Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
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Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 3 WLAN Radio Frequency Design Considerations
Planning for RF Deployment
Power Level and Antenna Choice
Power level and antenna choice go hand-in-hand to determine AP placement. Together, these two
variables determine where and how powerful the RF is in any given place in the environment. Along with
choosing the correct antenna to produce the required coverage area, Cisco recommends the use of RRM
to control the power level and provide the optimal channel/power plan. For more information, see
variables determine where and how powerful the RF is in any given place in the environment. Along with
choosing the correct antenna to produce the required coverage area, Cisco recommends the use of RRM
to control the power level and provide the optimal channel/power plan. For more information, see
.
An antenna gives the wireless system three fundamental properties:
•
Gain—A measure of increase in power introduced by the antenna, over a theoretical antenna that
transmits the RF energy equally in all directions.
transmits the RF energy equally in all directions.
•
Direction—The shape of the antenna transmission pattern. Different antenna types have different
radiation patterns that provide various amounts of gain in different directions.
radiation patterns that provide various amounts of gain in different directions.
•
Polarization—Indicates the direction of the electric field. An RF signal has both an electric and
magnetic field. If the electric field is orientated vertically, the wave is said to be vertically polarized.
magnetic field. If the electric field is orientated vertically, the wave is said to be vertically polarized.
A good analogy for an antenna is the reflector in a flashlight. The reflector concentrates and intensifies
the light beam in a particular direction similar to what a parabolic dish antenna does to an RF source in
a radio system.
the light beam in a particular direction similar to what a parabolic dish antenna does to an RF source in
a radio system.
Gain and direction mandate range, speed, and reliability; polarization affects reliability and isolation of
noise.
noise.
Omni-Directional Antennas
Omni-directional antennas have a different radiation pattern compared to isotropic antennas; the
isotropic antenna is theoretical and all physical antennas are different to the isotropic antenna. The
omni-directional antenna features a radiation pattern that is nearly symmetric about a 360 degree axis in
the horizontal plane, and 75 degrees in the vertical plane (assuming the dipole antenna is standing
vertically). The radiation pattern of an omni-directional antenna generally resembles a donut in shape.
isotropic antenna is theoretical and all physical antennas are different to the isotropic antenna. The
omni-directional antenna features a radiation pattern that is nearly symmetric about a 360 degree axis in
the horizontal plane, and 75 degrees in the vertical plane (assuming the dipole antenna is standing
vertically). The radiation pattern of an omni-directional antenna generally resembles a donut in shape.
Regarding antenna choice, you must consider the RF pattern produced by the antenna because the type
of antenna (omni or directional) affects RF coverage by focusing the bulk of the RF energy in a specific
direction, pattern, and density.
of antenna (omni or directional) affects RF coverage by focusing the bulk of the RF energy in a specific
direction, pattern, and density.
For example, the omni-directional antenna shown in
shows an omni-directional antenna RF
radiation pattern in the vertical and horizontal direction. This is an actual measurement, so it does not
follow the donut lines perfectly, but does show from where this shape comes. As described above, other
RF-affecting variables (people in the room, amount of devices stored in the facility, leaves on trees for
outdoor deployment, interference from different RF sources, and so on) may affect the real RF coverage
pattern.
follow the donut lines perfectly, but does show from where this shape comes. As described above, other
RF-affecting variables (people in the room, amount of devices stored in the facility, leaves on trees for
outdoor deployment, interference from different RF sources, and so on) may affect the real RF coverage
pattern.