The characteristics of a radio signal cause it to occupy a broad cross-section of space, called the Fresnel Zone, between the antennas. From figure shows the area occupied by the strongest radio signal, called the First Fresnel Zone, which surrounds the direct line between the antennas
Because of the shape of the First Fresnel Zone, what appears to be a clear line-of-sight path may not be. As long as 80 percent of the First Fresnel Zone is clear of obstructions, the link behaves essentially the same as a clear free-space path .
The following formula is used to calculate it:
H = 43.3 √ (D/4F )
Where,
H = Height of the First Fresnel Zone (in feet)
D = Distance between the antennas (in miles)
F = Frequency in GHz
Waves can be deflected by objects in their paths. If a wave from an outer band of the cone (see Radio Signals) is deflected back through the center lobe, it can either strengthen that signal or reduce its strength, depending on how the waves align when they collide. A glancing deflection changes the angle of the wave very little, so it remains generally in phase with the wave at the center lobe Within the signal span, there are zones where deflected signals are generally in phase with the center lobe signal, and there are other zones where deflected signals are generally out of phase with the center lobe signal. We refer to these zones as Fresnel (frnl) zones. The first Fresnel zone surrounds the center lobe where the RF signal is strongest. If more than 40% of the first Fresnel zone is obstructed, the RF line of sight is not sufficiently clear.
In the first Fresnel zone and all the odd numbered Fresnel zones, deflected signals are generally in phase with or the center lobe signal. In the second Fresnel zone, and all even-numbered Fresnel zones, deflected signals are up to 180 out of phase with the center lobe signal . Signals deflected from the second Fresnel zone can cause Inter Symbol Interference (ISI) which can result in great losses of the center lobe signal. To avoid this problem, must place the antenna at a height that is out of range from F2 deflections. (An antenna can be set too high as well as too low.) Where deflection and diffraction from ground-based objects cause interference, even a small relocation of the antenna often produces a substantial improvement.
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