Thursday, February 28, 2013
Albert Einstein, when asked to describe radio, replied: "You see, wire telegraph is a kind of a very, very long cat.
You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there.
The only difference is that there is no cat."
Saturday, February 23, 2013
There is a great deal of confusion about Faraday cages. Not only about how to build them, but also what they actually protect against. In this article, I will answer a few basic questions and perhaps debunk 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).
|Figure 1: Building a homemade Faraday cage: (a) gather tape, box, and aluminum foil, |
(b) cover box and lid completely with foil, (c) line box with cardboard and store items, and
(d) close Faraday cage
What does it do?
The cage shields the contents from both electrostatic fields (i.e., fields that don’t change over time) and non-electrostatic fields (i.e., fields that do change over time). It is particularly useful for protecting against an electromagnetic pulse that may be the result of a 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 emissions because the frequency content of such disturbances is at much lower frequencies—they don’t couple enough energy into small-scale electronics. Solar emissions do however disrupt radio transmissions, damage satellites, and like an EMP attack, can potentially destroy the electrical power grid.
How does the cage work?
The free carriers in the conductive material rapidly realign themselves 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 conducting layer be?
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 optimal shielding. 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. 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 what type of conductor is used?
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) were used in place of aluminum, the skin depth at 200 MHz would be 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?
Yes, as long as the holes are small with respect to the wavelength of the incident electromagnetic wave. For example, a one 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 also be used for making a Faraday cage. In practice, the lid or door usually causes the most leakage. Taping the seam with conductive tape greatly reduces 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 as covered in the book, Disaster Preparedness for EMP Attacks and Solar Storms.
Does the cage have 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.
Written by Dr. Arthur Bradley, author of the Handbook to Practical Disaster Preparedness for the Family, the Instruction Manual, and Disaster Preparedness for EMP Attacks and Solar Storms. To sign up for his free “Practical ” newsletter, send an email to firstname.lastname@example.org.