Accounting for the Fresnel Zone and Earth Bulge

 
A main consideration of any point-to-point design is the Fresnel zone. An electromagnetic
signal traveling between two antennas does not travel in a straight line.
The wave spreads out as it propagates.The individual waves that make up the signal
do not travel at a constant velocity. A pair of antennas define a three-dimensional
elliptical path for the radio waves that propagate between them.This elliptical path
is divided into several zones based on the phase and speed of the propagating
waves.These zones are referred to as Fresnel zones. Each Fresnel zone differs in
phase by up to half a wavelength, or 180 degrees.This Fresnel zone is commonly
thought of as line-of-sight (see Figure 1.9). Radio line-of-sight is not the same as
visual line-of-sight. In visual line of sight, a direct line exists between two points; it
is easy to think this way between two antennas in a point-to-point design.
However, radio line of sight is not a straight line between the antennas; it is more
of an ellipse. In a good point-to-point design, this ellipse should be calculated to
determine its size and clear of obstacles to provide a good signal. Because of the elliptical shape of the Fresnel zone, the antennas used in a
point-to-point design must be high enough to provide clearance of the Fresnel
zone’s radius at the midpoint. As the distance increases, other factors must be
considered such as the curvature of the Earth, where the line of sight becomes
difficult at 6 miles (for a 6-foot tall person) and disappears altogether at 16 miles
(for two structures at 10 feet) (see Figure 1.10). Paths over 20 miles are extremely
difficult to align and install, so take caution when recommending these types of
configurations.