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Name the three different modes of propagation of electromagnetic waves, explain, and using a proper diagram the mode of propagation used in the frequency range above 40 MHz.

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Last updated date: 22nd Mar 2024
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MVSAT 2024
Answer
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Hint: There are three types of modes of propagation of electromagnetic waves: Ground wave propagation, Space wave propagation and Skywave propagation. Electromagnetic waves in the frequency range of a few Hz to about $10^{11}$Hz are generally called radio waves. These waves are useful for the transmission of information from one place to another without the help of a wire.

Complete step-by-step answer:
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Ground wave propagation:
When the radio waves from the transmitting antenna propagate along the surface of the earth, to reach the receiving antenna, the wave propagation is called ground waves or surface wave propagation. Ground waves are radio waves that travel or propagate along the surface of the earth. These radio waves induce a current in the ground from which they pass. While propagating through the medium waves will have losses as well as absorption. Therefore there will be a loss of signal or power. The more the frequency of waves the more will be losses. That is why it is favourable only for low-frequency waves only. Generally, this kind of technique is used for broadcasting purposes.

Space wave propagation:
The radio waves from the transmitting antenna propagate through space, around the ground and reach the receiver either directly or by reflection from the ground, this kind of wave propagation I called space wave propagation. It is also called the line of sight propagation.
The radio waves which travel directly from the transmitting antenna to the receiving antenna are also called space waves. Waves in the MHz frequency range are used for space wave propagation. We know that our earth is spherical therefore to make the long-distance we need the height of the antenna as long as possible.

Skywave propagation:
The radio waves from the transmitting antenna propagate through the sky towards the receiver, either directly or by reflection from the ground, this kind of wave propagation is called skywave propagation. It is also called ionospheric wave propagation.
The refractive index of the ionosphere is less than its free space value and it decreases with increases in height. Therefore radio waves go under total internal reflection. Radio waves of frequency less than 3 MHz are observed in the ionosphere and of frequency greater than 30 MHz can pass through the ionosphere after suffering a small deviation.

Note: Students should also know that loss of energy in ground wave propagation is due to the diffraction effect. Ground waves are not used in FM to transmit the signal. In-ground waves, the height of the antenna is an important factor. In skywave propagation has some threshold value of frequency for the transmission.
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