Afar AR24027, Руководство оператора

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AR24027 Operator’s Manual
6-4
Designs such as the semi parabolic offer a choice of polarization. Mounting a semi parabolic antenna
with the radiating element horizontal provides horizontal polarization, while mounting the antenna
with the radiating elements vertical provides vertical polarization.
In setting up the AR24027 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.
6.3 Site Selection
At the high operating frequencies of the AR24027 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 must be able to physically “see” each other.
For this reason, a proper antenna site must meet the following criteria:
1. For optimum performance at maximum range, there must be a clear line-of-sight path among all
antennas that communicate directly with each other. At shorter ranges, some degree of
obstruction may be tolerated, but performance in the presence of obstruction is difficult to predict.
2. Elevating one or more of the antennas in the system increases maximum line-of-sight range,
called the radio horizon. If antennas are located at a greater range than the ground-level radio
horizon, a means must be available for elevating the antennas.
3. All antennas must be properly oriented, and a directional antenna must be carefully aimed at its
target antenna to ensure communication at maximum range.
4. All antenna cables attenuate (reduce) signal strength in proportion to their length. Since various
cable types offer different attenuation levels, maximum length depends on cable type. Since the
AR24027 is designed for outdoor installation, it can be installed in close proximity to the antenna,
reducing or eliminating cable losses.
Each of these criteria is discussed at greater length in the following paragraphs.
6.3.1 Line-of-Sight Path
Because high-frequency radio waves are attenuated by obstructions, a clear line-of-sight path between
antennas is required for optimum performance at maximum range. For shorter ranges, a degree of
obstruction may be acceptable. For example, at less than maximum ranges the radio has some ability
to “penetrate” trees and other foliage. On the other hand, geographical features (hills) and large
buildings are likely to interfere with communications, and antennas must be elevated to “see” each
other above such objects.
Because of the uncertainties of radio communication, it is difficult to predict the results in conditions
where obstructions exist. The only valid advice is to try the proposed configuration and be prepared
to move or elevate the antennas.