Faraday cage, shielding and grounding - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: June 09, 2020, 11:07:46 am
content: Hi,I need to protect a small experimental space so that it is immune to external noise as much as possible. I had few ideas which then raised a few questions:1) what is the best material? Copper mesh, metal mesh, solid copper, solid steel, other?2) is placing a Faraday cage inside another more effective? Or would it create some capacitor (or other) undesired effect?3) if yes to (2) above should they be connected to each other? Of would connecting each other create some waves bouncing around inside between them and create other effects that ultimately affect the inner most space?4) should I be grounding it or leave it floating? Or perhaps if (2) is a better solution should I ground perhaps the external one and leave floating the inner one?Thank you

author: Henrik_V
author_rank: Regular Contributor
timestamp: June 09, 2020, 12:37:02 pm
content: Two cases, isolated by 10-20 mm styrofoam , outer to earth, inner to guardouter case 5mm Aluminium ... and good contact of the plates... also will shield magnetic fields ...Another construction used here: two boxes made of transformer iron 'foil' (~2mm) , like shoe box , isolation again 10-20 mmIf you place heat generating parts in there add forced ventilation ... (aluminium mesh..) temperature monitoring is a good idea anyway

author: ricko_uk
author_rank: Super Contributor
timestamp: June 09, 2020, 02:06:31 pm
content: Thank you Henrik for your suggestions/solutions,few questions:1) why styrofoam and not just "empty space"?2) puraly out of curiosity, why 10-20mm gap and not less or more? Just practical construction or is it related to some waves/physics phenomenon?3) you mentioned "outer case 5mm Aluminium ... and good contact of the plates... also will shield magnetic fields ..." I don't think aluminium blocks magnetic fields. Are you referring to some special aluminium alloy perhaps?Many thanks

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 09, 2020, 02:14:26 pm
content: Quote from: Henrik_V on June 09, 2020, 12:37:02 pmAluminum is bad because it forms an oxide.Copper is probably best. You can solder the seams. Place an FM radio inside the box and you can test for leaks. I have used copper sheet or copper screen. The screen is nice because you can see inside the enclosure. It also provides some ventilation.What frequencies are you trying to shield?The door needs to make a good connection all the way around. Maybe multiple latches.Double shielding works better but is it worth it?Do you need to bring signals our power into your screen box? That's always a weak point for shielding.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 09, 2020, 02:53:51 pm
content: Thank you AhbushnellQuote from: ahbushnell on June 09, 2020, 02:14:26 pmAs broadband as possible, from Hz to GHz (WifI etc). It needs to be as quiet as we can make it without spending thousands of dollars.Quote from: ahbushnell on June 09, 2020, 02:14:26 pmIf it contributes then definitely yes, especially if the materials cost is only few tens of dollars.Any feedback on Henrik suggestion about styrofoam and connecting?Quote from: ahbushnell on June 09, 2020, 02:14:26 pmYes, but an analog one not a power one (battery is inside the enclosure for that reason). I just posted another post exactly about this if you have any suggestions they are very much appreciates, you might want to have a look at it here:https://www.eevblog.com/forum/projects/optically-transparent-electrical-isolation-of-analog-signal/?topicseenThank you

author: tggzzz
author_rank: Super Contributor
timestamp: June 09, 2020, 03:16:35 pm
content: Quote from: ricko_uk on June 09, 2020, 11:07:46 amThis is really an EMI/EMC issue. Many small details contribute to the overall efficacy of such enclosures.It would probably help to read one or more of the standard textbooks on the topic. One that has been through several revisions over the decades is Ralph Morrison's "Grounding and Shielding Techniques in Instrumentation". That might help, and if not there are others.It is at times like this that the demise of decent technical bookshops is felt. Once upon a time it would have been possible to go and skim a couple of books, then buy the one that looked most suitable.

author: trevatxtal
author_rank: Contributor
timestamp: June 09, 2020, 03:58:53 pm
content: How about a unused microwave oven ?Made safe of course.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 09, 2020, 04:21:31 pm
content: Most microwave ovens are not great. I've tested a cell phone inside a microwave and it still works. They are made to be cheap and good enough for purpose not a lab grade screening cage.Anyway for low frequency your limit will be magnetic shielding. You will need a magnetic material like iron, steel, or mumetal to get good attenuation for Hz range magnet fields and it will be very hard to get good low frequency magnetic screening over a large volume. Low frequency electric fields are easy to screen. For high frequency the limiting factor will be your signal feedthroughs and the seams -- especially the door. Real screenrooms usually have berillium copper finger springs all the way around the door and compress them with a lot of force. EMI gasket material is an easier alternative that can also be effective but may wear out with use and stop making good contact. Springs are good because they scrape away oxide and contamination each time they are used to make sure the contact is good.

