress and when completed.
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Ask Slashdot: Wooden Chasis and EMF

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  • by Anonymous Coward
    The two things to remember are to make the shielding continuous (i.e. no gaps in the corners, etc.) and to ground the shielding inside the case.

    If you fail to ground the shielding, you defeat the purpose of it, which is to keep the voltage potential constant.
  • by Anonymous Coward
    The ability to shield RF with a faraday cage is easily accomplished. The key element to keep in mind is what frequencies to I want to shield? In this case you are trying to keep frequencies of approximately 400mhz(or below from other chips) inside the box. Since the wavelength at this frequency is in the range of 70+ cm, you should
    if a wire mesh is used), have no gaps or openings
    bigger then about 10cm(probably overkill). I forget the exact numbers but generally as long as any gaps you have are less then 1/4 wavelenght at the frequency in question, you should do a fine job of shielding.

    In addition to wire mesh, any ferrous metal should do just fine, regardless of thinkess, since
    the idea here is bouncing light off a mirror, rather then stopping a bullet with a wall, to use a metaphor.

    One final note..if you're like most people, you are much more likely to encounter problems from RF getting into your computer then RF escaping (radios with a few watts of output power near unshielded hardrives can sometimes produce
    unpleasant effects)

    Jean B

    General Radiotelephone Lisc., Advanced Amateur.
  • by Anonymous Coward
    My days as a radar tech. tell me that all the other posters are correct.
    A faraday shield is an EARTHED conductive material that COMPLETLY surrounds something, and electromagnetis radiation from either side will strike the shield and be drawn to earth.
    If your equipments isnt earthed properly the shield does nothing, the electromagnetism will be re-radiated by the case.
    As far as a mesh goes i beleive that a mech will block lower frequencies but not higher frequencies. If the wavelength of the elecrtomagnetic radiation is less than the diamater of each hole of the mesh then the shield will do nothing at that frequency and higher.
    The above is how waveguides work (waveguides dimensions have to be a multiple of the wavelength)

    This is probably way too much information, but i couldnt resits dredging up some old memmories
  • by Anonymous Coward
    To do it right, you will need to cover every area possible (except for door openings) and you will need to make sure that _ALL_ of the fabric is _PROPERLY_ grounded to an _EARTH_ ground. This is not as easy as it seems.

    I have some experience in this area, some years ago I repaired monitors that had very noisy switching power supplies and the FCC "busted" me because we blocked out the cop station next door with our interference. The solution was to encase our monitor racks in screen that we grounded. I put a little radio next to the racks and kept it on, if I got interference on that then I knew something was wrong and had to shut down. It worked like a champ!

  • by Anonymous Coward
    One other thing to remember about figuring out what size mesh to use when shielding PC's...harmonics. The PC bus may be using a 100 Mhz clock or the CPU running off a higher clock but that is not a sine wave, it's a square wave & square waves have harmonics, the degree of which depends on the rise & fall times. But since even Ku band satellite dishes (12-14 Ghz) have a mesh around the size of a window screen, those should be fine (window screen, not satellite dish ;-) ).

    GOESMan (awaiting password)
  • If I remember my EMC class correctly (which is about 2 yrs ago) even metal computer cases hardly provide any EMC shielding.

    The problem lies in the fact that there are gaps in any computer case (disk drive, CD-ROM, anything that can be seen on the outside of the case). These gaps leak almost as much of your EM fields as having no case at all (at least at the clock frequencies used by your computer).

    The only normal case I have seen that shields _something_ is my Alpha XL case, which is lined with EMC springs around every gap. This prevents the computer from disturbing my screen too much (and yes, it even makes the screen wiggle when the case is closed).

    If you want EMC protection, you need a special EMC cabinet and knowledge about connecting cables to such a cabinet (you can't plug in cables just like into an ordinary computer).

    Marcel de Boer, 3rd year student EE/IT
  • Ugh. iMac's monitor isn't shielded very well. At the place I used to work, someone turned on the monitor for the iMac next to where I was working, and my monitor got funky for a little while. It's like I degaussed it for the first time in years or something.

    Oh, by the way, no real work gets done on the iMac--it's just there to verify Web sites and to, well, look cute and attract investors. Heh. No one deserves the torture of having to work on an iMac :)
  • Anything I say is my opinion and mine only, it is not shared with anyone for whom I work with.

    I do not work for Linux.com full-time yet. That should tell you what you need to know.

    I'm sorry for any inconvenience or grievance that I may have caused.
  • I would like to say that I apologise for the negligent posting on Slashdot.org regarding the shielding of the iMac. The shielding of the iMac is obviously well within legal tolerances, and using the iMac will suit users in much the same way as Windows or Linux. I did not mean to badmouth the reputation of Macintosh systems. Most of what I say in Slashdot is in fact a joke. I will endeavour to make the distinction between truth and simple teasing painfully clear; in the event that I cannot, I will post through other means or not post at all.

    I would like to let you know that I am not yet in the employ of Linux.com. I currently work for a separate company for which the iMac is located. And its uses are just that--to test Web sites for accuracy and nothing else.

    Once again, I apologise for any inconvenience or grievance I may have caused. Thank you for bringing this to my attention. If you have any questions or suggestions, do not hesitate to send me an e-mail. Thank you.

