waves radiated from an antenna, directed upwards at large angles, gets reflected by the ionosphere back to Earth is called sky wave propagation or ionospheric propagation. The corresponding waves are called sky waves (Figure . (b)). Ionosphere acts as a reflecting surface.
It is at a distance of approximately km and spreads up to km above the Earth's surface. Due to the absorption of ultraviolet rays, cosmic ray, and other high energy radiations like α, β rays from sun, the air molecules in the ionosphere get ionized. This produces charged ions and these Earth Home Atmosphere Radio tower (a) Skip distance T Sky wave coverage Skip zone Ionosphere Ground wave coverage (b) Earth wave Direct wave Receiver Transmitter Reflected (c) Figure . Propagation of EM waves (a) Ground wave (b) Sky wave (c) Space wave - - - - - - - - Unit electronics and Communication The range or distance (d) of coverage of the propagation depends on the height (h) of the antenna given by the equation, Rh ( .
) where R is the radius of the Earth. The distance of coverage is shown pictorially in Figure . . h Figure .
Distance of coverage EXAMPLE . A transmitting antenna has a height of m and the height of the receiving antenna is m. What is the maximum distance between them for line-of-sight communication? The radius of the earth is .
× m. d d h1 h2 T R Solution: The total distance between the transmitting and receiving antennas will be the sum of the individual distances of coverage. ions provide a reflecting medium for the reflection of radio waves or communication waves back to Earth within the permitted frequency range. The phenomenon of bending the radio waves back to Earth is nothing but the total internal reflection.
When the angle of incidence at the ionosphere is large, the sky wave returns to the ground at a long distance from the transmitter. As this angle is reduced, the wave returns closer and closer to the transmitter. If the angle of incidence