author: fourfathom
author_rank: Super Contributor
timestamp: June 09, 2020, 04:38:16 pm
content: Quote from: ejeffrey on June 09, 2020, 04:21:31 pmMicrowave ovens are designed to contain the microwaves, not all random RF fields. The door seals are usually a resonant gap that provides a virtual short-circuit at the specific microwave frequency used (2.45 GHz). While this gap works very well at this frequency, it will not provide much if any attenuation at other frequencies.So it's not a matter of cheap, it's just that they are designed for a specific purpose.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:45:29 pm
content: Quote from: tggzzz on June 09, 2020, 03:16:35 pmgo to a university Library

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:51:06 pm
content: Quote from: ejeffrey on June 09, 2020, 04:21:31 pmGood point. i have used steel enclosures for EMP systems. But you need to treat the doors. For instance flame sprayed zinc. The steel needs to have continuous welds at joints.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 09, 2020, 11:23:24 pm
content: Thank you all,1) assuming a cylindrical shaped Faraday cage, and instead of a door it has a lid like one of those round metal chocolates boxes. Does that lid that "locks" into the main part of the box by "interference fit" provides tight enough of a seal or not?2) does the metal thickness matter? If so, how does it affect the shielding?3) I assume using one of those metal chocolate boxes is not good enough. What are the main reasons?Thank you

author: tggzzz
author_rank: Super Contributor
timestamp: June 09, 2020, 11:46:35 pm
content: Quote from: ahbushnell on June 09, 2020, 10:45:29 pmThat is significantly more difficult than when I was a kid.But then when I was a kid I used Downing Street in London as a short cut, walking right past No10. Good luck with that now

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 10, 2020, 03:52:28 am
content: Quote from: ricko_uk on June 09, 2020, 11:23:24 pmThere needs to be pressure on the contact. A flat flange like used on a water pipe would be better. Like a top hat. I assume this is for an experiment and not for a product. You could have a slip fit. Use finger stock EMI gasket.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 10, 2020, 04:55:23 am
content: Quote from: fourfathom on June 09, 2020, 04:38:16 pmSome don't even shield enough to completely block Wifi.

author: tggzzz
author_rank: Super Contributor
timestamp: June 10, 2020, 07:10:09 am
content: Quote from: NiHaoMike on June 10, 2020, 04:55:23 amAfter a quick glance at the production quality of that video, I assumed that also reflected on the technical content. So I decided I couldn't be bothered to spend 7 minutes of my life looking at it.Last time I looked, in the early 90s (just as WLANs were becoming practical and before 802.11), domestic microwave ovens could leak up to 1W. Obviously that is enough to affect some WiFi comms, although the effect will be highly dependent on the geometry.It it certainly enough that I don't get my corneas close to operating microwave ovens that I don't know have been well treated throughout their lives.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 10, 2020, 01:22:16 pm
content: Thank you all for all the infos

author: pwlps
author_rank: Frequent Contributor
timestamp: June 11, 2020, 11:14:02 am
content: Quote from: ricko_uk on June 09, 2020, 11:23:24 pmThe thickness does matter : it has to be reasonably greater than the skin depth (https://en.wikipedia.org/wiki/Skin_effect). Otherwise you still have electrostatic shielding protecting against capacitive coupling (Faraday box) but you are not immune to noise coming via inductive (magnetic) coupling. Since the skin depth varies like 1/sqrt(f) magnetic shielding at low frequencies becomes difficult. This is well known in audio industry, simple coaxial cables (like RCA cables) are not sufficient at low frequencies this is why professional-grade equipment use differential transmission (XLR cables). As far as I know for very low frequencies a magnetic shielding using a mu-metal may be used (but I'm not specialist in this domain, I don't know the details).

author: graybeard
author_rank: Frequent Contributor
timestamp: June 12, 2020, 10:26:19 pm
content: I have used industrial emc enclosures fromRose Enclosures. They are made of a proprietary copper-aluminum alloy that seems to avoid oxidation at the mating surface, yet is easy to drill or machine.I have never attempted to measure their attenuation vs. frequency, but they have always been quiet enough for me to make the measurements I need. I have used them to measure currents in Si and III-V devices down to the 10s of fAs.Here is one I am working on now to use with my HP4145B. I'm waiting on some stubby drills to finish it because it does not fit under my drill press with full sized drills.Here is the inside of one I made 35 years ago. I don't remember what the brand of the RF absorbent material is I used to coat the inside was, but it was put there to reduce resonances that were corrupting my measurements.Here you can see that the gasket they provide is conductive. That conductive gasket plus the lip on the lid helps with shielding.. . . Chris

