Bruce Clark writes in a message to Scott Christensen
 BC> Not to mention, only a few miles of atmosphere to attenuate 
 BC> the signal. 
Atmosphere is usually a negligable loss factor at less than 1 GHz.  What 
*are* problems are trees, buildings, multiple reflections, very heavy 
precipitation and of course, natural obstructions.
  
You can get odd atmospheric conditions where signal ducting will happen, but 
this phenomonon takes energy that is normally radiated off the planet and 
returns it to the planet's surface where it can be received at distances 
greater than normal.
 
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