Mention three different modes of propagation used in a communication system. Explain with the help of a diagram how long-distance communication can be achieved by ionospheric reflection of radio waves.

Three modes of propagation of electromagnetic waves are:
(a) Ground waves
(b) Sky waves
(c) Space waves.

Sky wave propagation is used for long-distance communication by ionospheric reflection of radio waves. In the ionosphere of the Earth’s atmosphere, there are a large number of charged particles (ions). The ionosphere is situated about 65 km – 400 km above the surface of the Earth. The ionization of molecules occurs due to the absorption of the ultraviolet rays and high-energy radiation from the Sun. The ionosphere acts as a reflecting layer for a certain range of frequencies (3MHz-30MHz). The transmitting antenna sends the EM signals of this frequency range towards the ionosphere. When the EM waves strike the ionosphere, they are reflected back to the Earth. A receiving antenna at a remote location on the Earth receives these reflected signals.