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 12, 2020, 11:11:58 pm
content: Quote from: graybeard on June 12, 2020, 10:26:19 pmHow does that compare to using chem filmed aluminum?

author: ricko_uk
author_rank: Super Contributor
timestamp: June 12, 2020, 11:30:14 pm
content: Thank you Greybeard! Very interesting, the conductive gasket too, that could be a possible solution!Ahbushnell,what is chem filmed aluminium? I googled it and it looks like some process, is it possible to somehow apply it to a standard enclosure too?Thank you

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 13, 2020, 07:04:46 pm
content: Quote from: ricko_uk on June 12, 2020, 11:30:14 pmchem film is slang.Trade name is Alodine. You can get clear and yellow. I like yellow because it shows it's been treated. Plus it looks better.You can buy kits and do it yourself or have it done. We normally spec the coating the sheet metal house.https://usglassmag.com/bonding/alodine-aluminum-the-brand-versus-the-finish/

author: ricko_uk
author_rank: Super Contributor
timestamp: June 13, 2020, 09:44:35 pm
content: Thank you ahbushnell

author: rhb
author_rank: Super Contributor
timestamp: June 13, 2020, 11:49:35 pm
content: There are two issues: electric field and magnetic field. Copper will not block magnetic fields whereas low carbon steel sheet is fairly effective above 1 MHz. Screen drops in effectiveness as the frequency goes up. Low frequency magnetic fields require mumetal which is quite expensive as it is nearly pure nickel. Galvanized steel sheet is a good enough conductor to be effective for the electric field.A friend who is a retired level II certified TEMPEST tech told me that the commercial rooms he dealt with used plywood with galvanized steel sheet on both sides and a custom rib that held the panels together with bolts every few inches.I'm building a lab room approximately 15' x 11' which will have all the surfaces covered with 26 gauge galvanized steel sheet riveted to metal studs with all seams soldered. The door will use metal weather stripping on all edges.For a small shielded enclosure I spot welded piano hinge on a pair of steel radio chassis along with a trunk latch. Later it will get a layer of thermal insulation and a Peltier junction to control the temperature.https://www.eevblog.com/forum/metrology/shielded-test-chamber/msg1471450/#msg1471450A lot depends upon how big you need the chamber to be, whether there is significant heat being generated that needs to be removed and what type and level of environmental noise you have.I lived a mile or two from a 250 kW FM transmitter when I was in grad school. Drove me nuts! While I don't have that problem where I live now, it made me want a shielded room.A reasonably inexpensive option for a quiet bench would be a 5 sided steel box holding your test gear and a copper screened front panel that allowed you to see the displays and provide air circulation.You should make or buy an H field probe. Dave did a video on them and there are some really cheap ones from China. There is *no* justification for the $$$$ versions. It's nothing special, just a shielded loop so it only responds to magnetic fields. Combine it with an SDR to do spectrum analysis.For a small low cost enclosure with good magnetic shielding consider a large diameter section of threaded steel pipe with malleable iron caps. Thoroughly annealed if possible.You will need to *very* carefully filter all lines passing through the walls. The details of that depend very much on the signal. For fast signals make opto-isolators that send the signal through a hole in the enclosure and then put a metal cover over both sides.It can get very gnarly. So measure often and keep at it.Have Fun!Reg

author: ricko_uk
author_rank: Super Contributor
timestamp: June 17, 2020, 04:57:49 pm
content: Thank you Reg and all

author: ricko_uk
author_rank: Super Contributor
timestamp: June 18, 2020, 07:56:31 pm
content: Between copper mesh and stainless steel mesh, considering the mechanical sizes are the same, which provides best electric and electromagnetic protection?Thank you

