User's Manual

2.1.6. Fresnel zones calculation
The position of obstacles between points of the bridge can significantly influence the quality of the mi-
crowave link. The radio signal doesn't only radiate along the line of sight, but also in the area around
it, i.e. in the so-called 1st Fresnel zone. Within this zone 90 % of the energy is transmitted between the
transmitter and receiver antenna. This space has the shape of an ellipsoid. If it is disturbed the link has
poorer transmission properties and a higher quality antenna is required. For this reason the position
of the antenna can be just as important as its height above ground. 60 % of the 1st Fresnel zone is
considered as the most important.
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r
D
F
1
d
1
× ×
Fig. 2.6: Fresnel zone
The general equation for calculating the first Fresnel zone radius at any point P in between the endpoints
of the link is the following:
Where:
F
1
first Fresnel Zone radius in metres
d
1
distance of P from one end in metres
d
2
The distance of P from the other end in metres
λ wavelength of the transmitted signal in metres
The cross sectional radius of each Fresnel zone is the highest in the center of link, shrinking to a point
at the antenna on each end. For practical applications, it is often useful to know the maximum radius
of the first Fresnel zone. From the above formula, calculation of the first Fresnel zone can be simplified
to:
where:
r
max radius of first Fresnel zone (m)
reducing the radius to 60% get values listed in the following table that define the space particularly
sensitive to the presence of obstacles
17© RACOM s.r.o. – RAy2 Microwave Link
Implementation Notes