2: System and RF Planning
32
Verint Video Intelligence Solutions
The region outlined by this elliptical area is known as the first Fresnel zone. The Fresnel
zone is always thicker at the mid-point between the two antennas. Therefore what appears
to be a perfect line-of-sight path between the base and a remote station may not be
adequate for a radio signal; this is the difference between "visual" and "RF" line of sight.
In practice, it has been determined that a radio path can be considered an RF line-of-sight
path if it has a clear opening through 60% of the first Fresnel zone (or 0.6 F1). Here are
values for 0.6 F1 for various signal path distances and frequency bands:
For distances under 7 miles, the earth curvature effect is negligible. However, for greater
distances, you need to consider it in your calculations; for instance, for a 15-mile link in the
2.4 GHz band, the two antennas must be located 82 feet higher than the highest obstacle
in the RF line of sight between them (that is, 53 feet for the Fresnel zone plus 29 feet for
the earth curvature effect). Consult the customer service team for assistance.
A common problem encountered in the field and related to the 0.6 F1 clearance rule is
building obstruction. The proposed visual path may just barely clear a building but the RF
line of sight will not. In such a case, the signal will be partially absorbed and diffracted.
Increasing the height of the two antennas or the gain of the antennas are the only
alternatives to improve the link quality.
Distance
Values for 60% of the First Fresnel Zone
Earth
Curvature
Effect
2.45 GHz
4.9 GHz
5.3 GHz
5.8 GHz
mile km
ft
m
ft
m
ft
m
ft
m
ft
m
1
1.6
14
4.2
9.8
3.0
9.5
2.9
8.9
2.7
0
0
4 6.5
27 8.4
19.5
5.9
18.7
5.7
18
5.5
2
0.6
7
11.3
37
11
25.8
7.9
25
7.6
23.6
7.2
6 1.8
15
24
53
16
37.8
11.5
36.4 11.1
35
10.6
29 8.8
Note: At 2.4, 4.9, and 5 GHz, radio waves are highly attenuated by dense foliage. A link
established in the fall or winter season may be adversely affected in the spring and
summertime, if it is established below tree level.
Visual line of sight
First Fresnel zone (F1)
