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   Earth–ionosphere waveguide
   The Earth–ionosphere waveguide refers to the phenomenon in which certain radio waves can propagate in the space between the ground and the boundary of the ionosphere. Because the ionosphere contains charged particles, it can behave as a conductor. The earth operates as a ground plane, and the res...
   Found op http://en.wikipedia.org/wiki/Earth–ionosphere_waveguide
   
   

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