  • I'm not certain myself (been a while since I did anything electrical, and I never fully groked FCs anyway), but wouldn't this sort of thing work in both directions? ie fields outside don't affect the inside and fields inside doeen't affect the outside. I suspect it depends very much on your grounding.
  • Thought so. Thanks for the confirmation.
  • Posted by viperx2:

    The Diamond MX300 has a daughterboard, the MX25 I think, that is fully digital and I think I read that it is compatable with minidisk format, so I assume it is also DAT ready. They use the same cables and stuff. The MX300 was 50$ on pricewatch, and the MX25 is available from diamond for only 25$. I looked at this a while back, so my numbers are off. The MX300 works in linux, while the SBlive does not. The MX300 is truly a fraction of the price, and, from what I understand, a much better card.

    -- The more you tighen your grip.... --
  • Posted by iam1:

    I would agree with the person above who said to check out the electronics setup in guitars. I am not sure about the copper "fabric", but a thin sheet of copper shielding will do everything fine. As far as grounding or "earthing" it, your power supply would do that for you.
  • Posted by HeapDump:

    The cases we sheild at work have nickel acrylic sprayed on the inside. They say that the trend is towards copper, but only because it is easier to landfill in Europe. It is a big thing for CE certification, or so I am told.
  • by volkris ( 694 )
    I'm pretty sure the MX300 does not work under Linux but the SBLive does.

    Correct me if I'm wrong, because I would love to use an MX300 in this computer.
  • That sounds right... Even 10 GHz has a wavelength of a few mm, from what I can remember... Unless you're using that .25-inch or so mesh, you should be safe.
  • You can't overestimate the importance of beads. All the machines in my house have full metal cases, and kill TV reception, unless you put inductors on every single lead. (Especially the power cable, as I think RF goes down that and up into our TV amp.)
  • You should see the ground systems some hams have. But this is a lot more than what someone needs to install just for an "adequate" ground.

    It also depends on the conductivity of your dirt. A swamp is a lot better than a desert. :) The ARRL Handbook (Basically one of the bibles of amateur radio) has a LOT of information on this.
  • As far as if you can't see the problem, no one else will - I have to agree with you on that.

    But if there is a problem, it can possibly occur house-wide. The only thing I can think of is that RF is leaking through my AC mains and into our TV preamp. Whenenver any machine in my house is running, Channel 11 gets funky. Putting ferrit on the PS cable and a few others helped a lot, but it's still pretty bad.
  • If I remember my Electricity and Magnetism correctly, a farday cage is an equipotential. The idea is that since field lines (in either electric or magnetic fields) can't cross, then the fields inside the cage are contained inside, and those outside are contained outside.

    So if you design something which is almost an equipotential you get an OK shield. Now, I'd think a fine enough mesh (something magnetic is probably best) would be suitable. But IANAEE and IANAP (an Electrical engineer or physicist).

  • Heh, when I was little, for Christmas, my brother got a radio controlled car, and I got a Star Trek walkie-talkie, with a morse-code key button. It didn't take me long, listening to the walkie-talkie, to realize that I could hear muted signals coming through from my brother's car, so I tried the morse-code key, and I made my brother's car turn. He tried returning it, but they wouldn't take it back because it worked just fine at the store. I drove him nuts for weeks until he caught me in the act, and beat the crap out of me.

    "The number of suckers born each minute doubles every 18 months."
    -jafac's law
  • The Trident 4DWave NX based cards support this. They're still a little hard to find, but you can order them from Hoontech at http://www.hoontech.com [hoontech.com]. They offer quite a bang for the buck.

    Trident actually wrote a GPL'd Linux driver for this thing, so it's a good card to buy.
  • Ah yes, first year physics...

    v = f * lambda, where v = c (speed o' light), f = frequency and lamba = wavelength.

    c is more accurately represented as 299 792.5 km/s, or 299 792 500 m/s.

    At a wavelength of 6mm (0.006 m), we have:

    f = v / lambda = 299 792 500 m/s / 0.006 m = 4.99654 x 10^10 Hz

    So, it's actually closer to 50 GHz (I think you forgot to convert 6 mm to 0.6 cm in your equation).

    Then again, I'm in math - what do I know? :)
  • Lead is a serious poison, especially to anyone under the age of twelve.

    And it doesn't require the children to lick it to pick up a bad dose. Basically, the big danger of lead is that it let's off lead dust constantly. This dust is then inhaled directly into your blood stream.

    I speak as a father whose son has had high lead levels... It's not worth it, don't use it.
  • Reminds me of my father almost 20 years ago, building a keyboard with a wodden case for the TI99/4A. It looked like the Flintstone's computer and was a long - running gag in my family...
  • Dolby Digital, DiscMan and DAT all use SPDIF for digital I/O. More and more soundcards support SPDIF, but beware: some only sample and output at one speed (SBLive=48K). If you're going for analog I/O, spend money; some cards cost more for a reason.

    SPDIF runs over fiber or copper, converters are available. The SBLive! forums seem to have good discussion of SPDIF issues. There are also cheap SPDIF-only cards available. Some are supported under Linux, I believe.

    'twould be nifty to have a thinclient with speakers in each room, feeding off a MP3/netradio server. Of course, decompression lag would lead to echoey audio between rooms. Better to just use USB speakers.

  • Use copper-clad steel sheet if you can get it. That way you get the benefits of magnetic as well as electric shielding (an RF emission has both aspects).
    If you really want to cut emissions, get some tin-snips so you can cut sheet stock to shape. Look at current cases to see how they even have petaled shielding cupping the keyboard connector to prevent leaks there.
    You'll also want to get some clamp-on ferrite beads (All Electronics in LA is my source) for the cables. The beads are lossy inductors at the intended frequencies, so they eat signals that would otherwise be emitted (or injected: you're protecting the system from arriving interference as well as preventing it from emitting interference, after all).
  • You'll have to be more careful to have good airflow throughout that box, compared to an equivalent metal box, because the wood is a better thermal insulator than metal, so the case itself won't throw off as much heat.
  • As a ham radio operator, I appreciate the efforts of others to shield their interfering devices. Open boxes can, and do, make a mess of the radio frequencies all the way up through UHF. Shortwave listeners, scanner enthusiasts, and wireless communications users would appreciate your efforts, too.

