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PROPAGATION OF ELECTROMAGNETIC WAVES:In communication using radio waves, an antenna at the transmitter radiates the Electromagnetic waves EM waves, which travel through the space and reach the receiving antenna at the other end. As the EM wave travels away from the transmitter, the strength of the wave keeps on decreasing. Several factors influence the propagation of EM waves and the path they follow. At this point, it is also important to understand the composition of the earth’s atmosphere as it plays a vital role in the propagation of EM waves. A brief discussion on some useful layers of the atmosphere is given in Table. Ground wave: To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength of the signal at least /4. At longer wavelengths i.e., at lower frequencies, the antennas have large physical size and they are located on or very near to the ground. In standard AM broadcast, ground based vertical towers are generally used as transmitting antennas. For such antennas, ground has a strong influence on the propagation of the signal. The mode of propagation is called surface wave propagation and the wave glides over the surface of the earth. A wave induces current in the ground over which it passes and it is attenuated as a result of absorption of energy by the earth. The attenuation of surface waves increases very rapidly with increase in frequency. The maximum range of coverage depends on the transmitted power and frequency less than a few MHz.
Sky waves:In the frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth. This mode of propagation is called sky wave 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 details of which are given in Table. The degree of ionisation varies with the height. The density of 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. These phenomena are shown in the Figure. The phenomenon of bending of EM waves so that they are diverted towards the earth is similar to total internal reflection in optics.