User's Manual

AR24027 Operator’s Manual (rev A)
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3.2.1 Directionality
An antenna may be designed to receive and transmit in all directions. Such antennas are
omnidirectional. An example of an omnidirectional receiving antenna would be a television antenna
in a metropolitan area where each television station transmits its signal from a different location
relative to the receiver. Similarly, a centrally located television transmitter would use an
omnidirectional transmitting antenna.
The sensitivity and power of an omnidirectional antenna are unfocused; that is, they are spread
through a wide volume of space, so the advantage of being able to communicate in all directions is
traded off for limited sensitivity and power.
If it is determined that all signals of interest are coming from a definable direction, the
omnidirectional antenna can be replaced by a directional or sectoral antenna, which increases
sensitivity and power by focusing the beam in the desired direction.
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 horizontal 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.
3.2.2 Gain
“Gain” specifies the receive and transmit performance of any antenna compared to a standard
omnidirectional antenna (“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,” which describes the standard spherical radiation pattern.
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 standard omnidirectional antenna.