Describe wireless propagation modes and explain how a signal radiated along these modes ?Ripunjay Tiwari
Ans a signal radiated from an antenna travels along one of three routes- ground wave, sky wave, or line of sight (LOS). Generally we are concerned with LOS communication, a short overview of propagation modes are given below-
(i) Ground wave propagation – ground wave propagation more or less follows the contour of the earth and can propagate considerable distances, well over the visual horizon (see fig. 1.12.)this effect is found in frequencies up to about 2 MHz. several factors account for the tendency of electromagnetic wave in this frequency band to follow the earth’s curvature. One factor is that the electromagnetic wave induces a current in the earth’s surface, the result of which is to slow the wave front near the earth, causing the wave front to tilt downward and hence follow the earth’s curvature. Another factor is diffraction, which is a phenomenon having to do with the behavior of electromagnetic waves in the presence of obstacles.
Electromagnetic waves in this frequency range are scattered by the atmosphere in such a way that they do not penetrate the upper atmosphere the best-known example of ground wave communication is AM radio.
(ii) Sky wave propagation – sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBS and voice of America. With sky wave propagation, a signal from an earth-based antenna is reflected from the ionized layer of the upper atmosphere (ionosphere) back down to earth. Although it appears the wave is reflected from the ionosphere as if the ionosphere were a hard reflecting surface, the effect is in fast caused by refraction. A sky wave signal can travel through number of hope, bouncing back and forth between the ionosphere and the earth’s surface as shown in fig. 1.13 with this propagation mode, signal can be picked up thousands of kilometers from the transmitter.
(iii) Line -of-sight propagation- above 30 MHz, neither ground wave nor sky wave propagation modes operate, and communication must be by line of sight (see fig. 1.14) for satellite communication, a signal above 30 MHz is not reflected by the ionosphere and therefore can be transmitter between an earth station and a satellite overhead that is not beyond the horizon. For ground-based communication, the transmitting and receiving antennas must be within an effective line of sight of each other. The term effective used because microwaves are bent or refracted by the atmosphere. The amount and even the direction of the bend depends on conditions, but generally microwaves are bent with the curvature of the earth and will therefore propagate farther than the optical line of sight.