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Gain
“Gain” specifies the receive and transmit performance of any antenna compared to a standard
omni-directional 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 Wireless Interactive 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 omni-directional antenna.
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.
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.
Designs such as the semi parabolic offer a choice of polarization. Mounting a semi parabolic antenna
with the elements horizontal provides horizontal polarization, while mounting the antenna with the
elements vertical provides vertical polarization. Similarly, the orientation of the radiating element of
the parabolic antenna determines polarization.
In setting up the Wireless Interactive system, either vertical or horizontal polarization can be used, as
long as polarization is the same at both ends of each link. For any given pair of line-of-sight antennas,
it is essential that they both have the same polarization. Differences in polarization among antennas –
called “cross-polarization” – can reduce signal considerably.
Site Selection
At the high operating frequencies of a Wireless Interactive system, radio waves travel in a nearly
straight line-of-sight path. This is in contrast to the lower-frequency radio waves used for AM
broadcasting. These waves bounce between the ionosphere and the earth’s surface to travel long
distances and operate over and around obstructions. Higher-frequency radio waves do not behave in
this manner and are greatly weakened by substantial obstructions or the absence of a direct path.
Simply put, all antennas communicating with each other in the radio network must be able to
physically “see” each other.
