Certain Apple iPhones  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.

Sometimes, you might really want to make sure something is genuinely isolated from the world around it, even if the software running on it has other ideas. For the radios in phones (which can transmit and receive cellular, WiFi, Bluetooth, and near field communication signals and receive GPS location signals), we can accomplish this by encasing the device inside a small Faraday cage.

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? It can be hard to reliably tell without access to a fairly specialized RF test lab.

Matt tested three commercial pouches as well as three commonly-recommended makeshift shielding methods.

Read Matt’s post for the methodology and results.