Wi Lan Inc EB04 ユーザーズマニュアル
Calculating a Link Budget
APR 2002 Rev 03
139
Antenna Gain
To ensure the best range and interference suppression, the external antenna should be directional, focusing
the radio energy in one direction (toward the other end of the link) rather than onmi-directional. Use of a
directional antenna also reduces interference from other systems operating at the same frequency.
the radio energy in one direction (toward the other end of the link) rather than onmi-directional. Use of a
directional antenna also reduces interference from other systems operating at the same frequency.
Note: In some situations, you may want to use an omni-directional antenna in your system design. For
example, you would use an omni-directional antenna for a base station with remote sites situated in a
360º path around it.
360º path around it.
When you select a Wi-LAN approved antenna, pay particular attention to the gain specification. When you
select an antenna for a remote station, select an antenna with a gain that provides at least 15 dB fade margin.
select an antenna for a remote station, select an antenna with a gain that provides at least 15 dB fade margin.
Antenna gain is specified in either dBi or dBd. When an antenna is specified in dBd, add 2.14 dB to the value
to convert it to dBi.
to convert it to dBi.
Propagation Loss
Propagation loss is the attenuation (reduction) in RF signal energy as it travels through space. In most wireless
systems, losses through space are the major contributor to signal attenuation. When you know the intended
installation locations of the base and remote stations, determine the physical line of sight distance and then
calculate the RF attenuation as follows:
systems, losses through space are the major contributor to signal attenuation. When you know the intended
installation locations of the base and remote stations, determine the physical line of sight distance and then
calculate the RF attenuation as follows:
Fresnel Zone
It is essential that you locate your antennas at maximum above-ground height to ensure that all ground-based
obstructions are cleared from the Line of Sight path and the Fresnel Zone.
obstructions are cleared from the Line of Sight path and the Fresnel Zone.
The Fresnel Zone is the expansion of the RF signal radio angles in the vertical plane near the middle of the RF
path.
path.
Fresnel Zone
Formula:
Attenuation (dB) for 5.8 GHz band = 108 dB + 20log(d
km
)
where:
d
km
= Distance in Kilometers
108 dB = Pathloss Constant in the 5.8 GHz band
First Fresnel Zone
Line of Sight
Fresnel Radius
Ground
The maximum
Fresnel Radius
indicates that this
path must be kept
clear of
obstructions.
Fresnel Radius
indicates that this
path must be kept
clear of
obstructions.