Faraday bags: what they are, how they work, and when you need one

Your phone is a radio. Multiple radios, actually. It's transmitting right now, whether you told it to or not.

That's not paranoia. That's engineering. And if you're attending protests, crossing borders, holding sensitive conversations, or just tired of being tracked through every airport and shopping mall you walk through, you need to understand what a Faraday bag does and why software alone won't protect you.

Why can't you just turn airplane mode on?

This is the first place most people get comfortable and the first place they get it wrong.

Airplane mode is a software instruction. It tells apps and the operating system to stop using radios. It does not physically disable the hardware.

On iPhone 11 and later running iOS 15 or later, powering the phone off doesn't fully silence its radios. The device drops into a low-power state and keeps broadcasting a Bluetooth Low Energy beacon — behaving essentially like an AirTag — which any nearby Apple device relays to Apple's servers. Apple says this works for up to 24 hours after shutdown, and about five hours if the phone shut down from a dead battery. Your phone is off. You are still findable.

(You'll sometimes see this attributed to the Ultra-Wideband chip. It isn't — UWB drives Precision Finding at close range. The power-off beacon is Bluetooth. You can disable the behavior by turning off the Find My network, or by disabling Bluetooth in Settings. The Control Center toggle isn't enough.)

That's one phone. One OS version. The broader picture is more complicated than it used to be.

Every phone broadcasts what are called probe requests, scanning for Wi-Fi networks it recognizes. These used to be a reliable tracking beacon: they carried the device's permanent hardware MAC address, and retailers, airports, and analytics firms harvested them at scale to map where you went and how long you stayed.

Both platforms have since closed that specific door. Android randomizes MAC addresses by default as of Android 10, and iOS has used per-network private addresses by default since iOS 14. If you read an article claiming your MAC address is broadcast to every shop you walk past, it's describing a world that has largely been patched.

The door isn't fully shut. Researchers have repeatedly shown that devices stay fingerprintable through the rest of a probe request — vendor-specific information elements, sequence numbers, frame timing, and the particular set of capabilities a device advertises. Randomization defeats naive MAC tracking. It doesn't make you anonymous.

We covered Stingray devices in a previous article. The short version: law enforcement and bad actors use fake cell towers called IMSI catchers to force nearby phones to connect, then capture device identifiers and pinpoint location. A phone inside a Faraday bag is invisible to them. The fake tower broadcasts. The phone never hears it.

Software cannot fix any of this. The chips are always on. A Faraday bag physically removes the phone from the equation.

How does a Faraday bag actually work?

Michael Faraday figured this out in 1836. A conductive enclosure redistributes electromagnetic fields around its exterior, leaving the interior shielded. Nothing gets in. Nothing gets out.

A Faraday bag is that enclosure made portable. The fabric contains metallic fibers that create a continuous conductive shell around your device. Radio signals, all of them, hit the bag and stop.

The shielding quality is measured in decibels. Higher numbers mean stronger attenuation. One important caveat before any number: attenuation is frequency-dependent, so a bare "70 dB" on a product page is close to meaningless without the frequency it was measured at. Published fabric data shows the same material going from 82 dB at 10 MHz down to 68 dB at 3 GHz.

With that said, vendor chamber testing using the IEEE 299 method reports roughly 70 dB at 2.4 GHz for a single layer of nickel-copper fabric, about 86 dB for two layers, and only ~88-89 dB for three or four — steep diminishing returns for the added weight. Treat these as vendor figures rather than independent lab results, but the method behind them is the right one.

The two materials you'll see most often are nickel-copper ripstop and silver-coated textiles. Nickel-copper is the industry standard. It's stable, corrosion-resistant, and maintains consistent attenuation across temperature and humidity ranges. Silver has higher conductivity and can hit 90 to 95 dB attenuation, which is why you see it in military and forensic applications. It's also significantly more expensive.

The fabric choice matters. So does everything else about how the bag is built.

Where do Faraday bags fail?

The physics of electromagnetic shielding require a complete, continuous conductive enclosure. One gap undoes the whole thing. This is where cheap bags earn their price.

Seams are the first failure point. Standard sewing thread is not conductive. A seam sewn with non-conductive thread creates what engineers call a slot antenna, a gap that actually radiates signal outward instead of blocking it. Quality bags use conductive stitching or thermo-welded seams that maintain the shielding layer across the join.

Closures are the second failure point. The opening is inherently a gap. How the bag handles that gap determines how much signal leaks through.

