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What the Heck is The Surface Wave Propagation and How Does it Work?
When the transmitting and the receiving antennas are close to the surface of the earth, the space is negligibly small and the surface wave alone is of importance, thus the day time propagation of the broadcast signals takes place only through surface wave.
The surface wave propagation may be taken under two heads, short distance and long distance surface wave propagation. Short distance surface wave propagation is also called plane earth surface wave propagation. If the distance of the receiving antenna from the transmitting antenna is short, the curvature of the earth's surface may be neglected and the earth may be assumed to be plane in the intervening distance. If the heights of both the transmitting and receiving antenna expressed in the wavelengths are low, then the surface wave alone may be considered.
The electrical conductivity and the dielectric constant of the earth are widely different for the different kinds of soils. There are different values of the constants of few typical solids, for sea water, and for fresh water. At medium and long waves, capacitance reactance of the earth is considerably greater than the earth impedance. Hence the earth may be considered to be purely resistive.
In determining the value of the earth conductivity and dielectric constant for use in assessing the attenuation of surface wave due to loose in ground, suitably weighted average values of these quantities, should be used, the average being taken to a depth below the earth's surface to which there exist ground currents of appreciable magnitude. This depth to which radio waves penetrate that is because an appreciable ground current depends upon the frequency, dielectric constant and conductivity. This depth of penetration may be as low as a few feet at the shortest short waves and may increase to hundred of feet at long waves. Accordingly the earth's constant are not materially affected by the variations in the conditions at the ground such as rain fall etc.
When a plane electromagnetic wave is incident on the surface of the earth it gets reflected such that the angle of reflection equals the angle of incidence. The reflection coefficient depends on the many different quantities like dielectric constant of the earth, conductivity of the earth, the frequency, the plane of polarization of the wave and the angle of incidence. For earth having infinite conductivity in the case of perfect reflector, the reflection coefficient is unity. In the case of prefect reflector the horizontal components of electric fields of incident and reflected waves will exactly cancel each other at the surface of the reflector. On the other hand, the vertical components of the electric fields of the incident and the ground reflected waves do not cancel but get added up at the reflector surface for small values of angle of incidence.