Faraday cages are one of the most misunderstood topics in EMP preparedness, and the internet is full of wrong advice. This article puts the most common questions to Dr. Arthur Bradley, an electrical engineer and EMP expert, who explains how Faraday cages work, what materials and thicknesses provide real shielding, how to build a shield room, why most homemade cages fail, and what conditions determine whether a cage will protect your electronics. Also covers aluminum foil bags, personal Faraday cages, and the difference between EMP and lightning protection.
Faraday cages come up constantly in conversations about EMP preparedness and so does a lot of misinformation. I’ve seen everything from garbage cans lined with cardboard to elaborate shield rooms, and it’s hard to know what will actually work without a background in electrical engineering.
Dr. Arthur Bradley has that background. He’s an engineer, an author, and someone who has actually set up and tested Faraday containers. I asked him to answer the questions I hear most often, including some that get confidently answered wrong all over the internet. What follows is his expert take. Updated June 2026 by Lisa Bedford.
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In this article
- What Is a Faraday Cage and How Does It Work?
- How Does a Faraday Cage Actually Work?
- What Is Field Cancellation and Why Does It Matter?
- How Thick Does the Aluminum Need to Be? (The Skin Depth Explained)
- Which Metals Work Best for Faraday Cage Shielding?
- Can a Faraday Cage Have Holes and Still Work?
- The Best Everyday Containers to Use as a Personal Faraday Cage
- What Goes Into Your Faraday Cage? Free Checklist
- Does a Faraday Cage Need to Be Grounded?
- Do Aluminum Foil Bags and Anti-Static Bags Actually Work?
- How to Build a Faraday Cage Room
- Ready For More Useful Info From Survival Mom?
- Frequently Asked Questions
- Related Articles on EMP Preparedness
What Is a Faraday Cage and How Does It Work?
A Faraday cage (a.k.a. Faraday shield) is a sealed enclosure that has an electrically conductive outer layer. It can be in the shape of a box, cylinder, sphere, or any other closed shape. The enclosure itself can be conductive, or it can be made of a non-conductive material (such as cardboard or wood) and then wrapped in a conductive material (such as aluminum foil).
How Does a Faraday Cage Actually Work?
A Faraday cage works by three mechanisms: (1) the conductive layer reflects incoming fields, (2) the conductor absorbs incoming energy, and (3) the cage acts to create opposing fields. All of these work to safeguard the contents from excessive field levels. A Faraday cage is particularly useful for protecting against an electromagnetic pulse that may be the result of a high-altitude nuclear detonation in the atmosphere (a.k.a. EMP attacks).
Despite rumors to the contrary, a Faraday cage is not necessary to protect against solar coronal mass ejections because the frequency content of such disturbances is at much lower frequencies—they don’t couple energy efficiently into small-scale electronics, except through conducted paths (e.g., wires coming into the system). A better precaution against solar events is to unplug electronics and use quality surge suppressors.
What Is Field Cancellation and Why Does It Matter?
Field cancelation occurs when the free carriers in the conductive material rapidly realign to oppose the incident electric field. If the cage is made from something non-conductive, the free carriers are not mobile enough to realign and cancel the incident field.
How Thick Does the Aluminum Need to Be? (The Skin Depth Explained)
The conductive layer can be very thin because of something known as the skin effect. That term describes the tendency of current to flow primarily on the skin of a conductor. As long as the conducting layer is greater than the skin depth, it will provide excellent shielding because the absorption loss will be large.
The skin depth is a function of the frequency of the wave and the conductor material. As an example, consider that for a frequency of 200 MHz, the skin depth of aluminum is only about 21 microns. EMP pulses can have frequency content that ranges up to 1,000 MHz. Therefore, wrapping a box in a couple of layers of heavy duty aluminum foil (typically about 24 microns thick) provides the necessary conductor thickness to protect against high-frequency radiated fields.
Which Metals Work Best for Faraday Cage Shielding?
Not much. The conductivity of nearly any metal is good enough to allow the carriers to easily realign to cancel external fields. For example, if silver (the best conductor) is used in place of aluminum, the skin depth at 200 MHz is reduced to about 4.5 microns. Of course, the high cost of silver would prevent using it for such a purpose.
Can a Faraday Cage Have Holes and Still Work?
Yes, as long as the holes are small with respect to the wavelength of the incident electromagnetic wave. For example, a 1 GHz wave has a wavelength of 0.3 meters in free space. As long as the holes are significantly smaller than that dimension (i.e., a few millimeters), they won’t let in much of the incident wave. This is why fine conductive mesh can be used when constructing a Faraday cage. In practice, the cage’s lid or door usually causes the most leakage. Taping the seam with conductive tape helps to reduce this leakage.
The Best Everyday Containers to Use as a Personal Faraday Cage
Yes, there are many conductive enclosures that can be used, including ammo cans, metal garbage cans, anti-static bags, and even old microwave ovens. Each has its own level of effectiveness as covered in my book, Disaster Preparedness for EMP Attacks and Solar Storms. The key criterion is that the gaps and seams remain very small.
What Goes Into Your Faraday Cage? Free Checklist
Does a Faraday Cage Need to Be Grounded?
