Question

Two towers on top of two hills are 40 km apart. The line joining them passes 50 m above a hill halfway between the towers. What is the longest wavelength of radio waves, which can be sent between the towers without appreciable diffraction effects?

Answer 1

The hills will not obstruct the transmitted radio waves, if over the halfway distance (Z_f) between the hills, the spreading of the signal is limited to the distance (a) of the line of sight from the hills.