Wireless:SitePlanning

From TIER

1 Fresnel Zone
2 Earth Bulge
3 Free Space Loss
4 Atmospheric Absorption
5 Rain and Fog
6 References

Fresnel Zone

The characteristics of a radio signal cause it to occupy a broad cross-section of space, called the Fresnel Zone, between the antennas.

Figure 2-1: First Fresnel Zone

Because of the shape of the First Fresnel Zone, what appears to be a clear line-of-sight path may not be. As long as 60 percent of the First Fresnel Zone is clear of obstructions, the link behaves essentially the same as a clear free-space path.

H = 43.3 * Sqrt(D / 4F)
where
H = Height of the First Fresnel Zone (in feet)
D = Distance between the antennas (in miles)
F = Frequency in GHz

[edit]

Earth Bulge

When planning for paths longer than seven miles, the curvature of the earth might become a factor in path planning and require that the antenna be located higher off the ground. The additional antenna height needed can be calculated using the following formula:

H = D^2/8
where
H = Height of earth bulge (in feet)
D = Distance between antennas (in miles)

[edit]

Free Space Loss

Loss (in dB) = 92.4 + 20.log10(F) + 20.log10(D)
where
Loss = free-space path loss between antennas (in dB)
F    = frequency in GHz
D    = path length in kilometers

For 2.4 GHz: Loss = 100 + 20.log10(D)
For 5.2 GHz: Loss = 106.7 + 20.log10(D)
For 5.8 GHz: Loss = 107.6 + 20.log10(D)

[edit]

Atmospheric Absorption

A relatively small effect on the link is from oxygen and water vapor. It is usually significant only on longer paths and particular frequencies. Attenuation in the 2 to 14 GHz frequency range is approximately 0.01 dB/mile, which is not significant.

[edit]

Rain and Fog

Except in extreme conditions, attenuation (weakening of the signal) due to rain does not require serious consideration for frequencies up to the range of 6 or 8 GHz. When microwave frequencies are at 11 or 12 GHz or above, attenuation due to rain becomes much more of a concern, especially in areas where rainfall is of high density and long duration. If this is the case, shorter paths may be required.

[edit]