Thoughts Regarding Faraday Cage Techniques For An EMP
As many of you know, an EMP (electromagnetic pulse) could end life as we know it in approximately one hundredth of a microsecond. Faster than the blink of an eye. While some may argue the likelihood of such an event, lets focus instead on the methods one might consider to protect their electronic equipment in a Faraday cage.
But first, briefly, an EMP in the context of this article is a pulse of considerable electromagnetic energy (with a wide range of frequencies and amplitudes) resulting from a nuclear detonation at (high) altitude in the atmosphere. As the pulse travels away from the burst point at the speed of light, the radiation can be ‘collected’ by metallic and other conductors at a distance. The energy of the radiation can then be converted into strong electric currents and high voltages. With sufficient energy, particularly from the high-frequency components of the EMP, electrical and electronic equipment connected to (or associated with) the collector may suffer severe damage from a strong current and voltage surge.
The way that the electromagnetic energy is collected from the EMP is complex, because much depends on the size and shape of the collector, its orientation with respect to the source of the pulse, and the frequency spectrum of the pulse. As a general rule, the amount of energy collected increases with the dimensions of the conductor which serves as the collector (or antenna).
Long runs of cable, piping, or conduit
Large antennas, antenna feed cables, guy wires, antenna support towers
Overhead power, telephone, and cable TV lines
Long runs of electrical wiring, conduit, etc.. in buildings
Metallic structural components (girders), reinforcing bars, corrugated roof
expanded metal lath, metallic fencing
One thing we can do to protect electronic devices is to use a Faraday cage.
What is a Faraday cage? It is an enclosure (shield) formed by conductive material or by a mesh of such material. Such an enclosure blocks external electric fields by channeling electricity along and around, but not through, the mesh, providing constant voltage on all sides of the enclosure. Since the difference in voltage is the measure of electrical potential, no current flows through the space. A Faraday cage operates because an external electrical field causes the electric charges within the cage’s conducting material to be distributed such that they cancel the field’s effect in the cage’s interior. Faraday cages are named after the scientist Michael Faraday, who invented them in 1836.
An ideal Faraday cage is a conductive metal box (on all sides). Contrary to what many believe, the Faraday cage does not have to be grounded to do its job. If you have trouble with that statement, do some research (web-search) and you will discover the physics. The Faraday cage keeps the charge (EMP) on the outside (during that fraction of a microsecond till its gone) while a grounded Faraday cage will simply bleed the charge to ground during that same fractional microsecond. In either case, the electronics inside are protected (to the extent of the construction properties of the cage itself).
Note: Faraday cages are intentionally grounded when the intended use is to operate an electronic device inside (e.g. a Faraday enclosed room) while the electronic device itself requires a proper ground (and proper isolation techniques from the incoming power source).
With that said, a covered metal trash can will act as a Faraday cage.
Note: Any conducting surface of the electronic devices that you wish to protect cannot be touching the metal of the Faraday cage. An easy solution is to place a non-conductive material such as cardboard on the inside between the devices and the cage itself. Or you may simply wrap the electronic devices in a non-conductive material and place inside…
Metal screening material will serve as a building material for a Faraday cage. For example you might choose to build a box (or larger) while using wood materials. You might then wrap all sides with conductive metal screening material to achieve the Faraday effect.
You might be wondering about the limitation of the hole size in the screen material as it relates to its effectiveness at stopping an EMP. A rule of thumb is that the hole diameter must be less than 1/2 the wavelength of the radiation frequency to adequately attenuate (sufficient dB) so as not to damage the electronics inside the Faraday cage.
The EMP radiation frequencies are very wide ranging, however research papers indicate that it is the very high frequencies that are of concern (apparently up into the microwave region) for electronic device damage. If one uses the frequency of a microwave oven (2.4 GHz) for example, the wavelength is just under 5 inches. Half that is 2.5 inches. The holes in typical screening material are even much less than that (resulting in an increased attenuation at those frequencies).
Excerpt from QST August 1986, “EMP and the Radio Amateur” and condensed from NCS TIB 85-10, “The electric field strength of an EMP remains fairly constant in the 10 kHz to 1 MHz band and it decreases by a factor of 100 in the 1 to 100 MHz band and continues to decrease at a faster rate for frequencies greater than 100 MHz”.
So, it appears that EMP field strength decreases by at least an order of magnitude for each decade of frequency above 1 MHz.
With all that said, it appears to me that ordinary screen mesh size is more than quite adequate for protection, while solid metal is obviously the best.
Note: The lower frequencies of an EMP are more easily ‘collected’ by long ‘collectors’ like power lines, etc…
Will a shipping container make a good Faraday cage? I happen to have a 40-foot container nestled behind some trees on the property which was here when I bought it last year — and I had presumed that it was ideal for EMP. The problem is that while all sides and the ceiling are heavy steel, and while the steel front doors are electrically ‘hinged’ to the rest of the container (even though the door closes onto rubber gasket material), the bottom (underneath the wood floor) is not solid steel. Instead it is made up of steel cross-member beams. Whoops… An EMP could theoretically enter through the floor. It was nearly impossible to see underneath to verify this – but I managed. Theoretically one would have to line the floor with metal and electrically connect it (or weld) to the existing structure.
Will an old microwave oven make for a good Faraday cage? Yes, although I would suggest to cut the power cord. I’m not sure of the attenuation specs (dB) in that frequency range, but I’m sure it’s ‘just enough’ given today’s engineering to maximize profit 😉
Will a metal ammo box be good Faraday cage? If it’s all metal, and if the metal cover is hinged via metal pins (electrical conductivity), then yes. If in doubt (regarding the conductivity of the hinges) then remove the rubber gasket if it exists. If its painted and you’re removing the rubber gasket, be sure to scrape the contacting surfaces of paint.
Will a galvanized steel trash pail make a good Faraday cage? Yes! Seems like a very simple and effective method.
Will a metal gun cabinet make a good Faraday cage? Yes it should, so long as the door is conductivity hinged with the rest of the metal frame (which it should be via the metal pins and hinge assembly). Additionally, most gun cabinets are felt-lined which eliminates the problem of isolating a electronic device from the interior metal walls.
Okay, lets hear from you and your own ideas on this subject…
(Some data sourced from “The Effects Of Nuclear Weapons” by the Dept. of Defense)