Planning a radio link

There should be a line of sight, especially in the event of longer distances, between the antennas of the wireless devices. To keep the Fresnel zone free from any obstacles, it may be necessary to mount the antennas a few meters high. This area should also be free from any other obstacles.

The Fresnel zone

The radius of the Fresnel zone corresponds to the desired antenna height. It depends on the radio frequency and the distance between the antennas.

The wireless path may also work if there are obstacles within the Fresnel zone (house, tree, etc.). The decisive factor is the number of obstacles and the area they occupy in this zone.

In this case it is recommended that you perform test measurements. Inside buildings, in conventional automation environments, there is a predominance of reflections, which do not occur outdoors. They contribute to a good wireless connection even if the Fresnel zone is not free from obstacles.

Wireless path distance (d)Antenna height (r)
868/900 MHz
Antenna height (r)
2.4 GHz
Antenna height (r)
5 GHz
200 m4 m2.5 m1.5 m
500 m6.5 m4 m2.5 m
1000 m9 m5.5 m4 m
2000 m13 m8 m5.5 m
4000 m18.5 m11 m8 m
10,000 m29 m--
20,000 m41.5 m--
30,000 m50 m (900 MHz only)--

The table shows the radius of the Fresnel zone depending on the frequency and distance. This gives the mounting height for wireless devices or antennas.


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