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Antenna Properties and Ratings
An antenna gives the wireless system three fundamental properties—gain, direction, and polarization. Gain is a measure of increase
in power. Direction is the shape of the transmission pattern. The 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 would to a
RF source in a radio system.
Antennas are rated in comparison to isotropic or dipole antennas. An isotropic antenna is a theoretical antenna with a uniform
three-dimensional radiation pattern (similar to a light bulb with no reflector). dBi is used to compare the power level of a given
antenna to the theoretical isotropic antenna. The FCC in the U.S. uses dBi in its calculations. An isotropic antenna is said to have
a power rating of 0 dB, for example, zero gain/loss when compared to itself.
Unlike isotropic antennas, dipole antennas are real antennas (dipole antennas are standard on Aironet Access Points, Base Stations
and Workgroup Bridges). Dipole antennas have a different radiation pattern compared to isotropic antennas. The dipole radiation
pattern is 360 degrees in the horizontal plane and 75 degrees in the vertical plane (assuming the dipole antenna is standing vertically)
and resembles a donut in shape. Because the beam is "slightly" concentrated, dipole antennas have a gain over isotropic antennas
of 2.14 dB in the horizontal plane. Dipole antennas are said to have a gain of 2.14 dBi (in comparison to an isotropic antenna).
Some antennas are rated in comparison to dipole antennas. This is denoted by the suffix dBd. Hence, dipole antennas have a gain
of 0 dBd (= 2.14 dBi).
Note that the majority of documentation refers to dipole antennas as having a gain of 2.2 dBi. The actual figure is 2.14 dBi, but is
often rounded up.
Type of Antennas
Cisco offers several different styles of antennas for use in the 2.4GHz ranges. Every antenna offered for sale has been FCC approved.
Each type of antenna will offer different coverage capabilities. As the gain of an antenna increases, there is some tradeoff to its
coverage area. Usually gain antennas offer longer coverage distances, but only in a certain direction. The radiation patterns below
will help to show the coverage areas of the styles of antennas that Cisco offers, omni-directional, yagis and patch antennas.
Omni-Directional Antennas
An omni-directional antenna is designed to provide a 360 degree radiation pattern. This type of antenna is used when coverage in
all directions from the antenna is required. The standard 2.14 dBi "Rubber Duck" is one style of an omni-directional antenna.
Figure 1 Omni-Directional Antenna
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