Free Space Loss; Line of Sight
Posted: Sun Nov 01, 2020 9:01 am
A frequent variable in calculating path loss is to consider a parameter often called "free space loss." The description of "free space loss" is actually misleading. There is NO loss in the signal power when propagating in free space. What is actually occurring in the phenomenon of "free space loss" is better described as spreading loss. When a radio wave signal propagates, it spreads out. While the total power emitted at the source remains constant, the power density decreases as the signal spreads. The reduction in the power received at a remote antenna is not due to a loss in propagation, but just due to beam spreading.
Inherent in the assumptions of calculating "free space loss" is the concept of loss-less propagation of the signal through space. Propagation of the signal through media other than a perfect vacuum may introduce some signal loss. For example, propagation of radio wave through water, through the earth, or through glass suffer significant loss. Propagation through the atmosphere is almost loss-less, depending on factors like frequency, moisture content, particles in the air that might cause scattering, and so on.
In the formula for path loss in free space, the effect of distance is described by a factor of 20 log (d). This represents loss-less propagation and only accounts for spreading losses. In other formulas for path loss through atmosphere and over real terrain, the effect of distance is described by a factor of x log (d) where x is at least 30 or higher. This reflects the propagation loss through air and also the influences of terrain.
Inherent in the assumptions of calculating "free space loss" is the concept of loss-less propagation of the signal through space. Propagation of the signal through media other than a perfect vacuum may introduce some signal loss. For example, propagation of radio wave through water, through the earth, or through glass suffer significant loss. Propagation through the atmosphere is almost loss-less, depending on factors like frequency, moisture content, particles in the air that might cause scattering, and so on.
In the formula for path loss in free space, the effect of distance is described by a factor of 20 log (d). This represents loss-less propagation and only accounts for spreading losses. In other formulas for path loss through atmosphere and over real terrain, the effect of distance is described by a factor of x log (d) where x is at least 30 or higher. This reflects the propagation loss through air and also the influences of terrain.