Roll-top closures are the most reliable. Rolling the opening multiple times creates a compressed conductive path long enough that signals attenuate before they get through. Magnetic closures are quieter (no Velcro rip to announce you're bagging your phone) and work well for everyday carry.

The general rule is that shielding effectiveness falls as frequency rises: the same physical gap that's invisible at 700 MHz cellular becomes a leak path at 5 or 6 GHz Wi-Fi. You'll see confident claims online about a specific frequency at which Velcro closures start failing. We couldn't find a measurement behind any of them — they trace back to bag vendors, not labs — so treat that number as marketing. What is documented: peer-reviewed testing of commercial Faraday bags found several that failed to block certain bands. Closures fail. The specific threshold is not something anyone has published.

A bag is only as protective as its worst seam.

What happens to battery life inside a bag?

When a phone loses signal it doesn't sit quietly. It assumes there's a weak tower nearby, raises transmit power, and scans harder for something it will never find inside a working bag.

How much battery that actually costs varies enormously, and you should be skeptical of any specific figure — including the "under three hours" number that circulates widely. We went looking for a measurement behind it and found an anecdote. Modern basebands generally throttle back into a low-power state after failing to acquire service for a while, and at least one documented test showed only single-digit percentage drain over eight hours. Other devices do run down noticeably faster. It depends on the phone and the firmware.

The fix is the same either way, and it's what forensic handling guidance recommends too: put the phone in airplane mode or power it off before it goes in the bag. Then the modem isn't hunting, and the bag only has to do the job it's good at.

What bags are worth buying?

Three brands have consistent track records and enough real-world use to have meaningful reputations.

Mission Darkness, made by MOS Equipment, is the standard in military and law enforcement circles. Their bags use dual-layer TitanRF fabric with ballistic nylon exteriors. Some versions include a forensic window that lets investigators charge a device while keeping it isolated. That same feature is useful if you need to use the phone while preventing remote access.

SLNT, sold as Silent Pocket, takes a different angle. Magnetic closures, clean industrial design, no tactical aesthetic. Executives, journalists, and people who want protection without advertising it tend to reach for these.

GoDark builds for outdoor use. Heavy 600D polyester exteriors, roll-top closures, reinforced inner liners. The construction holds up to the kind of repetitive use that causes cheaper bags to develop micro-cracks in their metal coating over time.

That last point applies to any bag you buy. Conductive fabrics degrade with handling: repeated sharp creasing fractures the metallic plating, raising resistance and opening micro-cracks along fold lines and corners — the same places that take the most abrasion from devices sliding in and out. Rip-stop weaves hold up better than thin metallized films, but nothing is permanent.

You'll find confident numbers online about exactly how many folds cost you exactly how many decibels. Those come from bag vendors without published methodology, so don't plan around them. Treat a Faraday bag as a thing with a service life rather than a permanent fixture, and test it instead of trusting it. Put a phone in, seal it, call the phone. No ring means it's still working. That test costs thirty seconds.

When does a Faraday bag actually matter?

Digital hygiene is the baseline reason. Stopping passive Wi-Fi probing and Bluetooth beaconing requires no particular threat level to justify. Your location history is built from a thousand small broadcasts you didn't consent to. A bag during commutes or time in dense public spaces limits that accumulation.

Protests are a higher-stakes situation, though the specific legal picture here changed significantly this year and most of what's written about it online is now out of date.

A geofence warrant asks a company — in practice, almost always Google — for records of every device present in a defined area during a defined window. You don't have to be a target. You just have to have been there.

Two things have since happened. Google restructured Location History to store data on-device with shorter retention, completing the rollout in 2025, and now tells courts it can no longer respond to these requests the way it used to. And on June 29, 2026, the Supreme Court decided Chatrie v. United States, holding 6–3 that obtaining that location data was a Fourth Amendment search — you have a reasonable expectation of privacy in your phone's location information.

Read that carefully, because it is narrower than the headlines suggested. The Court did not hold geofence warrants categorically unconstitutional. It held that using one is a search, which means the Fourth Amendment applies, and sent the case back down to sort out whether this particular warrant was reasonable.

The practical upshot for you: the classic Google geofence dragnet is both legally constrained and largely unavailable now. That is genuinely good news and it is worth knowing. It also doesn't mean location dragnets are over — it means one specific technique against one specific company got harder. Bagging your phone on the way to and from a protest, not just while you're there, still keeps your device out of whatever the next version looks like.

