Electromagnetic waves traveling within earth s atmosphere are called terrestrial waves and communications between two or more points on earth is called terrestrial radio communications. Terrestrial waves are influenced by the atmosphere and earth itself. In terrestrial radio communications, electromagnetic waves can be propagate in several ways , depending on the type of system and the environment. As previously explained electromagnetic waves travels in straight lines except when earth and its atmosphere alter their path. Essentially there are three ways of propagating electromagnetic waves within earths atmosphere ground wave space wave and sky wave propagation.
The three modes of propagation possible between two radio terrestrial antennas. Path 1 a direct or free-space wave, path 2 a ground - reflected wave. Direct and ground - reflected waves together are called space waves. Path 3 a surface wave which consists of the electric and magnetic fields associated with the currents induced in the ground. The magnitude of the ground current depends on the constants of the ground and the electromagnetic waves propagation. The cumulative sum of the direction ground - reflected , and surface waves is sometimes referred to as the ground wave, which is confusing because a surface wave by itself is also sometimes called a ground wave. Path 4 is called the sky wave which depends on the presence of the ionized layers above earth that returns some of the energy that otherwise would be lost in outer space.
Each of the four propagation modes exists in every radio system;however some are negligible in certain frequency ranges or over a particular type of terrain. At frequencies below approximately 2mhz , surface waves provide the best coverage because ground losses increase rapidly with frequency .Sky waves are used for high frequency applications , and apace are used for very high frequencies and above.
A surface wave propagation is an earth guided electromagnetic wave that travels over the surface of earth. As a surface wave moves over earth s surface , it is accompanied by charges induced in the earth. The charges move with the wave producing a current . Since the earth offers resistance to the flow of current , energy is dissipated in a manner very similar to those in a transmission line. Earth s surface also has dielectric losses. Therefore surface waves are attenuated as they propagate . because energy is absorbed from the surface wave , the portion of the wave in contact with earth s surface is continuously wiped out. The energy is replenished by diffraction of energy downward from the portion of the ground wave immediately above earth s surface. This phenomenon produces a slight forward tilt in the wavefront .
Attenuation of the surface wave due to absorption depends on the conductivity of earths surface and the frequency of the electromagnetic wave. Surface waves propagate best over a good conductor. It is apparent that the best surface wave transmission occurs over seawater and that the highest degree of attenuation is over jungle areas. Attenuation over all types of terrain increases rapidly with frequency . Extremely high losses make it impractical to use surface waves for long -distance transmission of high-frequency electromagnetic waves.
Ground wave must be vertically polarized because the electric field in a horizontally polarized wave woulds be parallel to earth s surface and such waves would be short circuited by the conductivity of the ground. Earth s atmosphere has a gradient density which also causes the wavefront to tilt progressively forward. Therefore the wave propagates around the earth remaining close to its surface , and if enough power is transmitted, the wavefront could propagate beyond the horizon or even around the entire circumference of the earth.
However , care must be taken when selecting the frequency and terrain over which surface waves will propagate to ensure that the wavefront does not tilt excessively and simply turn over , lie flat the ground and case to propagate.Surface wave propagation is commonly used for ship-to-ship ans ship-to-shore communications, for radio navigation and for maritime mobile communications. Surface waves are used at frequencies as low as 15khz.
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