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Antennas
Overview
Antenna choice is a vital component of any wireless network deployment. Essentially, two broad types
of antenna exist: directional and omni-directional. Each type of antenna has a specific use and is most
beneficial in specific types of deployments. Because antennas distribute RF signal in large lobed
coverage areas determined by antenna design, successful coverage is heavily reliant on antenna choice.
An antenna gives a mesh access point three fundamental properties—gain, directivity, and polarization.
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•
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Antenna Options
A wide variety of antennas are available to provide flexibility when deploying the mesh access points
over various terrains. 5 GHz is used as a backhaul and 2.4 GHz is used for client access.
Table 5
Table 5
Part Number
AIR-ANT2450V-N
AIR-ANT2480V-N
Cisco Aironet 1520, 1130, 1240 Series Wireless Mesh Access Points, Design and Deployment Guide, Release 6.0
OL-20213-01
Gain—A measure of the increase in power. Gain is the amount of increase in energy that an antenna
adds to an RF signal.
Directivity—The shape of the transmission pattern. If the gain of the antenna goes up, the coverage
area decreases. The coverage area or radiation pattern is measured in degrees. These angles are
measured in degrees and are called beamwidths.
Beamwidth is defined as a measure of the ability of an antenna to focus radio signal energy
Note
towards a particular direction in space. Beamwidth is usually expressed in degrees HB being
Horizontal Beamwidth usually the most important one with VB being the Vertical
Beamwidth (up and down) radiation pattern. When viewing an antenna plot or pattern, the
angle is usually measured at half-power (3 dB) points of the main lobe when referenced to
the peak effective radiated power of the main lobe.
An 8 dBi antenna transmits with a horizontal beamwidth of 360 degrees, causing the radio
Note
waves to disperse power in all directions. Therefore, radio waves from an 8 dBi antenna do
not go nearly as far as those sent from a 17 dBi patch antenna (or a third-party dish) that has
a more narrow beamwidth (less than 360 degrees).
Polarization—The orientation of the electric field of the electromagnetic wave through space.
Antennas can either be horizontally or vertically polarized, though other kinds of polarization are
available. Both antennas in a link must have the same polarization to avoid additional unwanted
signal loss. To improve performance, an antenna can sometimes be rotated to alter polarization and
thus reduce interference. A general rule of thumb is that vertical polarization is preferable for
sending RF waves down concrete canyons, and horizontal polarization is generally more preferable
for wide area distribution. Polarization can also be harnessed to optimize for RF bleed-over when
reducing RF energy to adjacent structures is important. Most omni-directional antennas ship with
vertical polarization as their default.
lists the supported external 2.4- and 5-GHz antennas for AP1520s.
External 2.4- and 5-GHz Antennas
Model
2.4-GHz compact omnidirectional
2.4-GHz omnidirectional
Mesh Network Components
Gain (dBi)
1
5
8.0
19
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