Advertisements
Advertisements
प्रश्न
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?
उत्तर
The three different modes of propagation in a communication system are
(1) Ground wave
(2) Skywave
(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.
APPEARS IN
संबंधित प्रश्न
The outermost layer of the earth’s atmosphere is _______.
(A) stratosphere
(B) mesospher
(C) troposphere
(D) ionosphere
State three components of space wave propagation.
Describe briefly, by drawing suitable diagram, the space wave modes of propagation. Mention the frequency range of the waves in these modes of propagation ?
Mention three different modes of propagation used in communication system.
Explain with the help of a diagram how long distance communication can be achieved by ionospheric reflection of radio waves.
Depict the fields diagram of an electromagnetic wave propagating along positive X-axis with its electric field along the Y-axis.
Explain the ground wave propagation and space wave propagation of electromagnetic waves through space.
In He-Ne loser the most favourable ratio of Helium toNe on for satisfactory laser action is
Two waves A and B of frequencies 2 MHz and 3 MHz, respectively are beamed in the same direction for communication via sky wave. Which one of these is likely to travel longer distance in the ionosphere before suffering total internal reflection?
The maximum frequency for reflection of sky waves from a certain layer of the ionosphere is found to be fmax = 9(Nmax)1/2, where Nmax is the maximum electron density at that layer of the ionosphere. On a certain day, it is observed that signals of frequencies higher than 5 MHz are not received by reflection from the F1 layer of the ionosphere while signals of frequencies higher than 8 MHz are not received by reflection from the F2 layer of the ionosphere. Estimate the maximum electron densities of the F1 and F2 layers on that day.
