On devices like smartphones, software runs almost every aspect of the user interface, including how and when it’s powered on and off, and, for that matter, what being “off” actually means.
The difficulty of reliably turning software-based devices completely off is no longer merely a hypothetical issue. Some vendors have even recognized it as a marketable feature. For example, certain Apple iPhones will continue to transmit “Find My Device” tracking beacons even after they’ve ostensibly been powered off. Misbehaving or malicious software could enable similar behavior even on devices that don’t “officially” support it, creating the potential for malware that turns your phone into a permanently on surreptitious tracking device, no matter whether you think you’ve turned it off. Compounding these risks are the non-removable batteries used in many of the latest smartphones.
A Faraday cage severely attenuates radio signals going in or out of it. It can be used to assure that an untrustworthy device (like a cellphone) isn’t transmitting or receiving signals when it shouldn’t be. A Faraday cage is simple in principle: it’s just a solid conductive container that completely encloses the signal source, such that the RF voltage differential between any two points on the cage is always zero. But actually constructing one that works well in practice can be challenging. Any opening can create a junction that acts as an RF feed and dramatically reduces the effective attenuation.
There are somewhat pricey (USD40-USD80) commercial Faraday pouches made specifically for cell phones, and there are a variety of improvised shielding methods that make the rounds as Internet folklore. The question is, then, how well do they actually work?
Matt Blaze went about testing cellphone shielding:
I tested three commercial pouches as well as three commonly-recommended makeshift shielding methods. Read on for the results. (Note that I have no connection with any vendor mentioned here, and I do not endorse any of the products discussed for any particular purpose. Caveat emptor.)
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