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920MHz Band Wireless Device (Korea) USERS MANUAL EM-9121 Rev. 1
9.4 FRESNEL ZONE
A constant space is necessary for optimum receiving sensitivity of radio wave. This space is called Fresnel zone. When the Fresnel
zone is secured, the distance causes attenuation of radio wave (free space propagation loss). To secure the Fresnel zone, install
antennas more highly than the Fresnel radius with no obstacles (actually at least 60% clear of obstacles) between them as shown
in “Figure 9.4 Fresnel zone”.
Antenna
Antenna
Fresnel zone
Fresnel radius r
Distance between antennas d
d1
d2
Figure 9.4 Fresnel zone
The Fresnel radius r can be obtained by the following formula:
Formula 9.1
where r: Fresnel radius [m]
λ
: wavelength [m]
d, d1, d2: distance between antennas [m]
r =
(d1×d2)
(d1+d2)
λ
×
The wavelength in 920 MHz is 0.326 m. The Fresnel radius is obtained by Equation 9.1 as shown in Table 9.1 when the antenna
distance is 100 m, 200 m, 300 m, 500 m and 1000 m.
[Table 9.1] Fresnel radius
Distance between antennas
100 m
200 m
300 m
500 m
1000 m
Fresnel radius
2.85 m
4.04 m
4.94 m
6.33 m
9.03 m
The above table shows theoretical calculated values to obtain the Fresnel radius. Such a height is not always necessary for the
radio wave to reach.
It is very difficult to secure the Fresnel radius for indoor use. Furthermore, indoor radio wave reaches a receiver through various
routes, for example reaching directly a receiver antenna from a sender antenna, reaching after being reflected by walls, a floor, and a
ceiling, and reaching by coming around obstacles such as partitions. Therefore, actual reception state cannot be shown in the same
manner as a case where the Fresnel zone is secured. For indoor use, measure radio wave in each installing location in advance to
check the reachability of the radio wave, and determine feasibility of installation.
