Can Microwaves be used as Faraday Cages? Date: Apr 28, 2009 1:27:48 PM PDT Author: wilpatsmi Could old microwave ovens be used as Faraday cages in which to store small to medium size electronic equipment? Last modified by wilpatsmi on Apr 28 2009 1:28PM   Reply to this Post

Microwaves ovens as Faraday Cages. Date: Apr 28, 2009 7:03:23 PM PDT Author: jerrye A microwave oven can be an excellent faraday cage. Ideally, though, the entire power cord should be cut off, both to prevent the microwave oven from being accidentally turned on with electronics inside and to prevent the power cord from acting as an antenna to pick up EMP. One good test to see if you've got a good faraday cage is to tune a portable radio to a nearby strong FM station, turn the volume all the way up, and see if you can hear the station when it's inside the microwave oven (or other faraday cage). If you only hear hissing or nothing at all, then you've probably suppressed the voltage resulting from the electromagnetic field getting to the radio by a factor of at least a thousand. Before you actually store the portable equipment away for a long period of time, be sure to remove the batteries.   Reply to this Post

Faraday cage design and testing Date: Apr 28, 2009 9:37:31 PM PDT Author: peterg Well, almost. There are two modes of damage from EMP: damage caused by the spike in the electric field (the EMP itself), and damage caused by electrical pulses induced in wires (power wires, phone wires, etc.). You could turn a room into a Faraday cage, which would absolutely protect any radio within it from the EMP itself. If you brought an extension cord into the room, however, radios still wouldn't work, but any radio in the room would still be vulnerable to the second kind of EMP damage. Also, the lack of radio reception isn't really a proof that an enclosure is safe against EMP. The energy levels of an EMP are huge by comparison. An enclosure may attenuate RF enough to prevent radio reception, while not attenuating EMP enough to protect a sensitive device inside. So... the "radio test" isn't really a good test. The fact that wires can bring EMP-induced electrical pulses into an enclosure is why I don't recommend grounding any enclosure protecting electronics against EMP, as commonly advised. The ground wire probably increases the risk of damage. I think all anyone needs for EMP protection is a simple metal box-- a metal suitcase, an ammo can, or something home- made, as long as it seals up reasonably well. Real sheet metal is best. Steel, aluminum, and copper are all good. I don't think foil is ideal, but it's probably adequate for most electronic devices, especially since most electronic devices are only slightly susceptible to EMP anyway.   Reply to this Post

faraday cage construction Date: Jun 1, 2009 11:29:06 PM PDT Author: opsec I have a friend who used to work at Underwriter's Laboratory and she used to to do the tests for electromagnetic interferance on electronics. She knows about EMP. According to her, an EMP is very high in energy and frequency. The only material that can attenuate an EMP is a ferrous material. That is to say, something made of iron or steel. Copper, aluminum, and other conductive materials won't suffice. The outer hull of the ferrous container must be completely sealed. An ammo can won't work because the paint is an insulator and the high frequencies of an EMP can get into the can. The ammo can would block any frequencies of which the wavelenth is larger than the gap created by the paint between the lid and the box but it would be penetrated by the frequencies that have a wavelength smaller than the gap produced left between the lid and box. That's all it takes to fry everything inside. She also said that aluminum foil does work to make an electrical gasket. So the solution is to take an ammo can and sand off the paint on the engagement surfaces bewteen the lid and box and make an aluminum foil gasket so that there are no gaps left. Insulate the inside of the ammo can with some nonconducting material like foam rubber to keep the electronics from touching the walls of the ammo can. Keep the can itself on top of something nonconductive like a cinder block or a dry wood board and don't let anything touch the exterior so the box is electrically isolated. After the EMP hits, the outside of the box will have a static electric charge built up on it. After the EMP is past, you then ground the outside of the box against a water pipe or some other kind of ground to bleed off the static charge. Then it is safe to open the can and recover the contents. If anybody has any other designs for a faraday cage, I'd very much like to hear them. I hope this was informative. Last modified by opsec on Jun 1 2009 11:32PM   Reply to this Post

Faraday cage construction Date: Jun 2, 2009 4:34:28 AM PDT Author: jerrye Underwriters Laboratories is mostly concerned with EMI (electromagnetic interference), and most EMI in consumer equipment has historically consisted of the radiated magnetic fields from transformers. Transformers radiate both electric and magnetic fields, but the magnetic field predominates. Ferrous materials are used for magnetic shielding, but they are not so good for high-frequency electric fields. In nuclear EMP, the electrical field predominates. This is shown by the fact that during actual nuclear EMP events in 1962 over the Pacific and in the Soviet tests in Kazakhstan, there was a lot of electrical damage, but no magnetic tapes (which were widely used then) were damaged or erased. (For electrical engineers, this is shown by the 377 ohm impedance of free space, where an intense 50 KV/m electric field transient produces a much lower magnetic field component.) Different materials are most effective for nuclear EMP vs. low frequency consumer-equipment generated EMI. Ferrous materials are usually used for shielding the radiation from transformers, but they are less effective for high-frequency electromagnetic radiation where electric fields are the primary concern. Military buildings shielded against EMP generally rely on copper for the principal shielding material against the high-frequency electric field component of the EMP. Unless a person is an electrical engineer with a lot of experience in high-frequency shielding, they are likely to make basic mistakes in trying to construct their own shielded enclosure for nuclear EMP. On the other hand, a professionally-designed faraday cage that can be conveniently opened and closed is readily available in the form of the chamber of a microwave oven. The chamber of a microwave oven is shielded against both magnetic radiation from the transformer and high-frequency electromagnetic radiation from the strong microwave field. It is proven to work, as is evidenced from the fact that a person right outside an operating microwave oven isn't cooked, or even slightly warmed up. Microwave ovens have been around for so long that you can use an old one that you already own or buy one on the used market. Just remember to cut off the power cord. Last modified by jerrye on Jun 2 2009 4:42AM   Reply to this Post

faraday cage construction Date: Jun 2, 2009 7:36:32 AM PDT Author: jerrye Originally Posted by opsec: The only material that can attenuate an EMP is a ferrous material. That is to say, something made of iron or steel. Copper, aluminum, and other conductive materials won't suffice. At the risk of beating a dead horse, the above statement is so extremely wrong that I want to make sure that it doesn't propagate across the internet like so many misconceptions about EMP do. Most of the nuclear electromagnetic pulse contains a broad spectrum of frequency components that are similar to those of broadcast radio and television. The cable that brings the signal into your television is made of non-ferrous metals that are mostly copper. If the statement quoted above was correct, coaxial cables wouldn't work because all of the signal would be radiated away from the cable. For the same reason, the coaxial transmission line that carries a broadcast signal from a broadcast transmitter to the antenna on top of the tower is made completely out of non-ferrous metal, with nearly all of both the inner conductor and the outer shield being made of copper. On many occasions, I've personally measured the electromagnetic field just outside of these copper broadcast transmission lines carrying many kilowatts of high-frequency electromagnetic energy. Like engineers for decades before me, I've found that the copper makes a very effective electromagnetic shield. Last modified by jerrye on Jun 2 2009 7:40AM   Reply to this Post