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.

The iPhone 11 and later models running iOS 15 and above stay active on Apple's Find My network even when the phone is fully powered down. The Ultra-Wideband and Bluetooth Low Energy chips keep broadcasting a low-power beacon. Nearby Apple devices pick that beacon up and silently report your location back to Apple's network. Your phone is off. You are still being tracked.

That's one phone. One OS version. The broader picture is worse.

Every phone constantly broadcasts what are called probe requests, scanning for Wi-Fi networks it recognizes. These probes include a unique hardware identifier. Retailers, airports, smart cities, and private data brokers harvest those identifiers to map where you go and how long you stay. You never connect to their networks. You never agree to anything. They log you anyway.

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. A single layer of nickel-copper fabric typically gets you around 70 dB of attenuation. That handles most cellular frequencies but leaves room for higher-frequency signals like 5G and Wi-Fi to punch through at reduced strength. Two layers, around 86 dB, blocks standard cellular and Wi-Fi reliably. Three or four layers have diminishing returns and add weight without proportional benefit.

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 path so long and indirect that signals attenuate before they can travel through. Magnetic closures are quieter (no Velcro rip to announce you're bagging your phone) and work well for everyday carry. Double-fold Velcro closures are common on budget bags and perform acceptably at lower frequencies, but above 3 GHz, the seal leaks. That range covers 5G and most Wi-Fi bands.

A bag is only as protective as its worst seam.

What happens to battery life inside a bag?

This is practical and important. When a phone loses signal, it doesn't sit quietly. It assumes there's a weak tower nearby and starts ramping up transmit power trying to find it. Full power searching. Inside a bag that's doing its job, it will never find anything.

A phone left on inside an effective Faraday bag can drain its battery in under three hours.

The fix is simple: put the phone in airplane mode before you put it in the bag. The phone stops looking for towers. Battery drain returns to normal standby rates. The bag handles the rest.

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 is worth noting for any bag you buy. Repeated folding and compression gradually cracks the metallic plating in the fabric. All bags degrade. Higher-quality construction degrades slower. If you're using a bag regularly, test it periodically with a second phone inside trying to receive a call. No ring means the bag is still working.

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. Law enforcement uses geofencing warrants: a request for records of every device present in a defined area during a defined time. You don't have to be a target. You just have to have been there. Bagging your phone during travel to and from a protest, not just while you're at it, can keep your device out of that geographic net entirely.

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 different risk. Devices can be subject to scanning or cloning at ports of entry, sometimes without formal seizure or notification. A bagged phone is harder to interact with.

A note on who else uses this technology

The same Faraday bags protecting your privacy protect evidence in criminal investigations. When law enforcement seizes a phone, they bag it immediately. The reason is preventing remote wipe commands from deleting data before forensic extraction. Some bags are designed to keep the phone charged and isolated simultaneously, preventing the encryption lockout that happens when a phone dies.

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.