    Besides shielding your box, it's nice to put ferrite RF chokes on the cables, especially the video cable but preferrably all cables running out of the computer. The cables work as antennas, potentially leading some of the RF noise out of your shielded box and emitting it. Some cables have the choke molded in, if yours doesn't, get the clip-on kind from the Radio Shack. The more turns of wire through it, the better it works.

    I also use shielded CAT-5 wire on my 100-base-T network, and I use that same wire for my DSL connection, from the point where it enters the house.


    Bruce Perens K6BP

  • Yup. There's a physics major all right. The surest sign of a physics major is using TeX. Almost every other tech field has found some other way to express their complex equations, but in physics the equations are so unbearably complex you need to use TeX to sort it all out.

  • My old nemesis, we meet again. Your obstinance is nothing more than the indication of a weak mind. Checkov, set phasers to "Bar-B-Que".

    You certainly won't last long aganist our attacks, my friend.

    Go boilers!

  • "crisp packets"? Is that some new sort of networking that has packets which are crunchier than regular TCP/IP? (mmm... crunchy networking)

    Oh. Right. You mean "potato chip bags." Heh. I forgot they say things weird on the other side of the big pond.

  • Each part DOES need to be certified individually, but if you assemble them it changes the picture: how well do they work TOGETHER.

    If you assemble a system from parts and want to sell it, it needs FCC testing. JUST because the parts passed muster does not mean they will assembled.

    How do I know? We sell workstations with custom hardware and software, currently based off Intergraph GT1 TZ2000's. Even if we make no modifications we would need retesting, but once you add in custom PCI cards with additional processors there's some work to be done.

  • When someone turns something on, and your monitor goes flakey like that it's because your POWER system's stretched. Plug into another circuit, or MAYBE add a UPS, and tell me if it still happens.

    I'll ignore the rest of your comments since you knew they were unprofessional... but you probably think Mac users are people not "smart enough" to use Linux shouldn't use computers, riiight?

    A little tip - when you use the linux.com URL and reply-to address in stupid posts, it's no different than using company letterhead to for dumb faxes or snailmail. You'll figure it out soon enough...

  • Actually, the Apple 1 came *without* case - you were expected to supply your own.

    Some of the computer history sites show the same wood photo over and over, but that's just chance and distribution of the same photo.

    And at 1ish MHz, the thing didn't throw out much heat or (guessing here...) interference. Heck, my G3 300 MHz doesn't even need a CPU fan - just a tiny heat sink. By contrast my AMD K2/450 has a heat sink the size of a coffee mug, and a fan atop to boot...
  • ... btw: IIRC the Apple I ($666.66 mailorder price) featured a wood case.

    Though I doubt the grain was anything to write home about..
  • I don't know how serious you are about the sound quality, but you'll do even better with an optical link rather than a wired one. A wired link will still introduce noise into your DAC, though the better ones will do a good job of keeping that noise out of the analog.

    Out of curiosity, what DAC do you plan on using? I've seen some mighty cheap ones in Stereophile as of late... I even recall one that was less that $100! :)
  • by Ares ( 5306 ) on Wednesday June 16, 1999 @04:06AM (#1848855) Homepage
    I seem to remember that a 100 MHz signal has a wavelength of 3m (c/100Mhz). Likewise, a 500 MHz signal has a wavelength of 3/5m = 60cm. Even at 1GHz, the wavelength is 10cm. I'd say you're fairly safe using a mesh (at least for now).
  • Just stick aliminum foil to the inside of the
    case, just do not forget to connect it to ground,
    also connect the motherboard to ground. All the
    individual cards need to be grounded too.
  • Just stick aliminum foil to the inside of the

    case, just do not forget to connect it to ground,

    also connect the motherboard to ground. All the

    individual cards need to be grounded too.
  • First of all , there are three "S"s in CHASSIS.

    And secondly, you are all thinking too
    complicatedly (sic), just take some metal mesh
    (screen door?) and staple it to the insides of
    the wooden computer case. And yer it is a
    faraday cage, and yes it keeps emf out and yes
    it keeps emf in. I should know this crap, I have
    a degree in physics.
  • The problem with not shielding, is that it increases the EM pollution in the area. While this doesn't sound like a problem to you now, wait until you want to use some sort of wireless networking or other sensitive radio equipment. Having to cope with EM interference will either reduce your data bandwidth significantly or make the equipment more expensive because it has to filter out the extra crap.
  • Look for a product called mumetal (sp).
    It generally comes in sheets which are
    almost like thin cardboard.
  • quit using them bigish words, dude. they confuse me.

    and maybe i won't last long against your attacks, but i shall certainly at least die knowing i was defending the honor of my school...

    go irish!!

    (ha, take that! two exclamation points!)
  • When I was 14, my dad brought home some Compaq portable XTs (suitcase models) that he found in a dumpster. I got a couple working and sold them, but I kept the rest to fiddle with. I built what may have been the first wooden XT tower. It was nice, but even with three cooling fans, it got HOT!!