Sensitive meetings, whether legal consultations, medical conversations, or anything you'd prefer wasn't streamed to a third party, benefit from bagging every device in the room. A compromised phone can capture audio and video without any visible indication. Physical isolation is the only reliable counter.

Border crossings carry a different risk, and the rules are more permissive than most people expect. Under CBP Directive 3340-049A, a basic search — an officer scrolling through your device by hand — requires no suspicion at all. An advanced search, where they connect external equipment to copy or analyze the contents, requires reasonable suspicion of a legal violation or a national security concern, but that's CBP's internal policy rather than a warrant requirement. Courts are split on how much more is required for forensic searches, so the protection you actually have depends on which circuit you're in. Devices and copies can be held for days.

A note on who else uses this technology

The same Faraday bags protecting your privacy protect evidence in criminal investigations. SWGDE's mobile device guidance directs examiners to disconnect a seized device from its networks so data can't be remotely modified or destroyed — airplane mode if the screen is accessible, a Faraday bag if it isn't.

Two details from that guidance are worth stealing, because they're counterintuitive and they tell you something about how this technology actually behaves:

Examiners want the phone to stay powered on. A reboot re-engages full-disk encryption and makes extraction dramatically harder. Which is a useful thing to know in the other direction, too.

And if a bagged phone needs charging, the power source goes inside the bag. A cable running in from outside turns into an antenna and defeats the shielding entirely. That's the actual reason those filtered charging-window bags exist — it isn't a convenience feature.

The technology is identical on both ends of that equation. The same physics that preserves evidence on a seized device keeps your device invisible to anyone trying to reach it without your knowledge or consent.

Practical steps

Before you buy anything, test what you have. Put your phone in a zip-lock bag with a signal-blocking insert you already own, then call it from another device. If it rings, it's not shielded. This gives you a baseline.

When you buy a bag, verify the attenuation rating in the product specs. A bag without published dB ratings is either untested or performing too poorly to advertise. Look for dual-layer construction and either roll-top or magnetic closures. Check the seam construction in the product photos if they're available.

Once you have the bag, build the habit of airplane mode first. The bag and the power management work together. Neither is optional if you care about battery life.

Test the bag monthly if you use it regularly. A bag that looks fine from the outside may have developed enough coating damage to leak at higher frequencies. The call test takes thirty seconds.

Pick the use cases that match your actual threat model and start there. You don't have to solve every scenario at once. Start with the highest-risk situations you're currently in, and build the habit from there.

Frequently asked questions

Does airplane mode do the same thing as a Faraday bag?

No. Airplane mode is a software instruction to the operating system; a Faraday bag is a physical barrier. The clearest example is that iPhone 11 and later keep broadcasting a Bluetooth locator beacon for up to 24 hours after you power the phone off entirely. Software can't be trusted to silence hardware. That said, you should use airplane mode and the bag — see the battery question below.

Will a Faraday bag drain my phone's battery?

Possibly, but not as dramatically as commonly claimed. A phone that can't find a tower raises transmit power and scans harder, though modern basebands generally throttle back into a low-power state. Published figures vary wildly and the widely repeated "dead in under three hours" doesn't trace to any real measurement. Put the phone in airplane mode or power it off before bagging it and the question mostly goes away.

How do I know if my Faraday bag actually works?

Put a phone inside, seal it, and call that phone from another one. If it rings, the bag isn't shielding. Do this when you buy the bag and repeat it every month or so if you use it regularly — conductive fabric degrades along fold lines with handling, and a bag that looks fine can leak.

Do Faraday bags stop Stingrays and cell-site simulators?

Yes, and this is the one countermeasure that reliably does. A cell-site simulator works by getting your phone to connect to it. A phone inside a working bag never hears the broadcast, so there's nothing to connect to. Encrypted messaging apps protect your message content but do nothing about this — your IMSI and IMEI are exposed well before any app-layer encryption matters.

Are geofence warrants still a reason to bag my phone at protests?

Less than they used to be, and the change is recent. Google restructured Location History so it can no longer answer these requests the way it once did, and in June 2026 the Supreme Court held in Chatrie that using a geofence warrant is a Fourth Amendment search. But the Court didn't ban them outright, and location data reaches police through other channels — including purchases from private data brokers. The habit is still worth keeping.

What should I look for when buying one?

Published attenuation figures with the frequency they were measured at (a bare "70 dB" tells you nothing), dual-layer construction, and either a roll-top or magnetic closure. Seams and closures are where bags fail, not the middle of the fabric. A bag with no published specs is either untested or performing badly enough not to advertise.