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Faraday Cage & Microwave Mesh

  1. Feb 9, 2016 #1
    Hi pf,

    Please could someone provide an explanation of why the relationship between the wavelength and size of the gap is the significant factor for determining whether or not it blocks radiation in particular reference to the mesh on the door of a microwave oven. It seems very simple when a book says "the wavelength is much bigger than the gap" as it paints in your mind the idea of something that is trying to fit through a hole that is too small but after thinking about it if this was the case then surely the AMPLITUDE of the wave would determine whether it will fit through or not. Look at this diagram:

    upload_2016-2-9_22-10-47.png
    The circle is supposed to represent a single mesh hole from a microwave oven door. I really don't see how the wavelength could influence whether or not it would go through since this will only affect when the next peak arrives? I fear this analogy and picture is too simplistic but when someone tells you the hole is much smaller than the wavelength then surely this is the picture in most peoples minds. I am guessing the explanation as to why the wavelength is important is complicated but I would very much appreciate someone who could provide an answer.

    Thank you
     
  2. jcsd
  3. Feb 9, 2016 #2

    davenn

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    Hi Jimmy

    that's not really the way it works
    for a start, this quote from Wiki ....

    now secondly, we have to get into the theory of something called slot antennas and how the slot in a wave guide will radiate a RF signal from inside the waveguide to the outside. and this is where the hole ( slot ) size comes into play. Now I'm not an expert in this stuff, but I can give you the basics and hopefully enough for you to see what is happening :smile:

    The holes/slots act as a high pass RF filter ... so that no frequency will be reradiated from the hole below a certain frequency.
    The holes/ slots will radiate RF at a maximum when they approach one 1/2 wavelength in diameter/length
    This is where the hole/slot is a resonant size at the frequency being dealt with.
    At hole/slot sizes significantly smaller than the resonant frequency and the high pass filter effect will occur.
    At hole/slot sizes significantly larger than the resonant frequency, the RF will just leak through the hole

    just as an example ... here's a slotted waveguide for 2.4 GHz ( same freq as a microwave oven) see the size of the slots
    these are an electrical 1/2 wavelength long, compared to the tiny holes on a oven mesh

    Slottedwaveguide.jpg


    some links on slotted radiators and how they work

    http://www.radartutorial.eu/06.antennas/Slot Antenna.en.html



    will try and find more info is that isn't enough to get you started

    Dave
     
    Last edited: Feb 10, 2016
  4. Feb 9, 2016 #3

    marcusl

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    There's a simpler explanation, which is this: The amplitude of an electromagnetic wave has nothing to do with physical size, only with the intensity of the electric field. You might be picturing a transverse mechanical wave, like a vibrating string, where amplitude corresponds to physical displacement, but that picture is irrelevant to electromagnetics. The only characteristic size that an EM wave has is its wavelength.
     
  5. Feb 10, 2016 #4

    davenn

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    tho you didn't say anything incorrect ... unfortunately, it doesn't offer an explanation :wink:


    Dave
     
  6. Feb 10, 2016 #5
    Thanks. I did some more research on google and found a PowerPoint lecture from the San Diego Physics Department which I have attached. It seems to tie in with the wiki article you quoted if you look at slides 5-7. I want to check I understand what they are saying. They seem to be saying that the electric field of the EM wave causes the charges in the mesh to accelerate up and down which generates an electromagnetic wave back (i.e. reflection like a mechanical rope with a freely moving loop at one end). If the hole is too big then the induced currents do not have enough time to re-distribute themselves and cancel/reflect the incoming wave and therefore it goes through. If you wave the wavelength bigger there is more time for the electrons to re-distribute therefore the wave can be cancelled. Is that right?

    Sorry couldn't attach so here is the link:

    physics.ucsd.edu/~tmurphy/phys8/lectures/14_microwave.ppt
     
  7. Feb 10, 2016 #6

    davenn

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    Interesting, particularly page 7
    Haven't heard it described like that before. That gives a good layperson understanding of what is going on.
    The real deep physics of what is happening around the holes/slots in the metal is something that is out of my league
    for trying to describe :wink:
     
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