## Free Space Loss; Line of Sight

jimh
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### Free Space Loss; Line of Sight

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.

jimh
Posts: 12159
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

### Re: Free Space Loss; Line of Sight

Boaters often refer to VHF radio communication as being limited to "line of sight." This is not true. The VHF spectrum is considered to be from 30 to 300-MHz, and radio signals on these frequencies, while tending to travel only in straight lines, are subject to several effects which can bend or reflect the signals.

VHF signals are subject to reflection and scattering in the ionosphere's E-layer due to unusually high ionization and enhanced reflection. Propagation beyond line of sight distances by this method is known as Sporadic-E skip and can create paths of 600-miles or more.

VHF Signals are also subject to tropospheric scattering by the air itself, but this mode is highly dependent on the weather. With unusual conditions, tropospheric scattering can create VHF signal propagation distance of hundred of miles. Temperature inversions in the troposphere tend to bend radio waves back towards earth. If the refraction matches the earth curvature, long distance paths can occur. This method is known as Tropo-scatter or Tropo-ducting.

Diffraction of radio waves by objects in the path can also enhance the range. An intervening tall object can create refraction of radio wave and increase the line-of-sight distance significantly.

Even under normal conditions, variation in atmosphere density creates refraction, so the distance to a radio horizon tends to be greater than the distance to an optical horizon by a factor of about 4/3.

For some illustrations showing Sporadic-E and Tropo-ducting, visit

https://fas.org/man/dod-101/navy/docs/e ... opagat.htm

which presents some material from a Navy course in radio. See the illustrations under the sub-heading "Line of sight."