Question

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

Answer 1

Hint: The communication waves can be sent from one place to another in three ways: by propagating them near ground, through sky at high altitudes where layers of atmosphere reflect the incident electromagnetic waves or through satellites installed in orbit around earth.

Complete step-by-step answer:
Long distance communication is necessary for connecting different parts of the world. The transmission of information from one place to another is done via electromagnetic waves mainly radio waves. There are three ways through which long distance communication has been made possible.
1. Ground wave propagation: It is also called surface wave propagation. The radio waves travel near the surface of earth but this method is not useful for too long distances.
2. Sky wave propagation: The radio waves sent into the higher layers of atmosphere which reflects the waves back towards earth covering a larger distance than ground waves.
3. Space wave propagation: The radio waves are sent beyond the atmosphere towards the satellites revolving around earth which reflect the waves back over longer distances.
Sky wave propagation transmits radio waves over longer distances and has been made possible because of the ionosphere in the upper layers of earth’s atmosphere. The ionosphere consists of charge particles or ions and is situated at a distance of about 65km to 400km from the surface of earth. When radiation in the form of radio waves is incident on these layers, the ions absorb and re-emit this radiation reflecting it towards earth over longer distances.

Note: The ionosphere is used only for a certain range of frequencies because it can reflect the radio waves of certain frequencies only. The range of frequencies which can be reflected by ionosphere and transmitted using sky wave propagation is 3MHz to 30MHz.