Microwave Oven Used As A Faraday Cage?
Reader Question:
Can an old microwave oven be used as a Faraday cage?
Answer:
YES, an old microwave is perfect for use as a Faraday cage against EMP (electro magnetic pulse). In fact, it is a Faraday cage! A Faraday cage is an enclosure formed by conducting material or by a mesh of such material. A microwave oven’s very design is to enclose the electro-magnetic radiation of microwaves, and keep them from getting out. The reverse will also be true – they can’t get in.
Think of a Faraday cage as a reflector. A reflector of electro-magnetic waves. It reflects waves on the outside from getting in and waves on the inside from getting out.
A Faraday cage by its very definition does not have to be grounded to reflect or keep out electro-magnetic waves (they normally are not grounded). From inside the cage, it makes no difference if the conductive shell is grounded or not. The inside ‘doesn’t know’ about the outside with regards to electro magnetic radiation.
The effectiveness of the ‘reflection’ properties of a Faraday cage depends upon the wavelength of the electro-magnetic radiation in question, the diameter of the holes in the cage’s conductive material, and the conductivity of the material itself. Aluminum, or even steel window screen is “good enough” to prevent any significant electro-magnetic radiation.
Most purpose built “Faraday cages” that you buy are made out of copper screen instead of solid metal. As long as the holes in the screen are smaller than the wavelength of the frequencies you are trying to protect against, screen works just as well as a solid piece of metal.
An EMP is a broadband, high-intensity, short-duration burst of electromagnetic energy. In the case of a nuclear detonation, the electromagnetic pulse consists of a continuous frequency spectrum. Most of the energy is distributed throughout the lower frequencies between 3 Hz and 30 kHz. However the first effects of nuclear detonation are the very-high-frequency pulses, in the microwave range, and can work their way into Faraday cages if there are cracks, seams, or vents.
The frequency of a microwave oven is 2.45 GHz (gigahertz) and has a wavelength of 4.82 inches. Since the holes of the screen mesh of a microwave oven are small compared to the wavelength of the microwave itself, little radiation can leak out. There are also mesh screens on the sides of the oven cavity, one to protect the oven light while allowing it to shine into the cavity, the other to permit ventilation.
A microwave will indeed protect your electronic gadgets during an EMP, so long as you don’t press ‘START’…
We would like to involve you, the Reader, more!
So, we’re asking you to chime in with your questions, any questions, ideally relating to the general topics covered in this blog or anything to do with emergency preparedness; survival. From time to time we will post these questions and answers, while you, the questioner, will remain anonymous.
Email questions to the following and type “Reader Questions” into the subject line.
If you enjoyed this or topics of emergency preparedness, or are planning for disaster,
Read our current articles on Modern Survival Blog
If a microwave oven is used as an EMP shield (Faraday cage),I would suggest that the cord be cut and removed to prevent it from becoming an antenna.
Not a bad idea, but in theory, not necessary given the existing shielding of the chamber itself.
I recently received a piece of junk mail trying to sell me a book. One of the interesting facts they presented in the book is that a microwave can be tested to see if it leaks by putting your cell phone in it, closing the door, and calling your cell phone. It said that if the call went through, the microwave leaked. Knowing a microwave is supposed to be a Faraday cage, I thought I would test it. I tried to call my cell phone in four different microwaves at work (one of them brand new). The call went through for all four microwaves. If a call can go through, is it an effective Faraday cage, or is that comparing apples to oranges?
I’ve researched that cell-phone/microwave oven test on the internet, and can find no solid conclusion to its validity as a true test that means anything significant. I do know that cell phones operate at lower frequencies, however I would assume that a microwave shielding would also block lower frequencies by default – since the microwave frequencies themselves are higher and require smaller holes in the screen. My best opinion is that knowing there is a defined amount of ‘acceptable’ leakage of a microwave oven, and knowing that cell phones are designed to be able to operate at extremely low signal levels, that this is enabling some cell phones to receive signal, tiny as it may be…
Bought some aluminum attic radiant barrier foil a few years back, it’s like heavy duty aluminum foil with threads inside for strength. Tough to tear the stuff. After lining the attic, used the leftover to double line cardboard boxes as Faraday cages. Outer cardboard box, aluminum foil cage, inner cardboard box, aluminum foil cage, electronics in paper bags. The boxes sit in plain view in the basement without attracting any attention from guests. Not as easy to open as a microwave, but we needed something to hold more and larger items.
