Am I doing Faraday wrong?

March 22, 2024 12:53 PM

On a whim I ordered some Faraday cloth and tape. It came today and I don't know how to test if it really works.

This cloth from Amazon is military grade! blocks cell, wi fi, gps, bluetooth yada yada! I've seen photos of people using some kind of meter to test the effectiveness. I don't have that. I'm just going by the bars on my laptop/phone/iPad. I draped (two layers of) the fabric over my wi fi router, no effect. I wrapped my phone in two layers, then more layers and scrunched up the fabric until I could only see the bars icon on the top of the screen--a teeny tiny opening, no effect. Well, it might have gone down a bar.

What am I doing wrong or is it bum material that I should return? Note: internet install guy is coming tomorrow (switching providers) and maybe he has one of those meter thingies?

posted by TWinbrook8 to Computers & Internet (11 answers total)

You need to fully enclose the object with the cloth and seal it. Draping the cloth over your router or scrunching it up around your mobile phone won't do anything. If you want to test its efficacy, completely wrap your mobile phone in the cloth, completely seal it with the provided tape and then try to call your mobile phone from another line.
posted by slkinsey at 1:06 PM on March 22, 2024 [10 favorites]

From what I've read, any small gaps will permit signal. It needs to be fully sealed to have an effect.

Phones, wifi routers, etc. are pretty smart and will increase power if they detect signal loss so you need to block the signal entirely.

Also, cell and wifi each can operate on multiple frequencies, I'm not an expert but I think the Faraday effect needs to be matched to the desired frequencies that you want to block? (An analogy is that light and sound are also waves, like radio, and solid fabrics can block light but not sound.) You may only be able to block cell signal but not wifi (or vice-versa).
posted by jpeacock at 1:18 PM on March 22, 2024

A Faraday cage is a volume fully lined with a frequency-blocking material. Not just on the sides or top. You have to think of radio waves like water, not beams - if there's a hole in the material, they will pour in. So if you want to test, get a small box and wrap it completely in the tape/cloth so there are no gaps at all, including on the lid/folding bits. Turn the sound up on your phone and shut it in there. Now use another phone or computer to call your phone, email it, do anything that would cause it to make noise, etc. Wait at least 30 seconds, if the stuff works nothing should happen. Then when you open the box and take the phone out, the notifications should appear immediately.
posted by BlackLeotardFront at 2:00 PM on March 22, 2024 [1 favorite]

Response by poster: I forgot to say, this material also claims to block RF signals which is why I really wanted it (to sew a wallet). I thought blocking wi fi signals would be the same test. (Science clearly not my strong suit). Is a meter the only way to test for this?
posted by TWinbrook8 at 2:08 PM on March 22, 2024

Maybe a comparison Faraday cage would work to see how it's supposed to work?

Years and years ago my mum worked the census, a mix of online and paper pick up, so she got a text message when someone submitted online so she knew not to go back there to ask for the paper form. Her poor phone on the morning after census night!! So we put it in on a metal plate under a metal bowl and the stream of texts stopped, it was able to process the texts it had and we let it out in intervals until they were all through.
posted by freethefeet at 2:12 PM on March 22, 2024 [3 favorites]

Response by poster: Okay, I had a friend call my wrapped-up phone and it went to voicemail so mission accomplished? (where as the metal bowl on top of the faraday cloth did not...)
posted by TWinbrook8 at 2:51 PM on March 22, 2024

Here's Matt Blaze on these things and his extremely, uh, Matt Blaze testing of the various products available from Amazon in 2021. It's a very deep dive but it may be of interest.
posted by The Bellman at 3:07 PM on March 22, 2024 [1 favorite]

I have a cheap but effective Faraday bag. I have an iPhone and an Apple Watch. When one is in the pouch, the other cannot call it (I have dual sim) or see it on the Find My network. If you have Apple and multiple devices, that could be a way to check it.
posted by dobbs at 6:27 PM on March 22, 2024

You have to think of radio waves like water, not beams - if there's a hole in the material, they will pour in.

A somewhat better analogy is thinking of radio waves as if they were just another kind of ambient light, and your Faraday cloth as if it were blockout blinds.

Everyday experience tells us just how hard it is to make a room truly too dark to see in using blockout blinds: it only takes a very small gap around the blinds to let in enough sunlight that we're not bumping into the furniture. Draping Faraday cloth over your phone is like just lowering an awning outside the window: it certainly cuts the signal strength, but not beyond the phone's ability to adapt to that. To get the best out of it, you need to close up every last little gap.

This is also why tinfoil hats so completely fail to achieve their alleged purpose. Anybody who truly believes that their brain requires shielding from radio waves, and intends to use tinfoil for this purpose, would need to wrap themselves fully head to toe.
posted by flabdablet at 11:10 PM on March 22, 2024 [2 favorites]

Anybody who truly believes that their brain requires shielding from radio waves, and intends to use tinfoil for this purpose, would need to wrap themselves fully head to toe.

THANK YOU! Finally someone explains what I've been doing wrong all these years.
posted by slkinsey at 7:16 AM on March 26, 2024

Response by poster: In the end, I sewed a credit card sleeve and waved it over the reader at the coffee shop. It didn’t register so I guess that proves it works.
posted by TWinbrook8 at 8:19 AM on April 14, 2024