Thread by @mattblaze

Spending a lazy sunday afternoon testing faraday bags for phones. (Preliminary results so far: You don’t always get what you pay for, but you never get what you don’t pay for.)

Motivated by the fact that iPhones officially can’t be powered off, which, even if they implement really good privacy protections, will inspire other manufacturers to try similar things, often less carefully.

Some quick preliminary results, testing at 1, 2, 3, 4 , 5 and 6GHz: The expensive (~USD 40-60) phone-size bags from Mission Darkness (sold on Amazon) and EDEC (online store) work reliably well: >60dB attenuation at 1M distance, IF closed properly.

Recycled ziplock mylar antistatic bag: < 8dB attenuation. Cheap RFID blocking bag: ~9-12 dB. Metal biscotti tin: ~6dB, worst performance, but provided snaks during testing.

The pricy bags worked pretty well, if you can stomach the “tactical” design and marketing. 60dB was a lower bound limited by the fact that the local RF noise floor prevented more sensitive measurements. Will redo in an RF test chamber when I get a chance.

As a practical test, the pricy commercial bags all prevented detection/activation of an Apple AirTag within 6 inches of a phone, while the mylar bag, RFID-block bag, and biscotti tin failed to prevent detection from anywhere in the room.

Some caveats: I just tested one or two phone-size bags, not the complete line of products from each vendor. Sealing the flap is absolutely critical. The bags seem somewhat brittle, and probably susceptible to damage if folded or handled aggressively.

Basically, 60dB attenuation is sufficient to give me reasonable confidence that a low power device (such as Bluetooth) in those frequency ranges won’t be detectable at moderately close range.

Can you make something for less money than the commercial ones? Probably, but the problem is that properly testing it to give you assurance it works involves a ton of expensive gear. And then there’s the problem of being sure it stays working after your’e done testing.

OK, some more results on the commercial faraday bags, this time inside a shielded RF test chamber. The Mission Darkness and EDEC bags were quite similar, > ~100 dB at 1 and 3 GHz, ~90 dB at 4 and 5 GHz and ~85 dB at 6GHz.

That’s pretty damn good. The two brands were quite similar, with about +/- 10dB variation in the measurements depending on how well I folded the velcro closure. More than sufficient for attenuation for almost any signal coming from a device that you could fit in the bag.

For these tests I used a small R&S RF test chamber (intended for testing cellphones), hooked up to an R&S PR100 measurement receiver. For the signal source I used a cheap RF Explorer signal generator with a short wire antenna, which just fit inside the bags under test.

What about assuring that signals from OUTSIDE the bag can’t be received? That’s harder in general, since an external transmitter might be arbitrarily high power and in close proximity. But ~90dB is still a LOT of attenuation in practice for any reasonably distant signal source.