> wrap that in aluminium foil, making sure that the ends are folded over and pressed down hard to provide good inter-layer contact
I've tried this many times, it's impossible to prevent gaps without welding it shut. Obviously I wasn't testing with an EMP or nuke, but trying to block 2.4GHz WiFi... But that is well within the E1 range the author states.
I think the problem with folding is it's too uniform, it's still too easy for waves to propagate through the humanly imperceptible gaps with only a few reflections.
The only method I found that worked consistently was to wrap many layers randomly overlapping and crumpling previous layers. My theory as to why this works is through self interference due to creating a long signal path with highly randomised reflections... No idea if that would help cancel out EMP.
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.)
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).
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.
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.
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)
"(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.
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.
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.
"""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 :)
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".
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.
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.
You shouldn't need to prevent gaps entirely. You only need to make sure there are no holes larger than roughly the wavelength of the radiation you're trying to block. Which, for 2.4GHz wifi, is about 125mm. I think what you saw is that a single layer of foil isn't enough skin depths thick to block radiation sufficiently at that frequency.
1. google how many lightning strikes are there per day
2. google how many millions of miles/kilometers of electric wires is hanging in air all over the world providing people with electricity
3. do not google how many of those millions of lightning strikes PER DAY disabled those billions of miles of wires per day, by applying energy bigger than nuclear EMP. do not google that.
You don't need to google. Simple experience tells you which continent is well prepared to EMP problems, and which not. Where do you have to protect your consumer electronics from lightning strikes and where not? In Europe and Russia you don't have to.
I need to test flaky cell phone connectivity issues and tried the same thing. Aluminum foil did not cause packet loss. But a microwave (not running) in a building with a metal roof in a room surrounded by metal filing cabinets did the job.
I've tried this many times, it's impossible to prevent gaps without welding it shut. Obviously I wasn't testing with an EMP or nuke, but trying to block 2.4GHz WiFi... But that is well within the E1 range the author states.
I think the problem with folding is it's too uniform, it's still too easy for waves to propagate through the humanly imperceptible gaps with only a few reflections.
The only method I found that worked consistently was to wrap many layers randomly overlapping and crumpling previous layers. My theory as to why this works is through self interference due to creating a long signal path with highly randomised reflections... No idea if that would help cancel out EMP.