author: rhb
author_rank: Super Contributor
timestamp: June 18, 2020, 11:22:03 pm
content: I'd expect copper mesh to perform better, but I've never seen data to confirm that. I doubt that there would be significant magnetic field benefit from SS mesh.Low carbon steel as used for making galvanized sheet for HVAC duct work has fairly high permeability. I doubt that the permeability of stainless steel would provide much shielding even as sheet. Much less as mesh. Work hardening requires annealing after forming for best performance. At the price of mumetal, pretty much required to justify the expense.A bit of context as to the scale of enclosure you want to build would be very helpful. Along with purpose. Metrology requirements are very different from RF testing.I'm doing both RF and metrology, so I'll have a shielded room with shielded test chambers for metrology experiments. And size and budget matters. For cheap I'd cover things with aluminum foil with 4" wide aluminum foil duct tape on the seams. The capacitance per square inch is so large that it's a dead short at RF.If I were building a lab in a rented apartment, I'd staple aluminum foil to the walls, floor and ceiling, cover the seams with foil HVAC tape, solder a wire to a strip of wide copper foil tape and apply that to the aluminum to capacitively couple the foil to ground. Then lay a sheet of cheap vinyl flooring over the foil on the floor. ESD flooring is about $6/ft. Even a few square feet would cost more than the rest of the room treatment.Galvanized steel sheet is less than 1/2 the cost of copper screen. So I can't see using copper screen unless you need the airflow. I plan to use galvanized duct filled with copper scrubbers for the air supply to my room with muffin fans on the outside of the room.Reg

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: June 19, 2020, 12:32:30 am
content: QuoteYes, but...Having done extensive modeling in FEMM* I always found mu-metal to be very expensive compared to steel and never better except if weight is the most important thing, e.g. you're going into outer space (satellites) or you're the military.Otherwise you can just keep making your steel thicker or at worst use two separated steel sheets. Anyway, that's what FEMM says.*http://www.femm.info/wiki/HomePage

author: rhb
author_rank: Super Contributor
timestamp: June 19, 2020, 12:55:11 am
content: Amen

author: ricko_uk
author_rank: Super Contributor
timestamp: June 20, 2020, 07:52:12 pm
content: Thank you all

author: ricko_uk
author_rank: Super Contributor
timestamp: August 10, 2020, 12:31:52 pm
content: Thank youFour questions:1) what is the difference between stainless steel and galvanised steel when it comes to shielding (all three...: electromagnetic, electric and magnetic)?2) would copper be more effective of any of the above? RHB mentioned above but seems he is not 100% sure. Does anybody have any tested results?3) What about phosphor bronze? Any better in any way? What are its advantages when it comes to shielding?4) someone earlier mentioned skin effect. I know what it is but have no idea how it would affect the choice of thickness of the shielding material/mesh. Can someone please explain it further? Also is it better to have a thick or thin one?Thank you

author: rhb
author_rank: Super Contributor
timestamp: August 10, 2020, 02:12:11 pm
content: My uncertainty is a function of your ambiguous questions. You've not stated anything about purpose, dimensions, frequencies, field levels.Magnetic shielding is a function of thickness and permeability. Fully annealed, low carbon steels have the highest permeability. If you want greater permeability you need mu metal which is almost pure nickel and very expensive. The thickness required varies with frequency. The permeability also changes when the metal is deformed, so annealing is required for best performance.Electric shielding is a function of conductivity. Here also the thickness required varies with frequency, but not so much as the magnetic case. Copper is certainly more effective than galvanized steel, but it is a lot more expensive. Silver is better than copper. For a small enclosure the cost is nominal, but for a room it's too expensive even for commercial shielded rooms. Copper screen is effective at low frequencies, but loses effectiveness as the wavelength approaches the mesh openings.It should be noted that a superconductor provides perfect electric and magnetic shielding.There is a lot of information on the subject in this:https://www.amazon.com/Electromagnetic-Compatibility-Engineering-Henry-Ott/dp/0470189304I suggest reading it.In particular there is a lot of experimental data presented about the effectiveness of different materials and thicknesses.There is no single answer. It depends upon what you are trying to do. And the shielding material quickly becomes less important than how the seams and penetrations are handled.I am very sure about what I am doing. I'm using 26 gauge galvanized steel sheet as sold to the HVAC trade. It is the cheapest material, provides decent shielding of both the electric and magnetic field above a few MHz and is easily soldered to seal the seams. IIRC a 12' x 16' x 8' room will run about $1200-1500 for the steel. Copper screen would be 2-3x that and copper sheet probably 5-6x.Reg

author: ricko_uk
author_rank: Super Contributor
timestamp: August 13, 2020, 06:53:38 pm
content: Thank you Reg !!

author: Avishu
author_rank: Newbie
timestamp: August 13, 2020, 07:01:54 pm
content: Great Information.sad shayari image