Seeking an effective tin foil hat on the cheap
February 13, 2018 9:50 PM   Subscribe

I'd like to make/buy a pouch that can block cell and wifi signals. Ideally the materials will be inexpensive, but more durable than literally wrapping my laptop and phone in tin foil. Can you recommend fabrics that would work? Would a zipper closure undermine the Faraday cage-ness? Sewing it with ordinary needle and thread?

The only threat I'm trying to protect against is distraction by incoming notifications. In other words, I'd like this pouch to be effective, but no one will go to jail or face other harm if it fails.

Due to the aforementioned low budget and low risk nature of this experiment, I'm hoping to find a more affordable alternative to fabrics I've seen advertised as RF-shielding. That said, I'm open to recommendations from this market, particularly less-expensive options that don't have minimum order requirements of 10+ yards.

(Yes, I'm familiar with silencing notifications, airplane mode, do not disturb, turning off devices, apps like StayFocusd, not owning or carrying a phone, and many other options for reducing distractions without literally blocking cell signals. These are all great, but for now I'd appreciate the HiveMind's help on how to make a signal-blocking pouch without spending too much $$$.)
posted by brackish.line to Science & Nature (11 answers total) 3 users marked this as a favorite
There are plenty of plans on the interwebs for duct-tape-and-foil pouches. Cheap, fun, and simple, and you get to choose your color!
posted by RobotVoodooPower at 9:59 PM on February 13, 2018

You can get a faraday pouch for your phone on amazon for <$10. Something big enough for a laptop will be much pricier, though.
posted by mollymayhem at 10:11 PM on February 13, 2018

Wifi is ~2.4/5 Ghz, and cellular is pretty much all lower than that.

The aperture of any opening should be smaller than 1/2 wavelength to sufficiently attenuate the signal. What this means at 5Ghz is that the signal wavelength is 6cm. Anything smaller than that will work, and the smaller, the better. Lower frequency stuff will be longer, so 5Ghz represents our worst case.

Pretty much any metal screen or mesh will do. But a heavy foil, metal duct tape, or project sheet metal will also work and are not expensive. Just be sure to bond it all together electrically.

One thing you should be aware of though - electronics work to preserve batteries by lowering the radio output power to the lowest that still gets the signal out. Blocking the signal will cause the device to increase output power to the maximum allowed, reducing battery life substantially. This also increases considerations for cooling - another word for radiated power is "heat".

It is better in just about every way to just turn the device off.
posted by Pogo_Fuzzybutt at 10:24 PM on February 13, 2018 [15 favorites]

Mylar is cheap, and while its RF-blocking ability is a function of its thickness, it's like a flock of birds: cheap cheap cheap! Get some mylar thermal blankets (here's 315 square feet of mylar for less than $8) and layer them up, and glue them into a sturdy-enough pouch. Mylar is reasonable durable, but can be torn, so make it the radiation-blocking layer sandwiched inside something that can withstand the rubbing of your laptop and the outside world.

But yeah, turn the device into Airplane mode or power it off, and then wrap it. The first thing will prevent the overt network from reaching you, and the second thing will effectively prevent LEOs and other shady folk from reaching into the special parts of your phone that keep running when it's turned off but has a connected battery. Heat of a powered-on device is going to really cause a problem if you wrap it in thermal blankets.
posted by Sunburnt at 11:55 PM on February 13, 2018 [1 favorite]

I had in mind that a Faraday cage had to be grounded to work - not entirely correct as it happens - so I went searching and found this question on StackExchange. The fact that people are having trouble getting one to work, i.e., completely block phone and wi-fi signals, suggests that it's not all that easy to do in a "works every time" kind of way.

The problem with only partially working is that your phone and/or laptop will crank up their radio amplifiers, which will suck battery life and make them run hot, as noted by Pogo_Fuzzybutt. This does happen. I've experienced it.

If you are determined to go this way, you may be best to buy a bag or pouch because you'll be able to take it back if it doesn't work.
posted by mewsic at 1:12 AM on February 14, 2018

The fact that people are having trouble getting one to work, i.e., completely block phone and wi-fi signals, suggests that it's not all that easy to do in a "works every time" kind of way.

Yes, capacitive coupling is a thing. Basically, if you have a conductor (cell phone antenna) and another conductor (faraday cage) separated by some space what you have is a capacitor.

This is why, in the ideal case, you have the cage connected to a low impedance ground. However, this design is free floating, so that is impossible. In that case, you want to very carefully engineer all the facets of the design so that they are not resonant to frequencies between 500MHz and 5.4GHz.

That's basically impossible if you don't know what you are doing - and anyway, just making a big envelope out of foil or mesh will get you most of the way there.
posted by Pogo_Fuzzybutt at 7:30 AM on February 14, 2018 [1 favorite]

So as an experiment I just wrapped my phone in successively thicker layers of static and moisture shielding bags until it stopped receiving test emails I was sending to myself. It took 4 layers (of bags inside of bags) before it totally dropped out. I used one layer of moisture/esd bag (on left) and 3 layers of normal static bags (on right) as shown in this picture.

Those are just what I happened to have on my desk and you could probably do better by experimenting with different types of layers. Static bags come in large enough sizes to put a whole laptop in and are probably free for the asking, and even buying them new they are pretty cheap
posted by Dr. Twist at 9:46 AM on February 14, 2018 [1 favorite]

For the laptop, it's probably easiest to just to turn the WiFi off. My Toshiba has a keyboard button to do that, and if you don't have that, there there maybe a keyboard combination that works.
posted by SemiSalt at 2:06 PM on February 14, 2018

You could use the If This Then That app to silence it turn wifi off in specific circumstances.
posted by theora55 at 6:33 PM on February 14, 2018

Cheapest by far: those composite bags/boxes that things like coffee beans, dry pet food, shelf milk or soup come in. They are shiny (foil) inside, melded with a printable outside surface. Drop a phone in an empty, cleaned one and nothing gets through.
posted by Chriswill44 at 7:20 AM on February 15, 2018

Just as a datapoint, I stuck my phone inside one of the bags my Raspberry Pi shipped in. WiFi dropped from full to nothing, cell from full to one bar. You can buy those bags on Amazon by the box load. My phone got pretty warm in there pretty fast, though.
posted by sincarne at 7:08 PM on February 15, 2018

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