Faraday Cage to block WiFi - Page 1
Forum section: electronics / beginners

author: FrankT
author_rank: Regular Contributor
timestamp: February 21, 2021, 07:42:53 am
content: Apart from the obvious use to stop the government reading my mind, can I use a faraday cage to block the wifi signal on a microcontroller so that I can test the source code handling wifi connections dropping out?I tried this with a die cast aluminium enclosure. It wasn't entirely sealed; I just pushed the lid down over the USB cable to the microcontroller, leaving a small gap, but it didn't stop the signal.Will a faraday cage work? Do I have to be more pedantic about the seal? How do I run a cable out of the cage? I also have self adhesive copper foil that I can use; both kinds: with conductive and non-conductive adhesive, if that is a better option?Thanks,Frank

author: CJay
author_rank: Super Contributor
timestamp: February 21, 2021, 12:14:33 pm
content: I'm no way expert in EMC but the holes in enclosures I believe should be less than some fraction of a wavelength, quarter wave I think, you need to bear in mind that a gap between a close fitted 'lid' and body of an enclosure is also a hole so while it might only be a fraction of a millimetre wide, it could be several cm long.A seal needs to provide continuity between the enclosure pieces so you need either tape with conductive adhesive, a way to electrically connect it to the case or a conductive gasket.Your enclosure with a USB cable poking out, as you've found, is unlikely to be RF tight, not only because the lid wasn't fitted but because the USB cable might also have been radiating.To do that properly you'd need to spend significant effort and probably money on a pass thru but you would probably get reasonable results by cutting a notch in the enclosure to allow the lid to close properly and fit some ferrites on the USB cable on the inside of the enclousure and as close to the outside as you can.Finally, you need to earth the enclosure.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 21, 2021, 01:05:55 pm
content: Maybe, Maybe this might work....We had a problem with a high powered HF amplifier (hundreds of watts) in a vehicle getting signal into everything. The cast aluminum case was grounded very well. Somebody suggested wrapping it in Padded aluminum tape. I thought this was silly, but we tried it, leaving gaps for the fan...It Worked!!!The tape had a few mm of padding on it and was sticky (hardware store stuff), the outside aluminum was NOT grounded.Let me know if this works if you try it.Wally

author: Terry Bites
author_rank: Super Contributor
timestamp: February 21, 2021, 06:22:16 pm
content: The cage will not work if its not grounded and all parts bonded with low inductance straps (old coax braid)I found that i could put my 4G mobile in a stainless sterilizing tray with lid and with 5mm holes and it still rings.An earth on the box stopped it dead.

author: Gyro
author_rank: Super Contributor
timestamp: February 21, 2021, 06:55:34 pm
content: As this is the beginners section, it's probably best to clarify that a Faraday cage does not need to be grounded to operate.Yes, all panels need to be low impedance bonded, with any slots or apertures much shorter / smaller that the wavelength of the signal to be screened - this makes door seams particularly vulnerable - EMC chambers have sprung silver plated finger strips all the way around the door opening and bonded mesh on all ventilation ports.In the case of the sterilizing tray, the issue is the long slots formed by the openable lid, where the lid makes only occasional contact along the length of the slots, possibly infrequent welding of the perforated plates too. Sealing the lid seams with conductive copper tape ought to make it behave as a true Faraday cage. I suspect that what is happening in this case is that earthing the box (although I'm surprised that the earth that is low enough impedance at mobile phone frequencies) is achieving sufficient reduction in local field strength to prevent reception.

author: nali
author_rank: Frequent Contributor
timestamp: February 21, 2021, 07:02:57 pm
content: If OP is only concerned by WiFi then an old microwave oven might be the easiest place to start. OK they're a bit meh in terms of construction but at least they're optimised for 2.4GHz (that assumes we're talking 2.4GHz WiFi or can set the AP to 2.4) and you can improve by bonding panels etc.

author: Gyro
author_rank: Super Contributor
timestamp: February 21, 2021, 07:15:28 pm
content: If specifically looking at 2.4GHz, you may be right. Microwave ovens are constructed to have very narrowband filtering - they make use of a combination quarter wave traps which are resonant at the magnetron frequency and some lossy materials. It is the 1/4 wave traps (combined with the door spacing) that do the bulk of the work though. It's about the only way of getting around the EMC chamber type door grounding in a consumer product. [Edit: Attenuation is very good, but not perfect.]People are often surprised by the fact that a mobile phone will operate quite happily inside a microwave oven (assuming that it hasn't been turned on that is!)

