Ah, so! Some information, anecdotes about ducting here http://forum.mikrotik.com/t/60km-link/4241/1
Very hard problem to solve if indeed it is ducting. Surplus jet engines strategically placed along path? 
Ah, so! Some information, anecdotes about ducting here http://forum.mikrotik.com/t/60km-link/4241/1
Very hard problem to solve if indeed it is ducting. Surplus jet engines strategically placed along path?