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

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Solution

Distance between thetowers, d = 40 km
Height of the linejoining the hills, d = 50 m.
Thus, the radial spreadof the radio waves should not exceed 50 km.
Since the hill islocated halfway between the towers, Fresnel’s distance can beobtained as:
Z_P= 20 km = 2 × 10⁴m
Aperture can be takenas:
a = d =50 m
Fresnel’sdistance is given by the relation,
Where,
λ = Wavelengthof radio waves
Therefore, thewavelength of the radio waves is 12.5 cm.

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