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Jul 26th, 2011
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  1. I read through your EMF shielding tutorial, but am not familiar with
  2. the theory enough to apply it to a particular question. [1] It has
  3. often been said that putting a cellular phone inside a microwave,
  4. closing the door, and seeing if it will ring when called is a good way
  5. to test the integrity of the microwave's shielding/seals. I've been
  6. trying to understand the theory of how shields work in order to
  7. determine if this is true.
  9. - Cell phone seem to operate between 900-2000 MHz (.33m for 900 MHz,
  10. ~.15m wavelength for 1900 MHz)
  11. - Microwaves operate at around 2.45 GHz (~.122m wavelength)
  13. The answer seems dependent on whether or not microwave oven shielding
  14. also shields cellular phone frequencies, and I have not found a
  15. scientific consensus as to whether or not this is the case.
  17. In reading your explanation, I was drawn to the following formula as
  18. perhaps the answer to this:
  21. Thus, it would appear that if all other characteristics about the
  22. shield are kept constant, decreasing the frequency will increase the
  23. attenuation -- perhaps an effective shield for 2.45 GHz really isn't
  24. an effective shield for lower frequencies.
  26. However, this quote seems to imply that if an aperture is much smaller
  27. than the wavelength, the energy will not penetrate:
  29. ,---
  30. | Fortunately, apertures with maximum dimensions that are much
  31. | smaller than a wavelength provide very little impedance to
  32. | the flow of currents on a conducting surface.
  33. `---
  35. So, it would seem that an effective shield for 0.122m would also
  36. shield for the longer wavelengths of the cell phone range very well.
  37. Would you be able to clarify which of these interpretations is
  38. correct? I believe I have tried calling a cell phone inside a
  39. microwave and had it both ring and not ring. This might be a neat
  40. practical example of the theoretical article you have online, perhaps
  41. as an appendix. Just a suggestion.
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