    What I did was remove the plastic casing from a machine and use wire cutters to split the chassis in half. With the Compaq, the 9" monitor & PS were on the left and the drives and MB and cards on the right, each in their own separate cages. The I attached a board to the top of the MB cage and attached the drives and power supply (the PS was a bare board!) to the top of that. I attached a cooling fan behind the PS, cut a hole in the board and had another fan ventilate the MB, and had a third fan ventilate the 20MB Miniscribe HD. The outer casing was all plywood except for the base, which was 3/4" pine. The front bezel and back cover were screwed directly into the cages, but the top and side covers wrapped around with hinges and attached to the base with latches.

    As far as shielding, it was very necessary to have the entire cage. If I left the side panel off, the EMI was enough to make the TV fuzzy and to cause the cordless phone to lose sync with the base and not work until it was replaced on the base. For anyone making a case, don't forget what a great INSULATOR wood is and to shield well. In my case, the large amounts of heat were also a result of having old equipment (such as the completely stepper driven HD and the 8087 chip), but I'll bet things haven't improved much since then. The cages in the machine were not solid, but sheet metal with nickel-size holes cut at regular intervals. If you have an old machine, why not make the conversion?
  • I've used a a couple coats of carbon paint to shield against RFI in electric guitars and even good old heavy duty tin foil (what a bitch to tape down though). This might do the trick for you, you just ground it with a screw. Thin sheet metal would be best but if your case is solid hardwood the weight savings of paint would help.
  • I have used 4 to 5 five computers in the same room with the covers off with no problem. Sound was fine, moniters were fine, TV was fine, and radio tuner card was fine. Why would computers be bothered by the EMP created by computers, They are always inside the Faraday cage of the case.

    The amount of interference from AC wall power is worse in my experiance then the interference then a computers with there covers off.
  • I don't understand why a dual cpu system would matter. Aren't you going to put heat sinks with fans on the cpus which are metal and the fans power is a source of ground. As for the SCSI card you are worried about interference there well if scsi had a problem with interference then it would not run inside the case where there is lots of interference, or outside the case in places like computer rooms.

    I also remember reading in a computer manual some where that if you get interference to try moving stuff from behind the computer, they admit that the fan of the power supply is a great big EMP hole.
  • Of course, the square wave harmonics drop off (technically without actually ever reaching 0) as the frequency increases. You'll get some high frequency leakage, but I don't think it'll be that much of a problem.

    If I were an EE person, I'd even post the equation for the amount of signal power vs. frequency in a square wave. :-)

  • Heat and airflow from the fans would probably cause quite some lead in the air coming out of the thing, doesn't sound too healthy..

  • Okay, I've read everyone's comments, and I think that the copper mesh should provide sufficient shielding. I don't know how overboard on that I want to go, considering what else I'm trying to do...

    As I said, I hope to put up pictures of it in progress, and when complete. I just need to get to a scanner...

    There was one comment about heat, and one about the movement of wood. To those guys: I've got that pretty well covered.

    As of my current design, insulating against sound is my primary concern. I'm going to line the entire case with a really dense (1 lb. per sq. foot) dB blocking fabric, as well as baffle any ventilation openings. The sound concern is directly related to my ventilation solution. I've long since decided that a filtered positive pressure system is the only way to go. The implementation on the first case wasn't so hot - but I've learned from my mistakes. Both cases are supplied with cool air from a large 90mm AC fan (80? don't remember) of all metal construction. It's not a normal case fan, but rather specifically a high power 'pushing against resistance' fan. The only problem is that it's pretty damn loud, hence the complicated sound protection.

    As for the wood movement: Absolutely. I really enjoy joinery and fine woodworking, and realize that one can't make anything without keeping all the grain essentially parallel. For those who don't know, wood expands and contracts with the weather by varying amounts across its grain (perpendicular to the stripe pattern). If you were to make a joint in which the grains were not expanding and contracting in the same directions, then your joint will literally tear itself apart with time. Then what you made will crack, and or, fall apart. Case in point - on the old case I have a mahogany and maple board (grain running up/down; two strips of mahogany with a central 'racing stripe' of maple) working as the face of the box. In it is a slot for the CD tray. I took off the plastic tray's end cap, and mounted a pice of ebonized maple on it. That piece has grain running left/right (naturally), and hence doesn't significantly expand left/right. The face does move, though, and in the winter I was surprised to see that the extra room I had given it on either side wasn't enough, and that the CD tray wouldn't close unless I took the end cap off!

    I can go more into depth, and talk about this for many kilobytes, but I'll save that for the web site where I'm going to put all this up for everyone.

    Thanks to everyone for their help and comments (sorry, aluminum foil people, I'm going with a little more style than that... this *IS* a piece of furniture, after all). If you have any questions, feel free to talk to me. I love bouncing things off people, and I've given this project a LOT of thought and planning.


  • Running your computer without shielding is like turning your stereo all the way up. You are likely to disturb someone else's radio reception or amateur radio station. You might also be disrupting digital transmission (wireless printers, etc.).

    Unlike acoustic noise, it's a lot of work for people affected by radio interference to figure out first of all that it is happening at all (rather than that their radio or digital wireless system is broken) and where it it is coming from.

    Causing interference like that is also against FCC regulations, and people can track you down and have you fined.

    Unless you live on a desert island, if you run your computer open, sooner or later you are going to cause other people a lot of unnecessary headaches. So, please shield your computer.

  • It's hard to track down interference, so, yes, you can probably get away with it without someone knocking on your door.

    I think the question shouldn't be whether you are likely to get caught, but whether you are causing someone else trouble. If you run your computer unshielded, there is a good chance that you are.

