Cisco Aironet 1520 Series User Manual download pdf (Page 10)

dBm mW dBm mW

12 dBm 16 mW -12 dBm 0.06 mW

13 dBm 20 mW -13 dBm 0.05 mW

15 dBm 32 mW -15 dBm 0.03 mW

17 dBm 50 mW -17 dBm 0.02 mw

20 dBm 100 mW -20 dBm 0.01 mW

30 dBm 1000 mW (1 W) -30 dBm 0.001 mW

40 dBm 10,000 mW (10 W) -40 dBm 0.0001 mW

Outdoor Range

The range of a wireless link is dependent upon the maximum allowable path loss. For outdoor links, this is a

straightforward calculation as long as there is clear line of sight between the two antennas with sufficient

clearance for the Fresnel zone. For line of sight, you should be able to visibly see the remote locations antenna

from the main site. (Longer distances may require the use of binoculars). There should be no obstructions

between the antennas themselves. This includes trees, buildings, hills, and so on.

As the distance extends beyond six miles, the curve of the earth (commonly called earth bulge) affects installation,

requiring antennas to be placed at higher elevations.

Fresnel Zone

Fresnel zone is an elliptical area immediately surrounding the visual path. It varies depending on the length of the

signal path and the frequency of the signal. The Fresnel zone can be calculated, and it must be taken into account

when designing a wireless link (Figure 6).

Figure 6. Fresnel Zone

Based on both line-of-sight and Fresnel zone requirements, Table 3 provides a guideline on height requirements

for 2.4-GHz antennas as various distances. This refers to height above any obstacles located in the middle of the

RF path.

Table 3. Guideline on Height Requirements for 2.4-GHz Antennas

Wireless Link Distance (miles) Approx. Value “F” (60% Fresnel

Zone) Ft. at 2.4 GHz

Approx. Value “C” (Earth

Curvature)

Value “H” (mounting Ht.) Ft. with

No Obstructions

1 10 3 13

5 30 5 35

10 44 13 57

15 55 28 83

20 65 50 115

25 72 78 150

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 72 73

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Cisco Aironet 1520 Series User Manual Page 10