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At higher frequencies reflections from the E layer can extend coverage thousands of miles on signals that are not bounded by the ionsphere. This was noticeable on "low band" TV from Europe being picked up in Australia. Hams in the six meter band use this for long distance communication when the condition is favourable. At the higher VHF frequencies extended range is often had by "ducting" caused mainly by temperature inversion.
As the frequency increases the effect of signals being reflected off dust, Aurora, aircraft, balloons etc becomes more noticeable. In fact reliable communication can be had at UHF over hundreds of miles using High power digital techniques from back scatter reflections.
Barry
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