To start today’s discussion of EMP shielding, we are going to go back to 1755.

As you may know, our polymath founding father Benjamin Franklin conducted many experiments with electricity in the mid-18th century.

Franklin’s kite experiment is perhaps the most iconic, but in 1755, he discovered a way to shield objects from electrical charges. He did this by lowering an uncharged cork ball suspended on a silk thread through an opening in an electrically charged metal can. In his words:

The cork was not attracted to the inside of the can as it would have been to the outside, and though it touched the bottom, yet when drawn out it was not found to be electrified (charged) by that touch, as it would have been by touching the outside. The fact is singular.

Franklin had discovered the behavior of what we now refer to as a Faraday cage or shield. Faraday, a British scientist, is given credit for inventing the shield in 1836. To do so, Faraday duplicated Franklin’s original experiments.

It gives me pride knowing that one of our nation’s forefathers discovered something that may protect us from threats today – without even knowing it. That’s certainly the spirit this nation was founded upon.

What is a Faraday Cage or Shield?

In the simplest terms, a Faraday Cage or shield is an enclosure that blocks electromagnetic fields. They can be tiny, or large enough to hold people and even larger for massive equipment.

They are formed by a continuous covering of conductive material (typically metal) or using a mesh of conductive material. A shield uses a continuous covering while a shield created with mesh is typically called a cage. This is mostly semantics, with many calling all such units cages. Potato, potato.

Besides being of interest to those of us in the preparedness community, Faraday cages are used in many fields. They are used in certain medical procedures to keep from interfering with equipment outside of the cage. Law enforcement uses them to protect recovered wireless devices from being wiped or altered during investigations. These are just a couple of examples.

In addition, Faraday cages help many of us along in our preparedness journeys.

We know that a powerful EMP, especially one produced by a high-altitude nuclear weapon could mean our grid and electronics could all go down.

If this happens, having electronics shielded beforehand could be a life-saver.

However, we also know that our government has not heeded calls from the EMP Commission and others to harden our grid and critical infrastructures. Countries like China, Russia and even North Korea have all taken measures to do this while our government focuses on more important threats.

What this means is that without a grid, many of our electronics (especially ones that make communication easy in our modern world) will be useless. That means no cell phone calls, no texting, no email, no social media. An old smartphone used for storage of survival knowledge might be a good idea to shield though. Just don’t expect any service. GPS also will not work on any phone, as they don’t actually use satellites for positioning, but cell phone towers.

What should you shield in a Faraday Cage?

This will vary depending on your own personal situation, but I advise you to shield only the most critical items first. You can always add to your cage or add more cages as you go. Here is a list to get you thinking about your own needs:

  • Flashlight (especially LEDs; batteries do not need protection)
  • A small transistor radio (CB, digital scanner and ham radios too, if you have them)
  • A device you can store and access digital files on
  • Walkie-talkies and two-way communication devices
  • Night-vision and laser optics (binoculars, rifle scopes, spotting scopes)
  • Electronic medical equipment
  • Spare critical electronics for vehicles

This by no means an exhaustive list.

How to build your own Faraday cage?

It is quite easy to build your own Faraday cage. Many common items can be used or repurposed for EMP shielding.

Most ad-hoc cages are built with conductive metals. Two common items used are metal ammo cans and galvanized trash cans.

While all-metal items like these are great at protecting items inside from electromagnetic fields, they still will conduct electrostatic charges. This means that any electronics directly in contact with the metal could be fried if electrical charges are present. So it’s a good idea to insulate these containers to prevent this situation, with something like corrugated cardboard.

I’ve even seen a few Faraday boxes built out of storage containers and crates for housing large electronics or even entire vehicles. Many of these are made of steel, which is okay, but you need to make sure it has no openings. So, knowing how to weld (and insulate) on a commercial scale will be critical there.

Pro-tip: aluminum foil is your friend. Get the heavy-duty foil. You can use foil to make any container – like a shoe box – into a Faraday cage. You can even wrap a few layers of foil directly around a device for a quick, impromptu shield (be sure to insulate the device first and be careful to tightly seal with no gaps). They even make aluminum foil tape that can be useful in sealing up a cage made from an ammo or garbage can.

Which reminds me: the opening to your cage is your weakest link. Be sure to make sure the opening has no gaps – airtight if you can. You can do this by using gasketing material or extra foil. If you can avoid opening the cage until you absolutely need what’s inside, that’s the best course of action.

Final tip: consider “nesting” for extra protection. The more layers of conductive material we put between our electronics and any outside electromagnetic fields, the better protected they are. This is a good idea for particularly sensitive instruments and electronics with a lot of components.

An example of a nested Faraday Cage would be an ammo can inside an insulated trash can. The devices inside the ammo can would have two layers of nested protection.

How to test your Faraday Cage?

Now that we’ve gone through the trouble of building a Faraday Cage, we want to be sure that it will effectively protect the devices inside.

However, there is a lot of contention about the best way to test our cages for their shielding effectiveness. Obviously, we can’t test it under the actual conditions we are preparing for, so we make an approximation.

The most scientifically legitimate way of testing a Faraday cage that I’ve seen involved using an RF meter and placing it inside the cage to be tested. A good one can cost a few hundred dollars. Unless you’re a radio operator, this may be an unnecessary expense.

The simplest and easiest way to test your EMP shielding at home is to use a quality radio. Simply turn it on, tune it to a station with a strong, clear signal, then place it in the cage and seal it up. If you can still hear the station, your cage fails the test. But if it goes quiet or static, it passes this at-home test.

If my cage passes the test, is it EMP Proof?

The answer to this one is tricky. We’ll refer to a resource a reader like you sent in. It comes from a three-part series on EMPs and their effect on professional and amateur radio broadcasters.

Will a model ABC123 smart phone survive an EMP at a given location?


Every MegaHertz of the spectrum will have some energy in it from an EMP. Predicting the total energy arriving at a distant location requires modeling the propagation efficiency for every frequency. Then the modeler will add up all of the voltages from all of the frequencies to obtain the peak Voltage applied to the victim device.

Due to this infinitely large number of parameters which determine the strength of the Voltage imposed on any distant electronic device, it is computationally prohibitive to predict, with precision greater than the nearest order of magnitudethe level of stress which any particular piece of equipment will be forced to endure.

Thus, the answer to so many questions is "maybe."

While we might not be able to accurately model an EMPs effects on our particular cages and shields, I believe that some protection is better than nothing.

Government Doing Testing Of Its Own

While we’ve been busy testing our Faraday cages for this article, the federal government has been doing some more EMP testing itself.

From November 4-6, the Department of Defense, along with the Army Military Auxiliary Radio System (MARS) and amateur radio operators, conducted a simulated “very bad day” scenario. This kind of scenario is defined by those involved as any event in which the national power grid fails along critical forms of communication.

This simulated exercise also included a simulation for a Coronal Mass Ejection (CME) event, also known as a solar storm (which we know can produce EMPs).

According to a MARS operator, this simulation is a routine training event they conduct 4 times annually. This info surprised me, because I believe that the government is ramping down measures related to the EMP threat. We saw the EMP commission close its doors less than two months ago, along with other disappointing developments. But this makes me feel slightly better – knowing that routine exercises like these still exist.

That about wraps up our primer on Faraday cages. I hope you’ve found this to be inspiring and helps you along in your preparedness journey.

In Liberty,

Grant Miller
Preparedness Advisor, MPS
P.S. To learn more about self-reliance, follow MPS on Facebook or Twitter.

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