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Free Space Attenuation

  1. Dec 5, 2009 #1
    Hi All,

    I'm reading about the attenuation of radio waves in space and I understand that, in addition to the normal attenuation that is proportional to the inverse of the square of the distance, there is also a loss due to the medium.

    This surprised me as I thought that there was no loss in space due to energy absorbed by the medium. Is this loss because space is not actually a vacuum but contains a few atoms here and there and these atoms absorb energy from the radio waves?

    Thank you,

    Bob
     
  2. jcsd
  3. Dec 5, 2009 #2
    Space is a pretty general term, and non of it is a perfect vacuum. That doesn't even really make sense to talk about. It is more reasonable to speak of the mass density of interstellar space. http://en.wikipedia.org/wiki/Interstellar_medium
     
  4. Dec 5, 2009 #3
    But to be more precise, I'm interested in knowing what is meant by the attenuation of free space with regard to communications.

    Thank you,

    Bob
     
  5. Dec 5, 2009 #4
    I don't know any mechanism for a wave to be lossy in free space.

    And according to this:
    http://en.wikipedia.org/wiki/Free-space_path_loss

    It's just inverse-square law plus the frequency dependence of the receiving antenna. The EM wave itself is not being attenuated (ie no power loss).
     
  6. Dec 5, 2009 #5
    yes right, but space is never really "free space"
     
  7. Dec 5, 2009 #6
    "free space attenuation" would, by definition, not include what you are talking about.
     
  8. Dec 6, 2009 #7
    I understand what is going on and the problem is semantics.

    The amount of power arriving at the destination is not frequency dependent but is dependent only upon the square of the distance. This, to me, is a true loss due to distance in space.

    Apparently, once a certain amount of power reaches the destination, the amount of it that is utilized is dependent upon the frequency.

    Is there a logical reason why the frequency dependence is not better separated from the term free space since it really has nothing to do with free space as I see it?

    Thanks again,

    Bob
     
  9. Dec 6, 2009 #8
    The frequency dependency comes only in when the antenna is considered.
    So it looks like your right about that.
     
  10. Dec 6, 2009 #9
    I'd like to thank everybody for helping me understand this issue.

    Bob
     
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