Space wave propagation

(Author: Tymon Hytem | Article Views: 20 | Word Count: 472 | Published: Sep 7th 2007)

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By space wave we mean the radio wave which travel from the transmitting antenna to the receiving antenna through the space that is the earth’s troposphere. Earth’s troposphere extends from the earth’s surface up to about fifteen kilometers. The space wave consists of two components that is direct wave and the ground reflected wave. Frequencies above sixty mega hertz are never refracted back to earth by the ionosphere and ii is only under special circumstances that frequencies in the range of thirty to sixty mega hertz are so returned. Further the surface wave is attenuated extremely rapidly at that these high frequencies. As a consequence communication at frequencies above about thirty mega hertz is possible only through space wave propagation. When the transmitting and receiving antennas are both at the earth’s surface the two components of the space wave are equal in magnitude and opposite in phase. Hence these two components cancel each other at the receiving antenna leaving the surface wave as the only component of ground wave. This is the case of ground wave propagation at broadcast frequencies is medium waves but as the heights of the antenna expressed in wave lengths are increased the amplitude of the space wave increases rapidly and when the heights equal a few wave lengths or more the space wave becomes the principal part of the ground wave. In general the transmission is possible only up to or slightly beyond the line of sight distance. Hence the elevation of the antennas is the factor which determines the range. The horizon effective for the radio waves commonly called the line of sight is slightly beyond the horizon determined on the basis of a straight line path because of the refraction of the radio wave in troposphere. The dielectric constant of the troposphere is slightly greater than unity and varies with the concentration of water molecules having high dielectric constant. With the increase of height above ground the atmospheric pressure reduces causing corresponding reduction in the dielectric constant and refractive index. This decrease in refractive index with height causes refraction of the radio wave resulting in bending of radio wave towards the region of high dielectric constant that is towards the earth. Accordingly space wave propagation becomes possible up to range slightly beyond the actual horizon. The atmospheric pressure, the dielectric constant and the consequent refraction varies with time. But on the average the effect of refraction in troposphere may be accounted for by assuming straight line propagation and increasing the effective radius of earth. The curvature of earth produces two effects that is it reduces the difference in the path lengths between the direct and ground reflected waves by raising the point of reflection above that of a plane earth and as a result tends to reduce the strength of the received field.

About the Author

Tymon Hytem has worked in the electronics field for the past 15 years. He enjoys helping people decide on electronic gadgets from finding the right phone for your business and can help you choose the perfect Background Music for your business needs.

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