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

Distance between the towers, d = 40 km

Height of the line joining the hills, d = 50 m.
Thus, the radial spread of the radio waves should not exceed 50 km. Since the hill is located halfway between the towers, Fresnel’s distance can be obtained as:

Therefore, the wavelength of the radio waves is 12.5 cm.