Afar AR-24010E, Руководство оператора

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pulsAR radio Operator's Manual
Antenna Polarization
The electro-magnetic waves radiated from an antenna are typically “polarized” meaning that the
waves are constrained to oscillate in one specific direction. The most common polarizations are
vertical (the waves oscillate up and down) or horizontal (right and left).
Note that this polarization is independent of the direction of wave propagation. For example the
omni-directional antennas described above radiate the energy along the horizontal plane (waves
propagate preferentially in that plane) but the oscillation of the electro-magnetic field can still be
either vertical or horizontal (polarization).
In the case of directional antennas (yagis, panels, or parabolic reflectors), you can mount the antenna
with either vertical or horizontal polarization by simply rotating the complete antenna assembly by 90
degrees. Most omni-directional antennas on the other hand consist of a vertical radiating element
that can only be mounted for vertical polarization (although, as mentioned above, you can get omni-
directional antennas with horizontal polarization).
It is essential that the two antennas at both ends of one RF link have the same polarization.
Differences in polarization among antennas – called “cross-polarization” – will reduce the signal
strength considerably.
The choice of polarization – horizontal vs. vertical – is in many cases arbitrary. However, interfering
signals from such devices as cellular phones and pagers are generally polarized vertically, and an
excellent means of reducing their effect is to mount your system antennas for horizontal polarization.
6.2 RF Path Analysis
6.2.1 Line-of-Sight Requirements
At UHF and microwave frequencies, when you deploy an RF link between two distant sites you need
to make sure you have "line of sight" between the two antennas. But at these frequencies "line of
sight" does not simply mean that from one site you can "see" the other.
For short ranges, a degree of obstruction may be acceptable. The radio has some ability to “penetrate”
trees and other foliage, specially the 900 MHz models. On the other hand, geographical features
(hills) and large buildings are likely to interfere with communications, and antennas should be
elevated to see each other above such objects.
For links covering very long distances (exceeding 5 miles or 8 km) you also need to take into account
the following factors:

The curvature of the earth.

Fresnel Zone clearance.

Atmospheric refraction.
Figure 6.1 illustrates these concepts with an exaggerated representation of a long link. The following
sections describe these effects. You can use our free “Fresnel Zone Calculator”, shown in figure 6.2,
to make all the computations for the RF path analysis and determine if you have adequate antenna
height for your links. The calculator is available on our website at
http://www.afar.net/fresnel-zone-
calculator/
6-2