Does a Faraday Cage Block Cell Signal?

If there are two things on most prepper’s minds concerning distinctively 21st century threats it is the illicit hijacking of our devices’ signals and the looming, Doomsday threat of EMP events.

Concerning the latter, one of the only defenses we have for our vital electronics is placing them inside a Faraday cage enclosure, a container or room that will absorb and attenuate the powerful, circuit board frying waves of an EMP.

sealed faraday cage ammo box

It has been asserted that an easy test to determine if a Faraday cage is adequate to the task is simply placing your cell phone inside the proposed container and seeing if it can receive, or send, calls. Is this true?

Yes, a faraday cage suitable for protecting sensitive devices against a powerful nuclear or non-nuclear EMP should be of sufficient design and construction to also prevent cell phone signals from getting in or out, but exceptions exist.

Notably, signal blocking containers that could shield a cell phone from intrusive signals or from signal leakage may not be sufficient to protect it from an EMP.

Suffice it to say that this is a complicated and non-trivial topic for those who are not already fluent in electronics and signal theory.

Considering that the above threats are only going to increase in likelihood as time goes on it is in your best interest to learn everything you can about the topic if you are serious about protecting your devices going forward.

Understanding Attenuation Levels

To understand the protection afforded by a faraday cage or a signal blocking bag alike, you need to understand attenuation levels. Attenuation levels are measured in dB, or decibels, just like sound waves.

A sound wave is just another type of signal, and just like other signals we are concerned with, namely the propagation of EMP waves and the waves carrying the data emitted by our personal devices.

For the uninitiated, you can think of decibels as a unit of strength, or pressure. A higher decibel wave is generally more powerful and will require correspondingly powerful attenuation to block or degrade in a meaningful way.

Cell phones, in particular, emit and receive signals that are quite potent, and suppressing these signals requires attenuation anywhere from 75 dB to 100 db, which is quite a lot.

An EMP, depending on the strength and the device being protected, also requires substantial attenuation to prevent it from damaging the device in question, although particularly nuclear EMPs and non-nuclear EMP weapons can produce emissions so powerful it is difficult to test against them in any meaningful way at the consumer level.

Think of a Faraday cage like sunglasses.

Another way to think of the interaction of EMP waves and attenuation devices is sort of like the relationship to light and sunglasses. Sunglasses don’t work by “darkening” or weakening the light; instead they work by reducing the amount of light that gets through the glasses, like a filter.

In essence, the environment behind the sunglasses, namely your eyes, experiences a far lower intensity of light because less light is allowed through them.

Think of your Faraday cage or signal blocking bag in the same regard. That device, be it purpose designed and commercially procured or improvised from common, household materials will provide a certain amount of attenuation against an incoming signal or the emission of signals originating from within the container.

Generally, most setups are not sufficient to completely block either kind of signal. This means the possibility still exists that signals could reach your device or be transmitted from within the container to the outside.

Similarly, an incoming, damaging EMP wave could be reduced in intensity but is unlikely to be completely blocked.

The type of signal makes a difference, too.

The type of signal also dictates what level of protection is required to defend against it. Without cracking open a whole book full of technical jargon, let it be known that, generally speaking, when it comes to signals protection any opening in the container or room may only be 1/10 the size of the wavelength you are trying to defend against.

Let us take the case of the commonly improvised signal blocking bag for cell phones, sometimes advertised as a solution to EMP worries also.

Usually made from common household aluminum foil, if there was a single slit left in one of the folded scenes that measured, let’s say, a 16th of an inch wide and about four inches long, even that tiny opening would effectively render any protective value of such a setup moot.

That is because the typical wavelength of a moderate cell phone signal, depending on band, carrier and other details, is anywhere from six to 12 inches in height, meaning our thin slit that is scarcely as long as our finger is more than enough to compromise our protection factor.

Additionally, the thickness of the material in a question that our container is made of makes a difference. thicker is always better in this regard, as material that is too thin, especially conductive materials, can actually allow the signal to strike it, propagate through it and carry on past it, once again potentially rendering our protection moot.

Testing a Cell Phone is one thing, testing for EMP is another.

One practical problem that is difficult to overcome is the viability of testing. It is easy enough to test your cell phone for signal reception and transmission using any variety of purpose made signal blocking bag or box, Faraday cage or improvised version of either.

This can be done using commercially available testing equipment that isn’t too expensive, or the old, trusty standby of simply trying to call the device in question, or having it place a call and then quickly sealing it inside.

However, testing against a legitimate EMP threat is a wholly different beast. Most destructive EMP testing is performed against a lightning strike as a standard, and only the military and certain national laboratories have the capability of generating a nuclear detonation equivalent or non-nuclear EMP weapon equivalent in a repeatable and controlled environment to test the efficacy of various protective measures.

I caution all preppers who are looking for commercially available EMP protective bags or boxes online or anywhere else.

These products from various manufacturers advertise protection levels that we have no way of ascertaining ourselves, and furthermore most manufacturers have no way to subject their products to the level of testing that will guarantee their performance against the threat we are buying it to defend against.

It is bad enough to waste money on a product that cannot even do what it is advertised to do, but it is far worse to depend on it as a legitimate solution when the chips are down only to be proven frightfully unworthy.


A Faraday cage can be bought or made that is sufficient to block cell phone signals from reaching a device placed inside it or escaping from a transmitting device placed inside it, but not all Faraday cages will prove up to the task.

There are many factors to be considered, including quality of construction, thickness of material, and the strength of the signal that is being defended against. Only by carefully assessing the attenuation value of any Faraday cage can its suitability for protection be determined.

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