Microwave radio communication and Line of Sight.

In free space, radio signals propagate like light ie. they follow a straight line. Such a line between the transmitter and receiver, is referred to as Line-of-Sight.Microwave radio communication requires a clear line-of-sight (LOS) condition.Under normal atmospheric conditions, the radio horizon is around 30 percent beyond the optical horizon.Radio LOS takes into account the concept of Fresnel ellipsoids and their clearance criteria.

Free Space Loss
As signals spread out from a radiating source, the energy is spread out over a larger surface area. As this
occurs, the strength of that signal gets weaker. Free space loss (FSL), measured in dB, specifies how much the signal has weakened over a given distance. Figure 1 shows the formula to calculate FSL and what the theoretical loss would be at sample distances. The type of antenna used has no effect on FSL, since at any appreciable distance all antennas look like a pointsource radiator. Note that the difference in FSL between a 2.4 GHz link and a 5.8 GHz link is always about 8 dB,regardless of the distance. This is one of the reasons why 802.11a wireless local area network (WLAN) devices will have less than half the range of a 2.4 GHz WLAN device (e.g., 802.11b).