Question

Name the three different modes of propagation of electromagnetic waves. Explain, using a proper diagram the mode of propagation used in the frequency range from a few MHz to 40 MHz.

Answer 1

Three modes of propagation of electromagnetic waves,

(a) Ground waves,

(b) Sky waves,

(c) Space waves.

Sky wave propagation is used in the frequency range from a few MHz to 40 MHz.

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 certain range of frequencies (3MHz-30MHz). The transmitting antenna sends the EM signals of this frequency range towards the ionosphere. When the EM waves strikes the ionosphere, it is reflected back to the Earth. A receiving antenna at a remote location on the Earth receives these reflected signals.