  • Many of us run caseless w/o any problems. It really depends on what else you have around it. Neither wood nor plastic shield EMF. Sometimes running without a case is nice if you're lazy like me or just like blowing the dust around.
  • So, if I don't shield it and cause interference with my mobile phone the world will end? As long as you haven't got anything very sensitive I'd say it's it's OK to "cause interference", but of course, I'm just in tenth grade and know nothing about physics...

    Also, it's not like the FCC will knock on your door...
  • The computer in my living room has no case. Is that why my cheap Grundig speakers blew up when I connected them to my Onkyo sound equipment?

    Automatic gears are for women who don't know how to drive, the elderly and Americans...

    * Please note the irony in this post *
  • For example, the keyboards are milled from one single piece of wood, so that the age-lines go through the entire keyboard and the keys.

    That is one of the coolest things I've ever seen. It would be interesting to see one of the keyboards or mice after a couple of years of continuous use. I'm sure there would be all sorts of interesting wear and colouration.

    Is it possible to be a wood geek? Mmmm, hardwoods.
  • > Ugh. iMac's monitor isn't shielded very well.

    Not many monitors are shielded very well. All but the expensive industry-quality monitors are encased in nothing but plastic.
  • Never tried it myself, but I'd think a metal mesh of some kind would work about as well as solid metal -- after all, a Faraday cage doesn't have to be solid.

    But wait, all a Faraday cage does is ensure there's no EMF inside the cage, which is sort of the opposite of what you want... someone with more electrical engineering knowledge bail me out here?

  • >>No one deserves the torture of having to
    >>work on an iMac :)

    Well, there is one Steve who sure deserves it.
  • I would like a "music server" in my house. That is, connecting an old computer with no peripherals to a music system. I would control it by telnet or a web interface. Obviously, good sound quality is a high priority, and that might very well mean shielding.

    Actually, though, what I would really like is an all-digital sound card, with any analog processing completely cut out of the loop. Ideally, it would output a digital signal that a good home audio system will decode, such as Dolby Digital, if that is possible. Or maybe the DAT format.

    Anyone know how, with digital signals, shielding, or some combination of the two, you could make the best, cleanest audio signal possible? This wouldn't just be for MP3s, but also higher quality stuff. Any advice on this greatly appreciated.
  • I work at a printing company, and some of the packaging we have to do is done with wooden crates lined with a kind of aluminum foil where one side is, likely enough, aluminum foil, and the other side looks more like utility paper, or packing paper. The machinists in the building actually use the same foil to prevent an EM field, which causes nausea when working with the two pieces of equipment we do have that are highly electric. They were given this hint by some other printing company in Britain who uses the same devices.

    I'm not sure where you can purchase it exactly but with a little poking around I'm sure you could find it. One side looks like normal aluminum foil (a little higher gauge however) and the other like a brown paper bag. If you try it, lemme know if it works, I might just comission a computer that doubles as an endtable. :)
  • Copper fabric will corrode over time. This will make the size of the holes efectively larger as the individual wires won't be connected at each cross anymore. This is a problem with all woven wire shielding methods, even ones made of Stainless Steel wire. The other problem is you need to ground both ends of every wire or those wires you don't properly ground will become capacitive radiators.

    The way around this is to instead use perferated or expanded metal sheets. Perferated metal sheets are ones that have many holes stamped into them. Expanded metal sheets are done by slicing many short parallel slits into a metal sheet, then streaching the sheet to expand the holes. It can then be optionally rolled flat again. Either way it weighs less than a solid sheet, but still supplies EMI/RFI blocking if made of the right materials.

  • Well, the only one I know of (that's not a professional card) is the SB Live, full edition. With it's digital I/O card and the extra optical digital card, you can input digital, process the signal digitally, and output digital. It can even mix digital audio, internal digital CD, digital MIDI, one analog source, digital SP/DIF, and optical digital all at the same time. I use my computer as a piece of my stereo system, with the internal digital CD, two digital in's (MiniDisc and an external digital out CD player,) and MP3; all being processed for better sound (if you're running [god forbid] Windows, try using the "Concert Hall" environment for classical music.)

    And, of course, it's 48Khz digital SP/DIF out is great for recording to MD or DAT. (Or going to a high-end reciever)

  • I don't know if copper cloth with cut it, but reatively thin sheet metal should do it. This is what they use for a lot of plastic cased PCs Make sure that the sheets are well sealed and are electically contiguous.
  • Well I suppose what you want to do is reflect
    the radiation. But the fraction of radiation
    getting out would be approx. proportional
    to the %area of the mesh which is sold.

    Hmm, I wonder what the diffraction effects do?
  • Your computer will generate RFI, and very little EMI. Copper mesh will work well, and is far easier to connect to than aluminum foil. If you emit significant RF energy, you will run the risk of upsetting neighbors who are ham radio operators, or perhaps will interfere with TV reception.

    If you know of any nearby hams, they can help you with practical tips on how best to shield your case. A ham can also scan some of the radio bands looking for your noise, if willing.

    To check for TV interference, use a TV with rabbit ears (not cable) and check the low VHF channels (2-6). Those are the most likely to be affected. If interference is present, shut down your PC to be sure whether it is the source.

    Also be aware that the noise signature of a machine will vary as a function of what it is doing. Disk access operations will change the RF compared to purely CPU based operation, so do a variety of things with the machine while checking.

    Also, RF, as with any field energy, decreases as the square of the distance, so move the TV within 6 feet or so for initial checking. If you can't see a problem there, chances are no one else will, either.
  • While magnetic flux is shieldable, magnetic fields are not. Also, one must ground the shield to hold it at constant potential (Earth serves as the best source of such a voltage).

