PathAlign-R
Spectracom Corporation
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5
Wireless Supporting Information
5.1
Free-Space Loss
The Friis free-space propagation equation is commonly used to determine the attenuation of a
signal due to spreading of the electromagnetic wave.
Free space loss is given as:
Attenuation
(dB) = 92.467 + 20 log10(
f
GHz) + 20 log10(
D
km); or,
Attenuation
(dB) = 96.6 + 20 log10(
f
GHz) + 20 log10(
D
mi)
Where:
f
GHz = frequency in GHz, and
D
km = distance between antennas (link) in kilometers; or,
D
mi = distance between antennas (link) in miles.
5.1.1 Frequencies above 10 GHz
For frequencies above 10 GHz, there are several additional issues that affect propagation,
including:
•
Absorption due to gasses or water vapor
•
Attenuation due to mist, fog, or rainfall.
Many gasses and pollutants have absorption lines in the millimeter bands but, due to their low
densities, their effect is negligible in microwave and millimeter wave frequencies below 30 GHz.
Water vapor, though, has an absorption line at 22.235 GHz and can affect microwave
frequencies above 10 GHz. The amount of water vapor in the atmosphere at sea level can vary
from 0.001 gram per cubic meter in a cold, dry climate to as much as 30 grams per cubic meter
in hot, humid climates. In addition, the effects of precipitation can be significant at microwave
frequencies above 10 GHz. The attenuation due to rainfall is dependent on the size and
distribution of the water droplets. Because snowfall rates are generally less than rainfall rates,
propagation is less effected by snowfall. For both snow and fog, the attenuation loss is a
function of temperature and can vary by a factor of 3 between 0°C and 40°C [1].
Total transmission loss for a microwave/millimeter link is given by Freeman [2] as:
Attenuation
(dB) = 96.6 + 20 log10(
f
GHz) + 20 log10(
D
mi) + excess attenuation (dB) due
to water vapor, mist, fog, and rainfall.
Where:
f
GHz = frequency in GHz, and
D
mi = distance between antennas (link) in miles.
