does aluminium shield RFI - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: September 20, 2012, 09:05:40 pm
content: simple question. If I have an aluminium box will it keep in RFI/EMI generated within ?

author: prenato
author_rank: Regular Contributor
timestamp: September 20, 2012, 09:11:25 pm
content: My understanding is that it should as a "Faraday cage" only requires that the material be a conductor, which aluminium is of course:http://en.wikipedia.org/wiki/Faraday_cageBTW, Found this quotein the wikipedia article interesting:"A booster bag (shopping bag lined with aluminium foil) acts as a Faraday cage. It is often used by shoplifters to steal RFID-tagged items."

author: Bloch
author_rank: Unknown
timestamp: September 20, 2012, 09:14:12 pm
content: Quote from: Simon on September 20, 2012, 09:05:40 pmAll RFI/EMI ? no"Some" RFI/EMI yeshttp://www.omniscienceisbliss.org/rfid.htmlQuote

author: Simon
author_rank: Unknown
timestamp: September 20, 2012, 09:19:27 pm
content: so you wouldn't expect to be selling a military grade product in aluminium if it is to not emit rfi.

author: SeanB
author_rank: Super Contributor
timestamp: September 20, 2012, 09:27:57 pm
content: Really depends on how well you join the pieces. Weld all seams and have no covers it works pretty well. Seams, missing screws and big gaps and slots and you make antennas in it.

author: kg4arn
author_rank: Unknown
timestamp: September 20, 2012, 10:48:09 pm
content: Quote from: Simon on September 20, 2012, 09:05:40 pmWell the question may be simple but the answer is not so simple.It is going to depend on how much RF you want to keep in (or out) and at what frequencies. Higher frequencies need smaller dimensions to the boxes lid perimeter. It really can be amazingly hard to seal up an enclosure to RF at 30MHz and above.For really tight RF applications all power and control signals should pass into the boxes via feed through caps or filters.All RF signals should be coaxial from the board outward (the BNC or SMA connector at the box should be the coaxial crimp or solder type). And it helps to build the box from PC board and solder all the seams shut!!I don't know much about EMI and I am not an expert. These are the things that I have had to do with my sensitive radio equipment.

author: gxti
author_rank: Frequent Contributor
timestamp: September 20, 2012, 11:42:34 pm
content: Quote from: kg4arn on September 20, 2012, 10:48:09 pmWhy is that, is it a near-field vs far-field thing? Because a microwave oven, for example, is pretty big compared to the wavelength but a metal mesh on the door is sufficient to keep your eyeballs from boiling when you stick your face in front. Or maybe I'm misunderstanding and you meant that the gaps in the enclosure have to be smaller, which makes sense.

author: TriodeTiger
author_rank: Regular Contributor
timestamp: September 21, 2012, 02:05:14 am
content: Quote from: gxti on September 20, 2012, 11:42:34 pmFor what it's worth, it is designed toattenuatethe radiation from the microwave (which is why you may get interference on your cordless phone or wifi even with this "faraday cage screen"), and is less relevant to shielding EMI, as a cell phone for example could very well be effected by slightly attenuated signals.Aluminum will block presumably all electric fields (it is not permeable), and it might be worth it to test yourself, by placing your cell phone inside and calling it or similar. Maybe metal screws tightly bound, LCD screen/buttons covering ports snugly, etc. are enough to block what you are hoping to - but just "an aluminum box" - probably yes.

author: kg4arn
author_rank: Unknown
timestamp: September 21, 2012, 02:31:24 am
content: Quote from: gxti on September 20, 2012, 11:42:34 pmYes, I meant that the gaps in the enclosure have to be smaller at higher frequencies.What I didn't understand until it bit me, was that a lid on a box (like the Hammond 1590B for instance) is not very RF tight. That's even with the lid bolted down and these boxes have a supposedly RF shielding design with a tongue and groove around the lid.I don't think it's a near field phenomenon, but I'll defer to others with more electromagnetics knowledge.It's probably a matter of degree of isolation. I don't know what the shielding on the door of the microwave oven is, but if the oven was 1000W and the door provided 60dB of isolation, then only 1 mW would escape and that's probably low enough not to cook our faces.On the other hand, if I have a 1mW (0dBm) LO on the tracking generator for a spectrum analyzer, even 100dB of isolation isn't enough to keep it out of the SA signal path. A -100dBm signal at the first IF will contaminate the the SA front end at a level that is probably 30+dB above the noise floor (or the DANL).I'm headed to the kitchen with my spectrum analyzer now and I'll post the results from my microwave oven.

author: kg4arn
author_rank: Unknown
timestamp: September 21, 2012, 02:43:55 am
content: Here are the results from the door of my microwave oven.It is a 1500W (+62dBm) rated oven. I used a 4 inch diameter loop at then end of 36 inches of RG58 for an antenna.The loop was held up against the door where the handle is.The oven pulsed the RF so this trace is obtained with the Max hold function.It looks like the oven leaks +7 dBm making the isolation 62-7=55dB.