    - Jeff Sherman
    Physics undergrad, UT Austin
  • What's the big deal with EM worries? I run an O/C Celeron 400 with all kinds of sensitive parts (SBLive, DVD Decoder, etc.) just thrown caseless into a tiny little wood cabinet and I don't get the slightest bit of noise or interference in anything. I always figured the shielding bit was one of those stupid FCC regs that could be safely ignored.
  • You might just be forgetting Maxwell's time-dependant equations, then (specifically $\rot{\vec B}-\frac1c\pder{\vec E}t=\frac{4\pi}c\vec J$). As someone said further up, the problem with meshes is that they effectivly act as high-pass filters, quietly letting through any radiation whose wavelength is less than $\approx1/\sqrt{5}$ of the mesh length. You can check this easily: look through it :) .
    I find that most of the vox-populi notions about Faraday cages come from radio people, where you only have to worry about very low frequency (long wavelength) radiation.
    Hey, where is the `submit as TeX' button?
  • Nice try but lead is not a ferro magnetic media. Steel wire cloth (window screen) would work better, be lighter, less toxic. You can cut window screen with kitchen shears and staple it to the inside of the case.

    (Yes lead is a conductor, a poor one. Faraday cages are great for weak electric fields but leak magnetic fields unless a magnetic conductor is used to trap that nasty B field)

    While your children are not licking the inside of your case the heat from the components in the case will cause vaporous lead and the trace mercury found in all lead to out gas. In and enclosed area like the typical bedroom you could get heavy metal poisoning in a few months of exposure.

    Yup, some people use lead as a shield against radioactive particles. So it blocks Superman's x-ray vision. Funny thing is, things like cement, paraffin and water are better shielding per unit weight. It must be some 'other' property besides particle capture cross section that blocks Superman's sight. Otherwise he should not be able to see through a concrete wall.

  • 3-4 feet isn't enough in a lot of areas, I have a friend who just put a new ground rod in, and code required something like an 18' rod in his area, he had one heck of a time getting that all the way down.
  • My friend lives up in the mountains, the ground up there isn't very conductive. I know people who have amateur radio setups, I've heard about the grounding requirements.
  • I'd like to see how a finished one looks and how hard it is to make one. I'm getting ideas of a tower case that doubles as a couch endtable....
  • The emf-l mailing list comprises laymen, experts, and other concerned citizens. The topics are just about everything you could think of in the field (pun) of the ever-increasingly "blue" world that we're all living in. Unfortunately the list moderator and maintainer is (as of today) on a two week vacation which puts the list on hold as well. But, you may still join now (OH, the work load of the /. effect. Poor Roy...). Just send an email with your concerns to rbeavers@llion.org and surely there will be someone who is able to help you.


    PS. BTW, don't mail me, as my mail server is currently broken. DS

  • I work in television engineering and we deal with EMF issues on occasion. There are actually several issues to consider.

    First, there is low frequency EMF, this is basically magnetic fields. The funny colors on your monitor that clear when you degauss are the result of low freq EMF. Put a speaker or magnet near your monitor and you can see the effect.

    Second, there hum EMF, typically seen when a monitor is near a source of EMF at a nearby frequency. You may have seen this if you place a 60Hz monitor next to a 75Hz monitor. You will see hum bar rolling through the monitor.

    Thirdly, there is high frequency EMF, not typically seen unless dealing with sensitive electronics with low signal to noise ratios.

    For the absolute best shielding use mu-metal. It is specifically designed to block EMF. There isn't much better than mu-metal. Be prepared, mu-metal is expensive. See www.mushield.com [mushield.com] (best) or Cutting Edge [cutcat.com] (okay).

    If you can't afford that, the copper will probably work OK. Try to leave as few gaps as possible. Make absolutely sure you tie any shielding to ground. Other simple tips are making sure the power cords are plugged into grounded sockets (get a $5 circuit tester), use a good surge protector, and put those little ferrite clamp-ons on your I/O cables. Check out the EMF Do's and Don'ts here [emf-emi.com].

    I think the thing to worry most about shield ing is your monitor. It is the most EMF susceptible thing you are likely to have. If the built in shielding is good, you will probably be OK.

    But in all reality, you might try running with out the shield at first and see how it goes. If you don't see hum in your monitors, your speakers don't buzz, and the dog doesn't have his paws over his ears, you might be OK without it.

    Good luck.

  • And don't forget your tin foil hat.... I never take mine off. Those aliens... err electomagnetic fields, aren't gonna get me!!!!
  • So, is my PowerBook G3 a big antenna?

    No, it's quite a few moderately sized antennas and a couple thousand minutely sized ones.

  • Several places sell sheet lead about a 16th or 32nd of an inch thick for use in soundproofing. I've worked with it, and it is really easy to work with, incredibly flexible and easy to cut. The only problem with this is that it is *really* heavy. But for shielding, there is probably nothing better. Hell, superman can't even see through it.

    Before anybody says it is dangerous for children/animals, do your children lick the inside of your computer case? I thought not.
  • Reading the articles in this thread is certainly interesting me now as I am doing a similar project on and off.

