The degree and sense of refraction of wave is related to a factor known as K. Refraction normally bends the wave downwards, extending the range, but the refractive index of the atmosphere varies from place to place and with time and height. At times, during an atmospheric inversion for instance, and in some places on the earth the effect is reversed:
When no bending occurs, K = 1.
WhenK> 1, the bending is downwards, effectively increasing the Earth’s radius.
WhenK< 1, the bending is upwards, effectively reducing the Earth’s radius.
For a point-to-point link, the path between the transmitting and receiving sites should be a clear line of sight, although by making allowances for refraction, the Earth’s radius has, up to now, generally been considered to be effectively increased by the factor of 1.33. However, in the interests of reliability a decrease to 0.7 times the radius (the minimum K factor considered likely to occur) is now often used in link planning. When a link has been planned using a higher K factor, a temporary reduction of K not only reduces the radio horizon but effectively raises objects close to the path, possibly to the point where they become significantly close to the first Fresnel zone.
It is usual when planning a link to plot the profile of a path on paper with curved horizontal graduations to represent the amended Earth’s radius. The radio beam can then be drawn as a straight line between the antenna locations. Figure 11.2 is an example.