Jonathan, that is brilliant. Great idea.
Playing devils aavcodte here: is the lack of cell signal proof of positive EMP protection? Wouldn’t an EMP be stronger than a cell signal? I certainly don’t have the answer, but I’m curious. I read One Second After for the first time only a few weeks ago and have been much more interested in this topic since then. I highly encourage anyone to read that book if this is a topic in which they are interested. Thanks,Robert
I regret to inform you that a microwave oven is only partially effective as a Faraday cage, something you can test for yourself. Since EMP is nothing more than a high energy RF pulse, any Faraday cage which will shield you from EMP will also shield you from RF. The article states that most of the energy will occur below 30 KHz and while I suspect this frequency may depend on the actual type of EMP involved, this is still close enough to the AM band to use an AM radio for testing. Simply tune the radio to the strongest local station and put it in whatever you are testing. If you can still hear the station, you probably want more shielding. I just did this with my microwave and while the signal did get rather noisy, it could still be heard, even with the door closed. I am an RF Engineer and was rather surprised at this since I knew that a microwave oven was really just a radio transmitter and it can’t leak much, but apparently the shielding must be less effective at frequencies other than 2.4 GHz. Ovens and fridges actually appear to be more effective than a microwave. Most effective, good old aluminum foil and ammo cans, even with the rubber seal, although you do need to make sure that any metal like an antenna does not touch the foil or the steel of the can.
Thanks for doing the AM Radio test – very interesting. I am also somewhat surprised that the microwave shielding seems to be apparently somewhat ‘tuned’ or restricted? to 2.4 GHz while evidently allowing ‘some’ lower frequencies to get in. It would be interesting to know the attenuation (dB) versus frequency of their shielding. This would stand to reason why some of the cell phone tests are proving that lower freq. signals are getting in. I would think that it would cost more to develop/manufacture shielding like that.
In any event, there are better methods of do-it-yourself Faraday cage design, as you mention. Thanks for the comment.
@Ken.
I wouldn’t have expected the shielding on a microwave oven to do anything but protect the user from the oven. Usually. Increasing the capabilities of pretty much anything costs money. If you want a Faraday cage, build one, it’s not rocket science.
Be well.
@Mortimer, Sure, Faraday cages for simple protection are pretty simple. While discussing the context of the microwave oven and the curious test results of other frequencies… Given the relationship between frequency wavelength and shield mesh size, one would think that a product designed to hold in 2.4 GHz would also hold in lower frequencies, which in turn are longer wavelengths. The microwave oven does apparently restrict these frequencies, but the attenuation itself in dB is unclear. It’s almost like saying that a box designed to hold the weight of 100 pounds, will somehow NOT hold the weight of 20 pounds. Once you understand the relationship between frequency and wavelength, one raises an eyebrow of curiosity to this phenomenon of microwave oven shielding. Perhaps ‘A-nerd’ the RF Engineer will offer an opinion on this. The ‘Product Manager’ in me, mixed with my electronics background is curious…
Have to admit, this has been bugging me. What I found is that supposedly, a microwave is permitted to radiate up 0.25 Watts of power. The actual FCC regulation is probably in terms of milli-Watts per centimeter squared, but then I would have to figure someway to sum up the total radiation and the 1/4 Watt seems like a reasonable number. For 1000 Watt microwave, this would require an attenuation of 37 dB to to reduce it to 1/4 Watt. We all know that commercial companies don’t put anything more than what they have to for fear of reducing their profit, so it would reasonable to assume microwave ovens are probably between 40 and 50 dB of shielding. Its not bad, but it just isn’t enough to fulling attenuate the AM station to the point that it is inaudible.
Go to any old appliance store and buy old single door refrigerator or freezer, , take off the rubber gasket, so that the door metal contacts the main shell, then line the inner walls completely with cardboard or styrofoam, place your electronic on cardboard covered shelves, you can easily store radio’s ,lap tops,etc. use the electric cord to ground to a water pipe, do not ground it to house electrical ground . $50.00 dollar faraday cage!
Don’t overlook the ubiquitous metal file cabinet. Put your spare electronics in a metal file cabinet…your spare laptop, your spare handi-talki, etc.