author: SeanB
author_rank: Super Contributor
timestamp: September 21, 2012, 04:53:37 am
content: Microwave has a limit of 5mW RF emission. Detectable with a simple diode probe. The door has a built in 2.4GHz filter in the edge, a 1/4 wave cavity that provides a high impedance area at the door, otherwise you would need a copper contact area to provide the RF seal.

author: ptricks
author_rank: Frequent Contributor
timestamp: September 21, 2012, 01:58:23 pm
content: Quote from: SeanB on September 21, 2012, 04:53:37 amBesides all that the other thing to take into account is how the RF emission power drops off over distance. Going back to the OP question, any metal can block RF, the difficulty of the specific metal at blocking RF changes depending on frequency and signal strength and the desired attenuation.This post made me think of something I saw while browsing, RF shielded clothing ! Everything from caps to underwearhttp://www.lessemf.com/personal.html

author: Wim_L
author_rank: Regular Contributor
timestamp: September 24, 2012, 02:04:15 pm
content: The problem with aluminium shielding is a lack of conductivity. Aluminium metal is a very good conductor, but aluminium oxide is not, and the surface of the aluminium exposed to the atmosphere will always be oxidised. A simple aluminium lid on an aluminium can is unlikely to provide a sufficiently conductive contact surface. Welding it will work better (if you can weld aluminium), sharp-edged non-slip washers between the pieces everywhere there is a screw may also be acceptable (use lots of screws close together).If weight and cost don't matter much, copper is the easier way to get good shielding. Even steel may turn out to be superior (not as good a conductor as aluminium, but much easier to get good metal-to-metal contact as long as you're not in a corrosive environment).

author: AndyC_772
author_rank: Super Contributor
timestamp: September 24, 2012, 05:01:52 pm
content: Aluminium can be an extremely good shield. Construct a sealed enclosure with a 360 degree rfi gasket all round the lid, and for all practical purposes its emissions can be made undetectable.The problems are always a) the cables, and b) the gaps in the enclosure needed for connectors, or ventilation, or the spaces between screws etc.

author: free_electron
author_rank: Super Contributor
timestamp: September 24, 2012, 05:22:29 pm
content: And NEVER-EVER re-use self-tapping screws !If you drill a hole in box and lid and put in a self tapping screw you make a good gas-tight (no surface oxidation possible) connection. Take the screw out and put it back in and it will still be ok.Do this a couple of times and it goes to snot ! after 3 to four times its game over.We had this problem on a cabinet . after testing things and swapping boards repetitively we couldn't get it back to the orignal measurement. So we took a new cabinet : problem solved. the final cabinet was modified to have threaded studs spot welded and used nuts with non-slip washers attached to it

author: vk6zgo
author_rank: Super Contributor
timestamp: September 25, 2012, 03:53:29 am
content: Quote from: free_electron on September 24, 2012, 05:22:29 pmSo you did it the slack way,& it came back & bit you?

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 25, 2012, 06:59:25 am
content: this is my secret advice yet i'm giving you for free about a screw. if you want to put it back, dont turn it clockwise right away, instead counter-clockwise it until you hear or feel a click, then only you can clockwise it without risking damaging the thread, self tapping or not screws. it will good for the 100th times unless you overtighten it (so where is my $10 fee?)

author: SeanB
author_rank: Super Contributor
timestamp: September 25, 2012, 07:23:30 pm
content: You take Zimbabwe Dollars?

author: robrenz
author_rank: Super Contributor
timestamp: September 25, 2012, 07:56:04 pm
content: Quote from: Mechatrommer on September 25, 2012, 06:59:25 amAgree 100% I have used this technique for years and it is especially important on plastic self tappers with the high-low thread form. But this does not address what free_electron is talking about. That is an oxide resistance/contact pressure issue. The gas tight interference contact of a self tapper bludgeoning its threads into the virgin material of the mating part can only happen once. Loosen it and things will never be the same again. Using a new screw in the same hole does not change anything, oxides have already formed and the contact pressure is greatly reduced.

author: G7PSK
author_rank: Super Contributor
timestamp: September 25, 2012, 09:12:49 pm
content: Seemples. use the next size up self taping screw when the old ones start getting loose.Also make sure that you have a bonding strap or two on the lid fixed with star washers either side of the bonding strap in order to get an electrical connection between the lid and body as at high frequency the separate parts of the enclosure can behave as if they were separate parts as there is always a layer of aluminium oxide between them putting star washers under the screw heads is not always enough as the screw can be insulated from the main body by oxide and grease.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 27, 2012, 04:12:00 am
content: I have used aluminum tape to shield an electric guitar in order to drop the noise floor. I found that for joining the different sections, two pieces of pencil lead (the normal pencil kind) held on top of the join about 0.5cm apart and discharging a 6800uF cap charged to 18V into the leads worked quite well to spot weld the pieces together. (I assume it arcs through the oxide layer and melts a spot of aluminum together.) Thicker aluminum would need more power to spot weld, but you might be able to make a line of spot welds along seams to seal them up.