DIY Farraday Cages, or How to block the signal from your smartphone

by Nate Hoffelder

Nate has been blogging about gadgets for over a decade, first at the online forum MobileRead, and later at his blog, The Digital Reader. He prefers Android over iOS. Windows over MacOS, and is annoyed by redundant product names like Apple's Apple Watch.

May 23, 2023

So a few days ago I was watching a video on FB where a home inspector showed how he used a tiny device with LEDs to check if a microwave was working correctly. (The device had no battery, and was powered by the energy emitted from the microwave, so if the lights on the device activate outside of a microwave, that is a problem.)

The home inspector mentioned that a microwave was a Farraday cage because it contained the energy emitted when the microwave was turned on. This got me thinking about smartphones, and how one might block their signals.

I’m sure we’ve all seen movies and tv shows where a phone was put in some type of bag to block its signal (for example, the latest Terminator movie had the main character using potato chip bags for this purpose), but have you ever wondered whether that could work in real life?

I did.

Earlier today I decided to run a very basic test: I put my smartphone in various metal appliances, and then called it. If the phone rang, then I concluded the test failed. 

Note: I skipped some appliances (oven, dishwasher) because they did not completely enclose the phone in metal, and I also skipped testing potato chip bags because I am 99% they would not work (plus, I don’t have any). I also did not try any of the products (metal-lined bags and boxes) which are designed and built to do this job; I did not want to spend money just to satisfy my curiosity.

So here’s what I tried:

  • Saucepan w\lid
  • Microwave
  • Refrigerator
  • Washing machine
  • Dryer

None of these worked; the phone rang on the first try, and Google was able to give me my phone’s location.

To be perfectly honest, I thought the saucepan would pass the test, and the microwave. It is my understanding that the microwave signal is higher on the electromagnetic spectrum than a 4G smartphone signal, so anything that blocked microwaves should also block a smartphone signal. (Maybe I have that backwards?)

While I do not have a comprehensive set of items to test (I’d like to test a metal toolbox, but don’t have one), I can tell you about one item which worked: aluminum foil. Wrapping my phone in two layers of aluminum foil, and then loosely folding the edges, made it lose all connection with the cellular network (and also Wifi). It could not take a call, and Google could not tell me its location.

I would consider that a successful test.

EDIT: It belatedly occurred to me that that was not really a Faraday cage, just a quick and dirty alternative. If we wanted to make a real Faraday cage, I would take a plastic storage bin and either line it with or wrap it in  aluminum foil. Then, build a tight fighting door out of more aluminum foil.

This should achieve the same result as simply folding your smartphone in 2 layers of foil.

P.S. As I sit here writing up this post, it occurs to me that I could also test Bluetooth, RFID, and other signals. I would expect that if Wifi is blocked and 4G is blocked then the other signals will also be blocked, but I could be wrong. This is worth looking into, but I will save it for a later post.

Image by wuestenigel via Flickr

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