About the Fresnel Zone Chapter 21
WinLink 1000 User Manual Release 1.9.40 21-4
Figure 21-1: Fresnel zone
Fresnel loss is the path loss occurring from multi-path reflections from reflective surfaces such
as water, and intervening obstacles such as buildings or mountain peaks within the Fresnel
zone.
Radio links should be designed to accommodate obstructions and atmospheric conditions,
weather conditions, large bodies of water, and other reflectors and absorbers of electromag-
netic energy.
The Fresnel zone provides us with a way to calculate the amount of clearance that a wireless
wave needs from an obstacle to ensure that the obstacle does not attenuate the signal.
There are infinitely many Fresnel zones located coaxially around the center of the direct
wave. The outer boundary of the first Fresnel zone is defined as the combined path length of
all paths, which are half wavelength (1/2 ) of the frequency transmitted longer than the
direct path. If the total path distance is one wavelength (1 ) longer than the direct path,
then the outer boundary is said to be two Fresnel zones. Odd number Fresnel zones reinforce
the direct wave path signal; even number Fresnel zones cancel the direct wave path signal.
The amount of the Fresnel zone clearance is determined by the wavelength of the signal, the
path length, and the distance to the obstacle. For reliability, point-to-point links are designed
to have at least 60% of the first Fresnel zone clear to avoid significant attenuation.
The concept of the Fresnel zone is shown in Figure 21-1 above. The top of the obstruction
does not extend far into the Fresnel zone, leaving 60% of the Fresnel zone clear; therefore,
the signal is not significantly attenuated.
For more about Fresnel zone, see http://en.wikipedia.org/wiki/Fresnel_zone.
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