User's Manual

pulsAR radio Operator’s Manual
6-3
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 a pulsAR radio system, either vertical or horizontal polarization can be used, as long as
polarization is the same at both ends of each link. 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” – can reduce signal 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.
6.1.5 Antenna Orientation
Directional antennas must be carefully oriented towards each other. Orientation of directional
antennas is critical because their sensitivity is greatly reduced outside a fairly narrow angle.
Performance of the system can be seriously degraded by misaligned directional antennas. The pulsAR
radio has a built in feature that allows you to use an audio signal to assist in aligning the antenna.
Refer to section 0 for details.
6.2 RF Path Analysis
6.2.1 Line-of-Sight Requirements
At the high operating frequencies of the pulsAR radio, radio waves travel in a nearly straight-line
path. These frequencies 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 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, specially the 900 MHz
models. 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.
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 runs on a PC and is available on the CD and at our website.