Afar AR24027, Руководство оператора

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AR24027 Operator’s Manual
6-3
In practice, even omnidirectional antennas take advantage of directionality by focusing their
sensitivity and power in the horizontal plane. Rather than waste performance by sending signals into
space or into the ground, the omnidirectional antenna redirects its power and sensitivity from these
directions, increasing performance in the horizontal plane.
In point-to-point applications, where the direction of communication is known and fixed, a highly
focused directional antenna can be used to provide maximum sensitivity and power. In addition,
because of its decreased sensitivity in all directions but the desired one, the directional antenna
improves performance by rejecting signals not coming from the desired direction. This provides an
effective increase in signal-to-noise performance.
A sector antenna has a wider “spread” than a directional (generally between 60 to 120 degrees) which
makes it a cross between an omnidirectional and a directional. This is useful in a point to multipoint
configuration where multiple sites are grouped in the same general area. The installer can then make
use of the higher sensitivity and power but also take advantage of the wider beam pattern and
improved front to back ratio.
6.2.3 Gain
“Gain” specifies the receive and transmit performance of any antenna compared to a theoretical
“isotropic” antenna or “spherical radiator”. The objective of a directional antenna design is to
achieve gain, improving sensitivity and effective radiating power to increase range or data rate.
Gain is measured and stated in decibels, abbreviated dB. The decibel is a unit used to indicate the
relative difference in power between two signals. For example, a signal 3 dB greater than another
signal has twice as much power. The decibel is a logarithmic unit so each doubling of decibels
represents a fourfold increase in power. Since 3 dB represents a doubling of power, 6 dB represents a
fourfold power increase, 12 dB represents a 16-fold increase, etc. For antenna performance, the unit
used is dBi, “i” standing for “isotropic,”.
One type of directional antenna available from AFAR Communications Inc. is called a “semi
parabolic”. This antenna has a gain of 24 dBi, representing power and sensitivity levels 256 times
greater than those of a isotropic antenna.
For omnidirectional coverage from fixed locations, AFAR Communications Inc. provides collinear
antennas. The collinear design achieves gain by increased focus in comparison with the dipole
design. The standard collinear antenna used with the
AR24027 provides 9 dBi gain, representing an
eight-fold power and sensitivity increase.
6.2.4 Polarization
Another important concept for antenna performance is polarization. An antenna radiates radio waves
that vibrate in a specific plane, normally horizontal or vertical. Polarization refers to the restriction of
wave vibration to a single plane.
NOTE
Do not confuse polarization with directionality. The plane of wave
vibration has nothing to do with the direction of wave propagation.
For example, an antenna that focuses its energy in the horizontal
plane may be vertically or horizontally polarized.