    I have a server in the rec-room that does masquerading for my local network (I can even hook up my laptop using PLIP and use it whilst lying down on the couch. Man, am I lazy!). Though the main reason I put it in was for an MP3 player to hook into the receiver. Here's the specs for those interested:

    - K6-2/300
    - 64MB RAM
    - 2.5GB HD
    - Good ol' GUS (though you guys have gotten me interested in using one of my TB Montegos (they've have gone waaay down in price recently, I believe they're cheaper than the SBLive), which has SPDIF and 18-bit AD/DA converters, so that might get interesting, especially since I now have a DVD and am thinking of getting a digital receiver. I can record sound from DVD now, though)
    ATI All-In-Wonder Pro 8MB AGP (Waiting for gatos to get TV-Out... Then life will get more fun)
    - Generic (Packard Bell) PC remote. With LIRC, I can use any remote to control MP3 playing.
    - SuSE 6.1

    It also does SETI@Home for Team Slashdot as does the other computers on the network (except the laptop of course) :-)*

    So, if you want absolutely, positively, best playback on a system, get a digital receiver and either a TB Montego or a SBLive and you should be sittin' pretty without having to worry about shielding.
  • As long as you haven't got anything very sensitive I'd say it's it's OK to "cause interference"

    Well, as long as I don't throw anything poisenous I might as well chuck crisp packets and other litter around. But of course, I know nothing about the environment.


  • What you want is Tempest-grade sheilding. Do some research on tempest.
  • All computers emmit a certain amount of electomagnetic radiation. Now, let's take your monitor as an example. Obviously, it emmits visible radiation (light). But that doesn't travel very far. But your monitor ALSO emmits other kinds of radiation. These waves almost resemble the image on your screen, if you analyize them enough. These waves are usually much faster than visible light waves. That means some feds can park a black van accross your street, intercept the highspeed radiation ebing emmited from your monitor and regenerate the exact display of your monitor in the van.

    The United States Government started research on this phenomenon many years ago. They code named the project 'Tempest'. Today the Tempest project has been mostly declassified and information on Tempest can be found all over the Internet. I can find the URL for a really informative site on Tempest, unfortunatly. You could try certain channels on IRC. I beleive someone from #2600 (EFNet) has a huge site on tempest.
  • >You misspelled "higher"

    You misspelled "lower"
  • It is my understanding that your chassis ground (case) is an electrical reference point. nothing more. If you decide that your chassis is going to be a metal grate nailed to your wooden case, that is fine (or just use a nail). Connect the ground wire from your power supply to your chassis metal grate (or nail) and anything else you want grounded. This grate is then your ground (or chassis or electrical reference point). The chassis ground for those who don't have a case is the metal ring on the end of the ground wire connected to the power supply.
    The bottom line is that it is not manditory to have something to connect it to (though it can be handy). You can leave the wire loose if you want (but anything touching to it is then grounded).
    Make sure you are consistant with your ground (do not have multiple chassis points. you can only have one reference point) or you may run into problems

    As far as EMF goes, I don't believe it is that huge of an issue unless your circumstances are unusual. The EMF generated by your computer is so small that it does not interfere with itself or other equipment around it. AFAK.

  • You can buy snap-on chokes (basically coils) at Radio Shack to snap over all the cables leaving the case. These cut EM WAY back. You can test this with a cordles phone (old 49mhz type). Hold it (turned on) next to the cables at the back of your PC. You can hear the signals (sounds like new age music).

    Last night I was working on my sons remote control truck. I set it next to my keyboard and it took off! It was picking up signals radiating from the keyboard, or more likely the keyboard cable. It acts like an antenna.
  • >Look at current cases to see how they even have
    > petaled shielding cupping the keyboard connector to prevent leaks there.

    Yeah, look at cases in offices especially. The FCC type 15 rules are MUCH tighter for stuff in the office (more stuff to interfere with) than in the home. Work places don't always follow the stronger rules (the better shielded cases cost a lot more) but big companies tend to.

  • That's to keep the NSA from reading your e-mail, right? I read a book a while back about how British Intelligence listened in to Russian teletype traffic. They put a bug on the main ground rod at the Soviet embassy. The Russians, following good engineering practice (say what you want, they are damn good enginners) had tied all their grounds together to prevent ground loops. The Brits could get the encrypted copy and the clear copy, because their encryption gear was tied to the ground rod..... :)

  • FCC Section 15.102 directly addresses this issue.

    The big deal is poorly shielded computers are the biggest generators of radio frequency trash that other services have to deal with. Not ionizing levels, that's another story, altogether.*

    RF interference drives hams crazy (ier). They constantly have to track down leakage from bad casing, bad connectors, cables, etc. Don't care about them? How about hospital patients monitored by sensitive medical equipment? A clean RF environment is important for these devices.

    The noise generated by computers already extends well into the VHF range. With the number of computers rising and CPU speeds going up and up, the noise is going to collide with more services such as cell phones.

    C. L. Smith's Unclaimed Mysteries
    *All: "That's another story."
  • Actually it is the opposite. Office/Industrial
    equipment with oscilators over 10 KHz must be FCC Class A Aproved. Residential equipment must be Class B Aproved, which is generally 10dB more strigent for most frequencies.

    The theory being, residential environments have more radio and TV receivers than industial/office environments, and residences are typically closer together than factories and office buildings.
  • There is an exception to your "only on an island" theory. If you live in Minnesota, South Dakota, or the likes, you won't have a problem. There aren't enough electrical devices in these areas to worry about electrical interference. (Well, in most areas, anyway...)

    ------- CAIMLAS

  • Funny thing is, things like cement, paraffin and water are better shielding per unit weight.

    Paraffin? That's cool. What's the comparison between cement and water? If you used it in a reactor, say, would it glow blue from Cerenkov radiation? Could you use it in liquid form as a radiation-resistant heat transfer fluid? Does this mean that Superman couldn't see through the top of Liberace's piano?
  • The whole idea sounds pretty cool - I remember a company back in the 80's that was making cases like this for XTs and monitors - I guess a company still is (www.woodcomputers.com).

