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Microwave Faraday Cage

  1. Nov 9, 2012 #1
    So I tried out a small, fun experiment of putting my cell phone in the microwave (not turning it on, of course) to see if the WIFI Hotspot capabilities were hindered by the Faraday cage that insulates the microwaves. I had heard that microwave cages will hinder If not completely block 802.11 frequencies, but I was still able to pick it up through my notebook and access the Internet. Has anyone else tried this?
     
  2. jcsd
  3. Nov 9, 2012 #2
    Where did you hear that Faraday cages completely block EM? Have you researched this at all on your own?
     
  4. Nov 9, 2012 #3
    It turns out that hinder is true and completely block is not.

    What you could probably demonstrate is that the microwave reduces the range at which your cellphone is able to get 802.11 reception.
     
  5. Nov 9, 2012 #4
    I am surprised that it does not block the cell phone. In theory it should. At 2.4GHz, EM wave penetration into the metal is very shallow due to skin effect. Also the holes in the front panel is way smaller than the λ of 2.4GHz and that should block the EM wave.

    Case in point, some cell phone and GPS don't work as well in the more expensive cars that use glass with very thin layer of metal deposit that is used to reduce heat in the car.

    I do make it a point not to stand in front of the microwave oven while it is on though.
     
  6. Nov 9, 2012 #5
    I just did an experiment using my wife's cell phone in the oven!!! It sure did not ring, It is not even detected as the voice message came on the first ring. Then I took the phone out......uncooked, and it sure ring right away.
    I don't know microwave oven, is the EM wave from the oven polarized? If so, they might be able to play with the polarization so they can allow large cracks at the oven/door interface and still block the EM wave from leaking out. But if other source that is not polarize, there will be leaks. I don't know.

    Also EM wave is very directional( say from right to left horizontally inside of the oven). I examined my oven, the inner box is all metal except one 3"X4" on the right side that is cover with plastic, I bet that is where the EM source come out. So the majority of the EM is from right to left horizontal to the door. Maybe the oven is designed so the EM from the source is contain very well in that one direction, but putting an RF source that emits omni directional inside the oven will leak out.

    I don't know, but theory is simple, it is govern by skin effect and λ as my original post. From experience, the microwave oven is a very leaky cage at the door/body interface.
     
    Last edited: Nov 9, 2012
  7. Nov 9, 2012 #6
    Of course your wife's phone wasn't receiving the signal directly from your phone but from a cell tower. The signal from the cell tower may be quite weak.
     
  8. Nov 9, 2012 #7
    Exactly, that maybe the reason the 802 works because it is local, you stand next to it and transmit, you get through.

    I forgot to mention, regarding to the metal film tinted window, my stupid GPS never acquire a signal if I have it inside the car. My friend told me to stick it out the sunroof when I first turn it on in order to acquire the signal first. Then I can leave it in the car and it will track. But every time I turn it off, I have to do that all over again.
     
  9. Nov 16, 2012 #8

    First, there is a common misconception that a microwave oven is a Faraday cage. It is not. Not even a close approximation. Go in your kitchen and open the door of your microwave. Look at the mating surfaces between the door and oven. There is no bare conductive metal at all, usually power coated sheet metal or plastic on both surfaces. It is essentially *wide open* from an RF perspective except right at 2.4GHz.

    Attenuation at 2.4GHz is achieved through the use of metal features inside the door that make the slot between the door and the oven look like a quarter wave reactive choke. 2.4GHz is essentially reflected back into the oven from the door/oven seam. This is why the door is so thick.

    The amount of attenuation at 2.4GHz is chosen to address human safety limits for RF exposure, with no consideration for RF communications. It is generally 30-40dB,

    At 30dB, 600W on the inside, enough to cook your chicken, becomes 600mW on the outside, about the same amount your cell phone radiates.

    This is a *lot* of attenuation when viewed from a human safety perspective, but only a modest amount of loss from an RF communications perspective. RF link budgets for WiFi can be > 100dB.
     
  10. Nov 16, 2012 #9
    What is the frequency of the Microwave oven?

    I also think the transmitter of the microwave oven is directional that it goes from right to left, so you can have better isolation even with the leaky cage. But 802.11 is omni directional, so it can go through the body/door crack. What do you think?

    But the high dynamic range you mentioned make sense also.
     
  11. Nov 17, 2012 #10
    Microwave ovens are assigned 2.45GHz +/- 50MHz.

    Perhaps, but I would think that RF at this frequency in this small, multipath rich, cavity would form a complex set of standing waves dependent, among other things, on what is cooking.
    I would think it would be difficult to characterize the RF as traveling in a particular direction within the oven. On the other hand I am sure that the engineers who actually design these ovens are aware of many nuances (perhaps we will get lucky and one will chime in).
     
  12. Nov 17, 2012 #11
    Where did the cable of the Wi-Fi device pass through?
    - The cable itself brings RF in the cage
    - Maybe you left something open to let the cable pass.

    In microwaves, the residual openings define the quality of a cage.
     
  13. Nov 17, 2012 #12

    sophiecentaur

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    There a plenty of Wi Fi devices that are cordless. Ipod for a start.
     
  14. Nov 17, 2012 #13
    That's a very good point, I am not familiar in cavity, I forgot about the wall of the cage being perfect conductor ( so to speak) and EM wave bounce off and get very complex. But I look at the inside of my microwave oven, looks like the opposite wall from the transmitter is concave shape. Might that be acting like a concave mirror that focus the reflection back to the center of the cage ( where the food is)?

    But I would imagine leakage from the body/door crack would be very bad. If you stand facing the crack, it can be dangerous. Yes they have a mesh on the front of the door that the opening is less than 1/4 λ, but the crack along the door is long, way more than quarter wave.
     
    Last edited: Nov 17, 2012
  15. Nov 17, 2012 #14

    sophiecentaur

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    Actually, afaik, the standing waves are pretty stable and don't shift about. The oven cavity is not many wavelengths, actually. Some ovens use 'node stirrers', which are rotating vanes in the roof to try to stir the cold spots around by modifying the standing waves. It means you can put bigger dishes and longer objects in the oven than if you were using a turntable.

    The door gap is a pretty good seal, actually. It works by a smart method which involves quarter wave slots, which acts as a transmission line 'transformer', which 'puts' a very good open circuit across the door, independent of the actual resistance of the door gap. So currents cannot flow to the outside of the case. The door gap itself is not a quarter wave and the transformer only works over a narrow frequency range (which the magnetron uses).
     
  16. Nov 17, 2012 #15
    What is afaik?!!

    I don't know slotted stuff, how is the quarter wave slot works? I was hoping to skip the cavity, resonance, waveguide and slotted stuff, I guess I have to put it back on my to study list!!!
     
  17. Nov 17, 2012 #16
    afaik means "as far as I know". I have attached illustration of quarter wave choke within microwave oven, and an equivalent circuit (bottom of post). This is just one particular implementation, there are numerous patents on this.

    Exposure requirement (U.S.) is 1mW/cm^2 measured at any point 5cm from oven when purchased, and 5mW/cm^2 at 5cm distance at any time after that. So,yes, putting your face right up against the crack of the door is putting yourself in untested territory as far as exposure. At normal distances it is deemed to be safe.
     

    Attached Files:

  18. Nov 17, 2012 #17
    Thanks, interesting.
     
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