Science 101 Earth Curvature And Troposphere Propagation

The curvature of earth produces two effects firstly 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, secondly curvature of the earth causes the reflection at the ground to take place from a spherical rather than a flat surface so that the reflected ray becomes divergent and consequently becomes weaker at the receiving point. This tends to increase the field strength of the total space wave at the receiving point. These two opposite effects produced by the earths curvature almost neutralize each other. As a consequence, the assumption of a plane earth yields results which remain valid in spite of considerable curvature of earth. The condition in the troposphere keeps on changing with time. Hence the amount of bending of refraction of waves also changes from time to time. This influences the relative path length of the direct and the ground reflected components of space wave. Hence the field strength of the resultant space wave at the receiving point changes, i.e. fading of the wave results. Small but abrupt discontinuities in the dielectric constant of air existing at the boundaries of air masses of widely different characteristics result in mirror like reflections. Such reflections may cause signal to be received at the points beyond the line of straight range. Fading may also be caused.

The distance traveled by the direct ray in the troposphere from transmitting antenna to the receiving antenna and the field strength of this direct ray at the receiving antenna. If the reflection at earth is prefect, the magnitude of the field strength due to ground reflected ray will be approximately equal since the total distance traveled by the indirect ray is approximately equal to the distance traveled by the ray. However the reflection co-efficient at the ground is p then the magnitude of the indirect ray is equal. The direct ray and the ground reflected ray combine vector ally at the receiving antenna. The phase difference in the two field strength is caused by the following two factors, path difference and phase shift caused due to reflection from ground. We revel, some important properties of the propagation of very high frequency. The field frequency strength increases linearly with the height of the transmitting antenna, linearly with the height of receiving antenna, linearly with the square root of the effectively radiated power. The field strength varies with the wave length and inversely with the square of the distance. In all we can say earths curvature has many effects on troposphere propagation.