A Faraday bag, similar to a Faraday cage or Faraday shield, is a specialized bag or flexible container designed to block electromagnetic fields, including those used in wireless communication technologies. English scientist Michael Faraday invented the Faraday cage (on which the Faraday bag is based) in 1836.

A Faraday bag is a flexible version of a Faraday cage. The bag, pouch, or envelope form factor of the Faraday bag makes it particularly useful for storing electronic devices like smartphones, tablets, laptops, and key fobs as well as electronically readable items like credit cards and passports.

You might use a Faraday bag to protect an electronic device or digital medium from electromagnetic interference (EMI) and radio frequency interference (RFI), but it can also be used to block wireless signals to defeat tracking, surveillance, and other invasions of privacy.

For example, a Faraday bag can block signals used to erase, corrupt, alter, or read sensitive information stored on digital and magnetic media. It can also block mobile internet and WiFi signals to stop software “phoning home.” Faraday bags block signals indiscriminately, though, so devices that rely on wireless connections will not function properly in a Faraday bag.

How does a Faraday bag work?

To understand how Faraday bags work, it is important to first understand how radio signals and other forms of wireless communication and connectivity work. A basic grasp of the physics involved will help you to know when you can rely on a Faraday bag and when your intended use case exceeds its limitations.

Electromagnetic radiation and electromagnetic fields

Wireless communication works through the transmission and reception of electromagnetic radiation. This radiation has different properties based on its wavelength (or frequency: the higher the frequency, the shorter the wavelength). Some common types of electromagnetic radiation include (in order of decreasing wavelength):

• Radio waves
• Microwaves
• Infrared (IR) light
• Visible light
• Ultraviolet (UV) light
• X-rays
• Gamma-rays.

The light we see (from red to violet) is only a tiny, tiny slice of the full electromagnetic spectrum. Devices like cell phones, tablets, smartwatches, and laptops typically communicate in the radio and microwave ranges of the spectrum. In 1836, scientist Michael Faraday invented a way to reduce the effect of such radiation.

Faraday cages

Five years before inventing the Faraday cage, Michael Faraday discovered the phenomenon of induction. Electromagnetic induction refers to the electromotive force (emf) that arises across an electrical conductor in a changing magnetic field.

Put simply, if you hold a conductor—like electrical wire—in a rapidly changing magnetic field, you will induce an electrical current in the conductor. You could also quickly pass the conductor through a stationary magnetic field to achieve the same result; all that matters is that the conductor moves relative to the field.

A Faraday cage is an enclosed box made of a mesh of conductive material, like metal. Anything placed inside this box is shielded from fluctuating electromagnetic fields outside the box. This is because an electromagnetic field will cause electric charges within the cage walls to be arranged such that they cancel out the field’s effects inside the cage.

Like with simple magnetic induction, either the electromagnetic fields outside the cage will have to be fluctuating or moving quickly or the cage will have to be moving quickly through the fields. Otherwise, induction will not take place and the Faraday cage will not have an effect on the field.

So Faraday cages cannot block or reduce the effects of stable or slowly varying magnetic fields. The good news is that stable fields are generally not an issue: just as they do not interact with the Faraday cage, they are not likely to interact with whatever you put in the Faraday cage.

Faraday cages are not a one-size-fits-all solution. The size of the holes in the cage walls has an impact on the wavelengths of electromagnetic radiation that the cage can attenuate. If the holes are too large relative to the wavelength, then the waves will pass straight through. In this way, Faraday cages are “tuned” to a particular task.

Faraday shields

A Faraday shield is like a Faraday cage only that, rather than being constructed from a conductive mesh, it is made from solid pieces of conductive material. Think sheet metal or foil in place of wire mesh. Faraday shields are better at blocking or attenuating (reducing) a wider range of wavelengths of electromagnetic radiation than Faraday cages.

Faraday bags

A Faraday bag is simply a Faraday cage that has been designed and manufactured to be light, portable, and flexible. It is made using conductive materials, such as metalized fabric or wire mesh, that create a barrier to electromagnetic radiation just like the walls of a Faraday cage.

These properties and the form factor of a bag, pouch, or envelope make Faraday bags well-suited to storing and transporting electronic devices and storage media. A device placed into an appropriate Faraday bag will be virtually unreachable from the outside, but it will also be prevented from connecting to anything outside the bag.

Faraday bag use cases

Forensics

Faraday cages and bags are often used by law enforcement agencies to store confiscated devices and prevent people from wiping or tampering with evidence. This is something to which both Faraday cages and bags are very well suited. Evidence stored in this way is also protected from electromagnetic pulses.

Electromagnetic pulses (EMPs)

An electromagnetic pulse (EMP) is a burst of electromagnetic energy that can interfere with if not outright destroy electronic devices. An electromagnetic pulse can be caused by a nuclear blast or through natural processes, like solar flares. Given the intensity of EMPs, the goal here is to preserve electronics from damage.

