Microwave Oven Used As A Faraday Cage?

Reader Question:
Can an old microwave oven be used as a Faraday cage?

Maybe, to an extent, an old microwave oven may be re-purposed as a Faraday cage against EMP (electromagnetic pulse). In fact, its design is very much similar to a basic Faraday cage (with a probable caveat related to frequency / wavelength). 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 electromagnetic radiation of microwaves, and to keep it’s energy 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 electromagnetic 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 electromagnetic waves. 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 electromagnetic radiation. If the Faraday cage is grounded it will simply shed off any static charge.

The effectiveness of the ‘reflection’ properties of a Faraday cage depends upon:

– the wavelength of the electromagnetic radiation in question

– the diameter of the holes in the cage’s conductive material

– the conductivity of the material itself. Aluminum, or even steel window screen is “good enough” to prevent any significant electromagnetic radiation.

A purpose built Faraday cage may be made of copper screen. It doesn’t necessarily need to be a solid conductive material. As long as the holes in the screen are smaller than the wavelength of the frequencies that you are trying to protect against, a screen mesh 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 a Faraday cage 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 potentially (partially?) protect your electronic gadgets during an EMP, but perhaps only to an extent. A microwave oven is designed to protect at 2.45 GHz wavelength. Given that an EMP is broadband, there may be pulse radiation energy from other frequencies which may not be adequately attenuated to protect your devices if used as a Faraday cage. It’s all about dB attenuation (protection) versus frequency along with the strength of the source energy itself.

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  1. 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.

  2. 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?

    1. The microwave’s screen (what you see through in the door) is based on the frequency of the electromagnetic radiation inside. The higher the frequency, the smaller the holes in that screen must be. Microwaves operate at around 2.4 GHz. The current cell phone frequencies can go up to around 6 GHz, so the mesh would have to be smaller than what’s in a microwave door. That’s why the call went through. The new 5G system will operate at bands that go up to 24 GHz, so an oven is not a shield to those frequencies at all. However, the oven MAY prove useful in an emergency, because the EMP is very broadband – and if you’re several hundred or a couple thousand miles away from the burst, the power may possibly drop off enough by the time it got to you that only longer wavelengths will affect your area. It’s still better to use a solid conductive material, but an oven could work in a pinch. Best thing to put in a cage is a battery operated radio, any medical devices you need (i.e. insulin or BP monitors), rechargeable flashlights, etc. The cell phone system will likely be toast, but you could throw them in as well. Just don’t expect to get a call out. Even old land lines, which were, by their DESIGN, used to be EMP proof, use so much solid state equipment now that they likely will fail. Brush up on smoke signals and Morse Code. ;)

  3. 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.

    1. 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

  4. 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.

    1. 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.

      1. @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.

        1. Goo point.

          Given the relationship between frequency wavelength and shield mesh size, one would think that a product designed to hold in (attenuate) 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.

    2. Think about the magnetron as a doorway to the low-freq world…

      If I remember correctly, 1) there is no screen over the magnetron’s output port (duh!) and 2) there is high-voltage DC applied to the magnetron. That HV wire, rectifier and transformer probably passes through a ferrite bead (low-pass filter), to prevent microwave energy escaping. There is no need for a band-pass filter.

      AM band RF might just pass right through the choke, to the elements of the magnetron and into the “faraday cage”.

      My guess anyway.

      Another thought: if the maximum energy is emitted in the 3Hz to 30kHz range, then what’s to prevent the HV transformer from coupling to the EMP energy and firing-up the magnetron, even very briefly?… (e.g. just long enough to ruin radio equipment) Kids, do not test this at home

  5. 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.

  6. 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!

    1. The electric service is grounded (the correct term is ‘bonded’ but that’s an hour explanation) to the water system so the two are at the same potential plane.

  7. 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.

  8. Interesting. I think there may be differences in old and new microwave designs. I had read an article by an RF engineer a few years back that mentioned microwave ovens, if disconnected from the wall, are excellent Faraday shielded devices and thus good EMP protective devices. Maybe in the old days of microwave ovens this was true, but not so much today.

    As an experiment, I tried the good old cell phone test. Inserted in my wall mounted, plugged in microwave oven, I could call it and it rang. Ooops. Then I just wrapped the cell phone in kitchen aluminum foil, and voila, no more cell phone reception.

    I suspect what has happened is that the microwave manufacturers have cheapened their units, and provide just enough RF protection to keep your eyeballs from being fried. Sad, but that’s what happens in a declining economy like ours.

    1. I’ve been fixing microwaves for non profit recycling purposes, and it’s true, later, cheaper models are made out of much thinner and and cheaper materials. That being said, microwave ovens design parameters allow some EM radiation to pass through, in a very low amounts. When you measure older models (let’s say 80’s and before) they actually tend to leak a lot more around the door, which is really the only place you ever will get results in normal units, while new ones tend to have less leak than what our meter shows up.

      In any case, I think that generally acceptable levels are 5mW/cm2 when measured 5cm from the surface of the oven. These are way under any harmful amounts to human beings. Our meter starts from 1mW/cm2 (it’s a digital meter) and 99.9% of the after 90’s models do not register at all on the meter, but almost every older model does allow something like 5 – 10mW around the door.

      That being said, what it does mean against EMP, I have no clue.

  9. Place your cell phone in a microwave oven and call it. It will receive the call. A microwave oven is not an effective Faraday cage.

  10. What good would your cellphone be unless you could also fit a cell phone tower in your microwave.

    1. Don, a microwave oven is not generally a very good Faraday cage for EMP. Additionally you are correct in that if you are trying to protect a cell phone from EMP, it’s likely not worth the effort since the transmitters and electronic infrastructure will be likely be zapped…

    2. We are missing the point here. The reason for cellphone-in-microwave is to test effectiveness of the unit in not passing rf energy.

    3. I use my cell as a mini computer with 250gb memory. I store my most valuable data there. If I can protect this, I will always have my data.

      1. Buzzy,
        Yes, I do the same with my cell.
        The problem with that is, I get a new phone darn near every year.
        Broken screens, no recharge mode. And then I can no longer retrieve my stored info….
        and hadn’t taken the time to hand write all that important info.
        And forgotten passwords to different accounts. I’m still battling with a new phone and lost info.


        1. Joe c – Both Android and Apple phones have many options to backup the entire contents of your phone to online storage. If that makes you nervous, you can do the same thing with a little flash drive and keep it locked up. Passwords… perhaps the single biggest irritant in my life, just behind sanctimonious liberals. You can fix this forever with an app call “Last Pass”. Don’t even screw around with the ‘free’ version. The full access app is only a few dollars. You set it up once, lay it on your phone and every PC you use – and you don’t have to deal with passwords anymore.

  11. Wow! This is so cool!
    Back to the ubiquitous metal filing cabinet. I’m not up on the science and watts and hurtz, so would you line the filing cabinet with cardboard? Is it the reverse of a box lined with foil?

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