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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.
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 RF cables attenuate (reduce) signal strength in proportion to their
length. Therefore, the distance between the antenna and the radio is limited to a
cable length that does not exceed the maximum attenuation tolerated by the
system. Since various cable types offer different attenuation levels, maximum
length depends on cable type. Generally speaking, because most Wireless
Interactive systems are outdoor units with the output port connected directly to the
antenna, cable losses are negligible and the radio will compensate, but there are
limits to this compensation. See table 4-2 for sample cables and their respective
attenuation values.
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