author: rfclown
author_rank: Frequent Contributor
timestamp: February 22, 2021, 12:06:59 am
content: Quote from: FrankT on February 21, 2021, 07:42:53 amI would suspect that it's coming in through the cable. Not through the gap, but the cable acting as a antenna.RF boxes are made with various feedthrough connectors for RS232, USB, ethernet, etc. These special feedthroughs have filters in them to prevent this type of thing.

author: Syntax Error
author_rank: Frequent Contributor
timestamp: February 22, 2021, 01:08:10 am
content: Out of curiosity, I just placed a tablet inside the microwave oven and using the Android 'WiFi Analyzer' app, took a rough-n-ready measurement of my nearby access point. With the door shut the signal fell some 15 to 20 dBm. Which was low enough to disconnect the tablet from the AP, but not enough to drive the line into the noise.So it's RF quiet in there, but not silent.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: February 22, 2021, 02:05:29 am
content: I've used a cracker tin, aluminum foil, and a set it on my stove(it's grounded) to stop a keyfob once. It went through the wash and it kept setting off the alarm. The screw was stripped out so I couldn't just remove the battery.lWith Wifi, you can turn down the radio strength.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 22, 2021, 02:13:37 am
content: Quote from: JustMeHere on February 22, 2021, 02:05:29 amI would have used a hammer.

author: nali
author_rank: Frequent Contributor
timestamp: February 22, 2021, 09:29:49 am
content: Quote from: Syntax Error on February 22, 2021, 01:08:10 amOut of curiosity, so did I. My microwave is a cheap unbranded model and the signal dropped from -45dB to off the floor - the app just stopped showing "No signal" so YMMVIt's completely irrational but I got an uneasy feeling seeing my phone in the microwave... I suppose I could've turned it off.

author: tszaboo
author_rank: Super Contributor
timestamp: February 22, 2021, 10:31:00 am
content: Danish cookie boxes.Or metal paint can, favorite of Linear technology.if you need to have cables going into the box, than there is two things to consider:1. Its better to run in the low speed signals, like UART not the high speed ones, like USB.2. Ferrite beads are your friend. Buy clamp on ferrite beads, buy many and use many, run the cable through them as many times as you physically can. At an EMC test lab, they had a 1M long openable ferrite bead, made out of smaller cores. That was used for EMC testing to isolate the non DUT parts.If you have SMA connector for the antenna, you can always replace it with a 50 Ohm termination.

author: Gyro
author_rank: Super Contributor
timestamp: February 22, 2021, 10:40:09 am
content: Feedthrough capacitors work very well for supplies and low frequency signals too. The non-screw types are cheaper and solder nicely into tin plated cookie tins. Again, they are a mainstay of EMC test chambers.

author: CJay
author_rank: Super Contributor
timestamp: February 22, 2021, 07:44:18 pm
content: Quite pleasingly, I just put my work phone (iPhone 7, just don't) into the microwave and tried to call it. After one abortive attempt caused by my not remembering I'd switched the damned thing off, I still failed to be able to call it while it was in the microwave.

author: perieanuo
author_rank: Frequent Contributor
timestamp: February 23, 2021, 08:29:27 am
content: Quote from: Terry Bites on February 21, 2021, 06:22:16 pmi'll consider this a joke.a faraday cage operation and principle is pretty straightforward and clear and demonstrated for more than a century, it DOESN'T need to be grounded.as someone said, holes must respect maximum size and material (metal) should respect thickness and type, that's all

author: perieanuo
author_rank: Frequent Contributor
timestamp: February 23, 2021, 08:32:57 am
content: Quote from: CJay on February 22, 2021, 07:44:18 pmyep, those microwave ovens are double faraday cages with only weak spot from the door, because of the viewing area, still shielded with some holes in the shielding material.glass absorbs also some of the remaining GHz, not all.

author: Gyro
author_rank: Super Contributor
timestamp: February 23, 2021, 10:01:39 am
content: Quote from: perieanuo on February 23, 2021, 08:32:57 amNope, sorry, you haven't got the hang of that one at all. The viewing area is no problem, the window area is covered by a metal mesh that blocks all microwave energy. Just go and look at your microwave oven door and note that its periphery is effectively insulated by plastic mouldings and latch, apart from the two metal hinges, not an RF bond in sight.As I mentioned previously, the secret of a microwave oven door is the inclusion of a quarter-wave trap right around the periphery, that presents an RF short to the microwave energy trying to escape. It is tuned to form a notch filter that effectively shorts the energy at the Magnetron frequency and RF grounding the door at that specific frequency - it's really a very clever technology to be able to accurately achieve this in a mass produced consumer product.Here's the relevant patent...https://patents.google.com/patent/US4523069

