Author Topic: Faraday cage, shielding and grounding  (Read 9740 times)

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Offline ricko_ukTopic starter

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Re: Faraday cage, shielding and grounding
« Reply #25 on: June 18, 2020, 07:56:31 pm »

Between copper mesh and stainless steel mesh, considering the mechanical sizes are the same, which provides best electric and electromagnetic protection?

Thank you :)
 

Offline rhb

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Re: Faraday cage, shielding and grounding
« Reply #26 on: June 18, 2020, 11:22:03 pm »
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
 
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Offline RandallMcRee

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Re: Faraday cage, shielding and grounding
« Reply #27 on: June 19, 2020, 12:32:30 am »
Quote
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.

Yes, 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
 
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Offline rhb

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Re: Faraday cage, shielding and grounding
« Reply #28 on: June 19, 2020, 12:55:11 am »
Amen
 
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Offline ricko_ukTopic starter

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Re: Faraday cage, shielding and grounding
« Reply #29 on: June 20, 2020, 07:52:12 pm »
Thank you all :)
 

Offline ricko_ukTopic starter

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Re: Faraday cage, shielding and grounding
« Reply #30 on: August 10, 2020, 12:31:52 pm »
Thank you :)

Four 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
« Last Edit: August 10, 2020, 12:42:08 pm by ricko_uk »
 

Offline rhb

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Re: Faraday cage, shielding and grounding
« Reply #31 on: August 10, 2020, 02:12:11 pm »
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/0470189304

I 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
 

Offline ricko_ukTopic starter

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Re: Faraday cage, shielding and grounding
« Reply #32 on: August 13, 2020, 06:53:38 pm »
Thank you Reg !! :)

 

Offline Avishu

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Re: Faraday cage, shielding and grounding
« Reply #33 on: August 13, 2020, 07:01:54 pm »
Great Information. sad shayari image
« Last Edit: August 16, 2020, 03:46:57 pm by Avishu »
 


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