Name the three different modes of propagation in a communication system. State briefly why do the electromagnetic waves with a frequency range from a few MHz up to 30 MHz can reflect back to the earth. What happens when the frequency range exceeds this limit?
The three different modes of propagation in a communication system are
(1) Ground wave
(3) Space wave
In the frequency range from a few MHz up to 30 to 40 MHz, long-distance communication can be achieved by the ionospheric reflection of radio waves back towards the earth. This mode of propagation is called skywave propagation and is used by short wave broadcast services. The ionosphere is so-called because of the presence of a large number of ions or charged particles. It extends from a height of ~ 65 Km to about 400 km above the earth’s surface. Ionisation occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules.
The ionosphere is further subdivided into several layers. The degree of ionisation varies with the height. The density of the atmosphere decreases with height. At great heights the solar radiation is intense but there are few molecules to be ionised. Close to the earth, even though the molecular concentration is very high, the radiation intensity is low so that the ionisation is again low. However, at some intermediate heights, there occurs a peak of ionisation density.
The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape. The phenomenon of bending of em waves so that they are diverted towards the earth is similar to a total internal reflection in optics.
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