FL WLAN EPA
4-2
PHOENIX CONTACT
8099_en_02
4.1.2.1
Marginal conditions for planning a wireless path
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 required to mount the antennas at a height of some meters. This area should also be free
from any other obstacles.
Note:
the wireless path may also work if obstacles are within the Fresnel zone (house, tree
etc.). In most cases the decisive factor is the number of obstacles and the area they occupy
in the Fresnel zone. However, there is no clear calculation basis for this. It is therefore
recommended to perform test measurements.
Indoors: reflections (which do not occur outdoors in this way) usually dominate in the
conventional automation environment and ensure a safe wireless connection even if the
Fresnel zone is not free from obstacles.
4.1.3
Free space attenuation of the wireless signal/Fresnel zone
The following table lists the values for free space attenuation which experiences a wireless
signal at different distances.
Keep the Fresnel zone free from any obstacles.
For WLAN 802.11.b,g the values apply for 2.4 GHz and for WLAN 802.11 a,h, the values
apply for 5 GHz.
Table 4-1
Fresnel zone - mounting height for the WLAN devices (antennas)
Wireless path distance
(d)
Antenna height
2.4 GHz (r)
Antenna height
5 GHz (r)
100 m
1.7 m
1.2 m
200 m
2.5 m
1.7 m
300 m
3.0 m
2.1 m
400 m
3.5 m
2.4 m
Table 4-2
Free space attenuation at 2.4 GHz and 5 GHz
Distance in m
Attenuation at 2.4 GHz
Attenuation at 5 GHz
5
54.0 dB
60.0 dB
10
60.0 dB
66.0 dB
20
66.0 dB
72.0 dB
30
69.5 dB
75.5 dB
50
74.0 dB
80.0 dB
100
80.0 dB
86.0 dB
110
80.8 dB
86.8 dB
120
81.6 dB
87.6 dB
150
83.5 dB
89.5 dB
200
86.0 dB
92.0 dB
250
88.0 dB
94.0 dB
300
89.5 dB
95.5 dB
