Electronics > Projects, Designs, and Technical Stuff
Faraday cage, shielding and grounding
tggzzz:
--- Quote from: NiHaoMike on June 10, 2020, 04:55:23 am ---
--- 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.
--- End quote ---
Some don't even shield enough to completely block Wifi.
--- End quote ---
After 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.
ricko_uk:
Thank you all for all the infos :)
pwlps:
--- 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
--- End quote ---
The 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).
graybeard:
I have used industrial emc enclosures from Rose 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
ahbushnell:
--- Quote from: graybeard on June 12, 2020, 10:26:19 pm ---I have used industrial emc enclosures from Rose 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
--- End quote ---
How does that compare to using chem filmed aluminum?
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version