FARADAY CAGE
There is a great deal of confusion about do it yourself Faraday cages. Not only about how to build them, but also what they actually protect against.
In this article, I answer a few basic questions and maybe shed a few myths.
What is a Faraday Cage?
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).
What does it do?
A Faraday cage works by three mechanisms:
(1) The conductive layer reflects incoming fields.
(2) The conductor absorbs incoming energy.
(3) The cage acts to create opposing fields.
These work to protect the contents from excessive
energy 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 attack).
Despite the rumors , a Faraday cage is not necessary to protect against solar coronal mass ejections, because the frequency content of such disturbances is at a much lower frequency — they don’t couple energy efficiently into small-scale electronics, except through conducted paths (e.g., wires coming into the system). The best precaution against solar events is simply to unplug electronics and use quality surge protectors.
Field cancellation and how it works:
Field cancellation 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 should the conductive layer be?
The conductive layer can be very thin because of something known as the skin effect, however I prefer a thicker layer to prevent tears and rips in the conductive layer. 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.
Does it matter which conductor type is used?
Not really. The conductivity of nearly any metal is good enough to allow the carriers to easily realign to cancel external fields. Take silver as an example (in my opinion, the best conductor) is used in place of aluminum, the skin depth at 200 MHz is reduced to about 4.5 microns.
Faraday cage: Can it have holes?
Yes, as long as the holes are small
with respect to the wavelength of
the incident electromagnetic wave.
Take a 1 GHz wave that has a
wavelength of 0.3 meters in free
space. As long as the holes are
significantly smaller than that
dimension (a few millimeters),
they won’t let in much of the
incident wave. This is why a fine
conductive mesh can be used when
constructing your homemade
Faraday cage. In actuality, the
cage’s lid or door are usually the
cause of most leakage. By taping the seams with conductive tape (e.g., aluminum tape) helps to reduce this leakage.
Can you use existing conductive enclosures?
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. The key criteria for them
is that the gaps and seams remain very small.
Should you ground the cage?
There is a great deal of debate regarding
the need for grounding a Faraday cage.
Grounding of the cage (i.e., connecting it to
some earth-based source of a charge) has
little effect on the field levels seen inside
the box. Grounding the cage primarily helps
to keep the cage from becoming charged
and possibly re-radiating. The bottom line is
that an ungrounded cage protects the
contents from harmful electromagnetic
fields as well as a grounded one.
Anti-static bags:
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.
How to Make Your Own Faraday Cage at Home
Larger Faraday Cages Inside a Structure:
Storing a larger set of electronics might require an entire room. We call these “shield rooms.”
because they do an excellent job of filtering out interfering signals, providing in excess of 100 dB of shielding. An easy way to create a Faraday shield room can be made inside a house simply by lining a small closet with a few layers of heavy-duty aluminum foil, covering all four walls, the floor, the ceiling, and the inside of the door. Make sure to overlap and tape the seams using either conductive or regular cellophane tape. There can be not be any conductive penetrations into the room, or it will seriously degrade the shielding and possibly not even work. Make sure to cover all electrical outlets, light switches, etc., with aluminum foil. Do not plug anything into the electrical outlets. Make sure to also lay a piece of plywood or cardboard on the floor so that it can be walked on without damaging the aluminum foil and also to store your items on as they cannot be allowed to touch the shielding directly. The majority of rooms built in this way have been shown to offer as much if not more than 50 dB of shielding up to several hundred MHz.
This is pretty technical on some issues but just remember that you can use almost anything to build a Faraday cage including a steel trash can with a very tight fitting lid. It does a pretty good job of acting as a Faraday cage. I have found through research that it varies greatly depending on how well the lid fits. One thing that really seems to help is to tape the seam of the trash can with conductive tape.
I have found the best way to test one is simply to put a cell phone in and try and call it, if it is working, then it shouldn't ring. Another way is to find a AM radio tune it to the strongest AM station you have around you and when the signal stops, it is working. Once it is actually working it does not matter the that much on the strength of the energy field so much, as long as the room or cage is protected from being damaged, and as long as the skin effect is maintained, your supplies should still be good.