author: ptluis
author_rank: Frequent Contributor
timestamp: February 23, 2021, 01:57:34 pm
content: why not using a jammer on the frequency you want to block? when I want to block all RF communications from different devices, for whatever reason, I use a lead coating box, it's 100% effective, no coms go out or in. Faraday cage is not 100% effective. The most effective is a jammer, you can block specific frequencies without blocking all coms, but it's an expensive device.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 23, 2021, 02:49:03 pm
content: Laminated aluminum foil plastic bags.I made a "Faraday blanket" for an instrument using some.Easy to make or just use an existing ziploc bag made from the same material. If you are careful enough, you can scrape away at the plastic to exposure the metal.As I recall, the one I made was heavier duty. I took two sheets of laminated aluminum plastic sheet and put some aluminum foil between them. It worked very well and was very robust - I just rolled it up when I didn't need it.

author: FrankT
author_rank: Regular Contributor
timestamp: February 23, 2021, 08:42:25 pm
content: Thanks for the replies.I tried creating a 3d printed enclosure that has a layer of copper foil on the inside and outside, and a lid that seals both layers. I helped a bit, but didn't stop the signal. Here's the RSSI plot (ignore the saw tooth) which shows in the box, then out.I think I'll give the blanket a try. I remember reading this technique used by videophiles to cover the video card in their PCs, sandwiching layers of aluminium foil and paper.In the meantime, I'm just disabling my wifi for my whole house when testing.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 23, 2021, 10:23:17 pm
content: For all practical purposes you do need to bond and earth it or there will be some resonant nodes you didnt see comming. These will happily reradiate in the GHz, at DC the cage will accumulate charge and you'll zap your semicondcutors with a scope probe or finger. For safety reaons all exposed metalwork in your lab (or house) needs to be at the same potential, preferably 0V.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 26, 2021, 01:33:27 am
content: If it's coming in through the cable, as I mentioned before, all the extra shielding in the world won't help. You either have to have filtered feed throughs, or try ferrite clip ons over the cables.

author: radiolistener
author_rank: Super Contributor
timestamp: February 26, 2021, 12:55:13 pm
content: Quote from: FrankT on February 21, 2021, 07:42:53 amYes, it works, but you're needs to understand shielding don't remove radiation, it just add attenuation for E, M and EM fields.First, you're needs to be careful with distance between your device and the shield. It should be about λ/2 to eliminate E and M field influence in a near field region.For 2.4 GHz, just keep your device for at least 63 mm distance from any surface of your shield box.And the second, you're needs to use sufficiently thick shield surfaces. More thick leads to more attenuation.I think 1 mm of copper or steel will be good enough to break communication at 2400 MHz.Quote from: FrankT on February 21, 2021, 07:42:53 amIt depends on the cable type you're want to out of a shielding box.For DC current, you can use feed-through capacitors.Also you can use ferrite filters on both sides of the shield surface to reduce common mode currents.

author: perieanuo
author_rank: Frequent Contributor
timestamp: February 26, 2021, 03:47:09 pm
content: Quote from: Gyro on February 23, 2021, 10:01:39 amyou're right in theory, but have you seen/tested in a lab the amount of radiation escaped from the front door? it's not 0, even with the correct sized holes.where did the oven leaks more? the door obviouslysome people talk theory, some consider the real life. i had the same arguments with my boss in france, he always told me the theory then he returnet from emi testing to redo the jobunfortunately, we must consider real life, otherwise we won't pass approval tests with some theory on paper.don't take this personally, it's just as it is

author: Gyro
author_rank: Super Contributor
timestamp: February 26, 2021, 06:55:34 pm
content: Quote from: perieanuo on February 26, 2021, 03:47:09 pmHa, yes that's an easy one, the door...but much more specificallytheedgesof the door, where the microwave traps are located, rather than the window. The traps have averyhigh energy attenuation at the 2.45GHz magnetron centre frequency but are not perfect. If you check out the service manual for any commercial microwave oven, you will find that the required leakage tests and energy measurements are carried out using a probe around the edges of the door. There is nothing more real life that a Microwave oven in actual operation, it is theory in action.I don't want to make a big issue of this anyway. My point was that a microwave ovenisn'ta Faraday cage. It is a cavity with a door which is not physically sealed (in fact has a very specific gap) and is only protected by a narrow band steep notch filter at a specific frequency, but effectively open to RF at all other frequencies.As you say, it's just as it is.