NEC-LIST:Progation at around 1 GHz

Hallo All ,
              I have a communication link around 1 Ghz. At one end I have a
directional antenna and the other end I have an omni low gain antenna. I
find in general although I have line of sight communication , I still get a
lower signal than expected for free
space. How can I improve this ? What is the expected degradation in such a
link ? The received signal changes during the day.
I understand that I have some fading effect. I am transmitting from a high
hill . The path length is about 50 Kms.
What sort of safety figure should one use to ensure good communications for
such a link?

Thanks for any help.
                           Zvi

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