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Weather influences
Snow, rain or hail only have a small effect on the wireless signal. With rainfall of 50 liters per
square meter, i.e., a cloudburst, attenuation of around 0.02 dB/km occurs at a frequency of
2.4 GHz, for example.
Strong wind does not influence the wireless signal, however, it does mean that the antenna
must be securely fixed in place. Especially when using directional antennas with a small
opening angle, you should make sure that the antenna cannot be moved by the wind. Mov-
ing the antenna away from its original position, even by a few centimeters only, may result
in a partial loss of the wireless signal on long transmission paths.
Figure 8-15 Wireless path with strong wind
8.12 Fresnel zone
A certain area between the transmitting and receiving antennas on the wireless path is re-
ferred to as the Fresnel zone. There should be a line of sight between the antennas, espe-
cially when covering large distances. In order to stay within the Fresnel zone, it might be
necessary to install the antennas at a height of a few meters. This area should also be free
from any other obstacles.
The ideal wireless path with a direct line of sight between transmitter and receiver is not al-
ways possible. In real-life applications, obstacles that affect the wireless channel often have
to be taken into account. The wireless path can work even if obstacles such as houses and
trees are within the Fresnel zone. The decisive factor is the number of obstacles and the
area they occupy in the Fresnel zone. In this case, test measurements should be performed.
Inside buildings, e.g., in conventional automation environments, there is a predominance of
reflections. They contribute to a good wireless connection even if the Fresnel zone is not
free from obstacles.
The following figure shows the Fresnel zone between two antennas. The required mounting
height for the antennas depends on the radius of the Fresnel zone.
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