There is a great deal of confusion regarding grounding of a Faraday cage. Grounding of the cage (i.e., connecting it to some Earth-referenced source of charge) has little effect on the field levels seen inside the box. Grounding primarily helps to keep the cage from becoming charged and perhaps re-radiating. The bottom line is that an ungrounded cage protects the contents from harmful electromagnetic fields as well as a grounded one.
Do Aluminum Foil Bags and Anti-Static Bags Actually Work?
Anti-static bags are readily available to protect electronic components against electrostatic discharge. They can be purchased in many different sizes, including some large enough to hold radio equipment. While they do offer shielding from EMP, not all products are created equal.
Testing confirmed that products certified to MIL-PRF-8170 and/or MIL-PRF-131 offer the greatest protection from an EMP. The results from testing three different types of bags are provided in Disaster Preparedness for EMP Attacks and Solar Storms. When selecting an ESD bag, consider not only the shielding effectiveness but also the physical ruggedness of the bag. A tear or large hole can compromise the bag by allowing EMP energy to enter.
How to Build a Faraday Cage Room
Storing a larger set of electronics might require an entire room. Engineers who work in electromagnetics often use “shield rooms” to conduct experiments because they do an excellent job of filtering out interfering signals, providing in excess of 100 dB of shielding. A poor-man’s shield room can be made by lining a small closet with heavy-duty aluminum foil, covering all four walls, the floor, the ceiling, and the inside of the door. Overlap and tape the seams using either conductive or regular cellophane tape.
There can be no conductive penetrations into the room, or it will seriously degrade the shielding. Cover all electrical outlets, light switches, etc. with aluminum foil. Do not plug anything into the electrical outlets. Also, lay a piece of plywood or cardboard on the floor so that it can be walked on without damaging the aluminum foil. Rooms built in this way have been shown to offer more than 50 dB of shielding up to several hundred MHz.
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Frequently Asked Questions
A simple test: put a cell phone inside, seal it completely, and try to call it. If it rings, the cage is not shielding adequately. This tests radio frequency shielding which is a reasonable proxy for EMP protection, though not identical. For more rigorous testing you’d need specialized equipment, but the cell phone test is a practical starting point.
Yes, the same shielding principle applies to both. A coronal mass ejection (CME) produces a geomagnetic storm rather than the fast-rise E1 pulse of a nuclear EMP, but both can induce damaging currents in electronics. A properly built Faraday cage protects against both.
Yes, this is called a shield room. The construction principles are the same as a small cage but applied to an entire room: continuous conductive material covering all six surfaces (walls, floor, ceiling), with special attention to seams, doors, and any penetrations for power or ventilation. Shield rooms are used in government facilities and testing labs. A properly built one can protect everything inside from both EMP and RF signals.
A CPAP machine would need to be stored inside a properly built Faraday cage to be protected. Like most other machines, the electronics inside are vulnerable to EMP. The challenge is that the machine is only protected while it’s inside the cage, which means you’d need a backup unit stored away from your primary one. Some people keep an older, simpler CPAP model as a backup specifically for this reason. It’s also worth checking whether your model has a DC power option because some can run on battery or solar, which is useful in a grid-down situation regardless of EMP concerns.
Related Articles on EMP Preparedness
- The Basics of EMP: What is it? How likely? How to Prepare?
- EMP Survival & The First 15 Things You Must Do Immediately After an EMP
- How to Make a Faraday Cage
- What’s in Your Faraday Cage? A Common Sense Guide to Preparing For an EMP
- Why and How to Protect Your Gear From EMP
Written by Arthur T. Bradley
I keep a few small electronics (handheld HAM, flashlights, personal fans, 6000 mah solar lithium power source, loaded USB’s – encrypted, etc.) in a large, isolated anti-static bag that is put inside of a metal pot with a clamped lid. I have a duplicate pot with electronics.
To test the effectiveness of my setup, I put the cell phone on vibrate and put it in the pot. It took a few combinations of pots and bags to find something that would prevent the cell phone frequency from spilling through.
I am reasonably (not completely) confident that the items in the pot can survive a decent surge.
Two is one and one is none. If the bad guys know that some people prep, they might delay a second device and use it at a later time to wipe out the additional electronics.
Of course, anyone should be ready to go primitive at any moment. An earthquake, tornado (or hail storm/flood), or other natural disaster can leave you without all the toys.
Are any generators emp proof?
All generators contain electrical components, so, theoretically, they would all be vulnerable to an EMP.
Which electric devices do we need to worry about surely all of them do not have computer “chips”,…CPAP som have memory card some dont? Any recommendations
Great question and one that applies to a lot of people with any type of medical device. A CPAP machine would need to be stored inside a properly built Faraday cage to be protected from an EMP because it contains vulnerable electronics. The challenge is that a CPAP is only protected while it’s inside the cage, which means you’d need a second unit stored away, or a non-electric alternative like a travel CPAP with manual backup capability. Some people keep an older, simpler CPAP model as a backup specifically for this reason. It’s also worth looking into whether your model has a DC power option. Some can run on battery or solar, which is useful in a grid-down situation regardless of EMP concerns.