    If you are going with an all wood case, I would line it with thin copper or steel sheet (heck, aluminium foil would prob. be ok), and solder at the edges, or use metallic tape. I would forget the fabric stuff. Build the case so that the back side/bottom is a normal metal case (with the front, top and sides being wood), and use "L" angle aluminium to screw the wood down to the PC chassis to complete the shielding.

    Another thing to consider is to just "fake" it - bond/screw the wood to the metal case - in other words, build a facade case around a normal metal/plastic one - shielding would be taken care of by the regular case. Unless this is all to impress any woodworking buddies or something...
  • Funny thing is, things like cement, paraffin and water are better shielding per unit weight.

    Well, this is true, but there are usually volume considerations as well, in which lead does considerably better. Lead also holds a shape better than water does, and is unlikely to short circuit equipment. And if you are using paraffin, better keep your machine cool, or you'll have the same problems as with water.

    (First /. post!)

  • Amateur grounding is probably the easiest, most effective, and least-understood method of messing up your equipment, your home, and probably yourself.

    If you need better grounding, or think you do, and don't understand it well, then get a good electrician.

    With that said:

    You can get a 3-4 foot copper clad steel rod

    You would want a steel-clad copper rod, not the other way 'round. Exposed copper will quickly oxidize into an amorphous, non-conducting blob. The steel should be galvanized, of course. Aluminum or a similar "rustless" covering is good, too.

    If you need to add a better ground to your house ...

    Then you need to remove the inferior ground from your house first. This is a must-do. If you have >1 ground point, imagine what happens when lightning hits that shiny new ground rod you installed...

    There's suddenly a huge positive potential at the rod, which will dissipate as quickly as possible wherever it can. It takes a while to do that, though, because the earth doesn't have a high density of delocalized electrons. It takes lots of earth, and some time, to sort out the potential difference introduced by a strike.

    With just one ground point, you're pretty much OK -- there aren't enough idle electrons in your equipment (compared with how many there are in the earth surrounding the rod) to do much damage.

    With a second ground point, though, there are *zillions* of handy electrons near the earth around the other ground rod.

    They will flow from the old ground, up to your house, through your equipment (depending on exactly how things are wired), and out to the new ground. Before it burns out your wiring and sets your home on fire, this current will be very large.

    Incidentally, this is also why it's not good to be near a lightning strike and standing on open ground. If your feet are planted, you have two ground points. There can be ~1e5V potential difference between one foot and the other as the charge dissipates. Without heavy rubber soles, that difference will resolve itself by sending current through your low-resistance legs and crotch. If you're holding a tree or metal rail or other somewhat-grounded thing, then add "heart, lungs, and spine" to "legs and crotch" above. Not fun.


  • The key in shielding is to surround whatever you wand shielded with a conductor. EM waves don't penatrate conductors very well. I'm not even sure you need to ground it, but it is a good idea for other reasons if not this one (unless you like getting electicuted). As a previous post mentioned, the smallest opening should be smaller than the wavelength of the EM radiation, in this case a few cm should do it.

    Take a look at your mocrowave door sometime. It uses the same idea. THere is nothing special about the glass, it just has a piece of metal with small holes in it. The EM (microwave) frequency is about 2.5 GHz IIRC. Those little holes are smaller than that wavelength, so nothing gets through.
  • I haven't had a lid on my box for a long time now.

    The only noticeable effect is that not one AM radio in the house works when my computer is on. But that also happens when my Win box is swtiched on (and I rarely have the lid off that).

  • by S_Walker ( 60381 ) on Tuesday June 15, 1999 @09:39PM (#1848931)
    Wire mesh is just fine for basic emf and rf shielding...
    It should be roughly contiguous, although, that is hardly worth pulling out a micrometer of magnifying glass.

    Just make sure each seperate piece of mesh when assembled makes good electrical contact with the frame? (if you have some sort of framework on this) or other conductive bus. To test if this is roughly adequate,
    1. get a multimeter (measures voltage/resistance and usually current, sometimes mislabeled a voltmeter)
    2. set it to measure resistance or continuity.
    3. touch 1 probe to one portion of mesh, the second to a point opposite...as in as far as you can get inside the case from point 1
    4. look at the resistance readout.
    5. It should read 0 ohms (if yours has a floating needle, it may be a bit off, not to worry) If it reads anything > 1 ohm, you don't have a significantly conductive material to be useful as a shield/ground...
    (For those who will argue here...I just finished measuring the foil from a pack of marlboro light 100's that came out to 200 mOhms , or 0.2 Ohms, in case you cared.)

    Now for the final key point. Ensure that the power supply is properly grounded, your shield is properly grounded, and that your new case is plugged into an adequately grounded 3 prong receptacle.

    For clarification...if you don't plug it into a grounded receptacle, what you've just configured is essentially a floating common attached nicely to a power cord which sould make a passable antenna for rebroadcasting all the stuff you just collected.

    If you need to add a better ground to your house...i.e. you get lines in the tv when you run the vacuum...talk to an electrician unless you are comfortable in a circuit breaker box.
    You can get a 3-4 foot copper clad steel rod, with copper wire, to act as a grounding rod. have the electrician hook up the copper wire to the ground bar in your circuit breaker box, then you attach it with a clamp to the grounding rod.

    Note..you're going to need a sledge hammer...preferably a big one...
    Apply the sledge hammer to thetop of the grounding rod until only about 3-4 inches sticks out of the ground....
    now attach that wire, and your house ground should be much better.

    Whew...wrote a lot more than I planned...but you get the idea.

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