Solar flares

A solar flare, or coronal mass ejection (CME), is a burst of solar energy that can overpower the Earth’s magnetic field with enough excess energy to interfere with electronics on the Earth’s surface. A large solar flare could trigger a geomagnetic storm that could overwhelm power grids and communication infrastructure.

A grounded Faraday cage is the best protection against the most energetic solar flares, but an appropriately specced bag may be enough for smaller flares. Keeping a spare cell phone or other communication device in a Faraday cage or behind a Faraday shield can be a good idea depending on the risk involved.

Car theft

Recently, one of the more common uses for Faraday bags has been to protect car keys and car key fobs from car thieves. Cars with keyless entry can be vulnerable to remote relay theft, a method that allows for car theft without any damage, lock picking, or the need for a car key. Small Faraday bags are ideal for this use case.

Privacy and security

Airplane mode is not an effective way of taking a mobile phone offline. With the number of backdoors known to exist on the firmware (chip) level, powering down a smartphone might not offer tracking protection either. For phones without removable batteries, a Faraday cage in one form or another offers the best protection.

Faraday shields, cages, and bags can also be used to protect access cards and other media from data theft. Without RFID protection, bad actors can contactlessly scan ID and access cards and gain unauthorized access to physical locations and computer systems. Credit cards are also vulnerable to such attacks.

Electromagnetic interference

A less common but equally important use case for these technologies is the elimination or reduction of electromagnetic interference caused by high voltage power lines and other sources of electromagnetic waves. This is especially important when making sensitive measurements, like those used in analytical chemistry.

Should you use a Faraday bag?

Whether you should use a Faraday bag depends on what you need to protect, the threats you perceive, and the risks associated with not using a Faraday bag. Not everyone needs to use a Faraday bag and in some cases doing so will actually do more harm than good (like exhausting the batteries of always-connected devices).

Here are some things to consider when deciding whether to use a Faraday bag:

Physical limitations of Faraday bags

Without getting too technical, there are many factors that affect how effective a Faraday bag is at attenuating electromagnetic radiation. Higher frequencies (lower wavelengths) are more difficult to attenuate. The waveform also affects attenuation—differently shaped waves interact differently with Faraday bags.

Ultimately, a high-powered transmitter placed close to or inside a Faraday bag is far more likely to penetrate it than a lower-powered transmitter located further away. High-powered, near-field transmissions like HF RFID (high frequency radio frequency identification) are particularly difficult to attenuate, for example.

Practical limitations of Faraday bags

Physical limitations aside, there are some practical considerations you should take into account before deciding if you need a Faraday bag. The additional protection a Faraday bag provides comes with some drawbacks. Many boil down to your being able to protect electronic equipment or use it, but not both.

Placing a device that is trying to pair with other devices over Bluetooth or connect to the internet or local WiFi networks into a Faraday cage or bag will cause that device to ramp up its power output to establish a connection. This will quickly exhaust the device’s battery and can cause it to overheat.

This means that something like a mobile phone will have to be turned off or put into airplane mode before being placed in the Faraday bag. This can cause problems in emergency situations or if you need to locate your phone remotely. Not everyone reaches for a Faraday bag to protect their devices, though.

If you have good reason to worry about being digitally surveilled by government agencies or other technologically sophisticated actors, blocking communications to and from your mobile devices may actually draw more attention to you. It’s not normal for these devices to go completely dark.

When a bad actor has full and persistent access to your device, they can see when you put it into airplane or offline mode. If it suddenly disappears off their radar altogether, they may switch to other avenues of surveillance to see what you’re up to. Of course, the chances of all this happening are extremely small.

How to use a Faraday bag

Make sure the device is in airplane mode if it has it, manually turn off its antennas (WiFi, Bluetooth, mobile internet, etc.) if it doesn’t. Power down the device if you don’t need to have it on while it’s in the bag—this will drastically reduce the chances of overheating and the damage that goes with it.

How to make sure your Faraday bag works like you expect it to

Just as using airplane mode can give you a false sense of security, so can using a Faraday bag. If the stakes are high, you should test and then regularly retest your Faraday bag to make sure it really is making communication with the devices you place inside impossible.

Try every wavelength and waveform you can. For example, in the case of a cell phone, turn on all of its antennas, place it in the bag, and try to:

• Send an SMS to it;
• Call it;
• Pair with it over Bluetooth;
• Connect to it over WiFi;
• Connect to it using NFC.

Go through analogous steps for a laptop, tablet, or other connected device. 

If you keep the bag in a single location where it doesn’t get jostled, bent, or crushed, then you won’t need to retest it very often. If you carry your bag around with you, you should check it more often and especially if it begins to fray or show other signs of wear and tear.

What else do you need to disappear completely from the internet? Check out this post.

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Updated on: July 4, 2023