Electronics > Projects, Designs, and Technical Stuff
Faraday cage, shielding and grounding
ahbushnell:
--- Quote from: ejeffrey on June 09, 2020, 04:21:31 pm ---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.
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Good 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.
ricko_uk:
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
tggzzz:
--- Quote from: ahbushnell on June 09, 2020, 10:45:29 pm ---
--- Quote from: tggzzz on June 09, 2020, 03:16:35 pm ---
--- Quote from: ricko_uk on June 09, 2020, 11:07:46 am ---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 :)
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This 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.
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go to a university Library
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That 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 :(
ahbushnell:
--- Quote from: ricko_uk on June 09, 2020, 11:23:24 pm ---
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
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There 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.
NiHaoMike:
--- Quote from: fourfathom on June 09, 2020, 04:38:16 pm ---
--- Quote from: ejeffrey on June 09, 2020, 04:21:31 pm ---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.
--- End quote ---
Microwave 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.
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Some don't even shield enough to completely block Wifi.
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