[Workaccount2]

Let me tell you something from first hand field experience with faraday cages...

They attenuate signals, they do not block them. The common verbiage is to say "faraday cages block EM radiation", so people naturally assume that it blocks EM radiation. But I learned the hard way while doing compliance testing that no, they do not block EM radiation, they just weaken it (and it's highly frequency dependent on top of that.)

[lostlogin]

From messing about with MR scanners:

An MR Faraday cage attenuates the RF signal about 100db (according to the engineer who built it). Phones work as long as the door is open 1mm or so. Blue tooth works through the cage just fine. Wifi doesn't work very well anywhere near the cage.

MR scanners get nice pictures with the scan door open, but the if the scanner next door has its door open (so 2x scanners running with door open), images are wrecked.

Also, the coastguard sends you grumpy letters if you leave the door open and scan (at 3T).

[accoil]

Why does the coastguard get involved?

[lostlogin]

I think that 3T scanners are effectively transmitting on one of their frequencies in my region.

We image hydrogen which has a precessional frequency of 42*3MHz at 3T (though most ‘3T’ MR scanners seem to be more like 2.8-2.9T).

So they must use something at around that frequency. All I know for sure is that we got asked to stop blasting that RF out. Sometimes it is convenient to open the scan door while imaging.

[genewitch]

126-128MHz if i am reading what you said correctly is aviation (aircraft use 108-136MHz, AM) - but i am unsure of what that notation means and the AI explanation made my eyes glaze.

I'm unsure why the FCC wouldn't be the ones to complain; but i've never managed to annoy a government with my radio-work yet, so i am not sure who calls who to send someone to knock on the door.

either way https://www.ntia.gov/sites/default/files/publications/januar...

also i only glanced the the US band plans, if this setup isn't in the US, then you'd have to find the relevant band plan for your area (although the US band plan for HF will be fairly accurate, HF is hemispherical on a bad day and global on a good day)

[lostlogin]

> I'm unsure why the FCC wouldn't be the ones to complain

Are you mentioned - US only.

I’m in New Zealand.

[genewitch]

https://www.rsm.govt.nz/assets/Uploads/documents/radio-spect...

as far as why the coast guard, that's a dive too deep for me today. Interesting story, though, on your part!

[jerf]

"(and it's highly frequency dependent on top of that.)"

Well, sure. Can people inside the cage see outside? (Or a hypothetical person for a small cage.) If so, then clearly, not all frequencies are being blocked. A lot of "Faraday cages" are explicitly designed for radio and deliberately let other frequencies, particularly the visual range, through.

In fact we all have direct experience with that. Our microwaves use a Faraday cage to keep them in. But we can still see through the mesh, and you can tell that the inside can see out because outside light can go in and bounce back out. (That is, while there's probably a light in your microwave, it's obviously not the sole source of light.) Blocks microwaves well, but visible light goes right through the holes.

[wat10000]

They let out enough to interfere with radios operating around 2.4GHz. They'll attenuate the stuff, quite strongly if built well (the only reason interference is a problem is because the oven is 3+ orders of magnitude more powerful than a typical 2.4GHz radio), but it's not a total block.

[giantg2]

Anyone interested can test this with an RF bug finder, even the homebuilt ones that just increase the intensity of an LED when near a source will work to demonstrate the leaks.

[grogenaut]

"""General rule of thumb is that the opening in a Faraday cage should be smaller than 1/10th of the wavelength that should be blocked. For example, in order to block EM fields with frequencies of 10 GHz and lower, the hole size of the Faraday cage should be smaller than 3 mm."""

wavelength of red in inches: 2.46063e-5 wavelength of 2.4ghz in inches: 4.25 wavelength of xrays in inches: 7.87402e-7 (upper end)

You could easily see through a 4.25 mesh, that's almost chain link

you could not see through a 2.4e-5 mesh, that's call fabric, unless you can see through clothes in which case I'm not going near you. xrays can see through that :)

[washadjeffmad]

That seems intuitive, though. EM radiation is either reflected or absorbed, and optimizing for that requires both a pretty complex understanding of RF behavior and generally knowing that materials are generally radiopaque and radiolucent at different frequencies and wattages.

Sometimes we're trying to keep things (eg- information) outside from getting in, and other times we want to prevent things inside from getting out. There are practices to optimize for both that don't rely on "blocking".

[tomxor]

> EM radiation is either reflected or absorbed

By interfaces yes, but it can also be cancelled out through destructive interference as a side effect of reflection, which is my theory of how a "big ball of crumply aluminium" is so effective compared to less chaotic solutions.

[widforss]

Rough surfaces increase reflection in non-specular directions and decrease it in the specular direction. I have never heard that it would facilitate destructive interference.

[b112]

Every time my friends make fun of my hat, every time I think of shedding the 'Luminum Life, something convinces me to stand fast.

Thank you brother.

Thank you.

[Onavo]

Well, I am not sure how you expect redneck prepper types to pick up on enough RF theory to manufacture homemade metamaterials.

[Wobbles42]

We aren't all rednecks. Or maybe we kind of are but some of us have engineering degrees.

[grogenaut]

a lot of people think if you get your hands dirtied you're a redneck... I guess I'm a redneck.

[milkshakes]

yes, this has been studied: https://web.archive.org/web/20100708230258/http://people.csa...