#9 Viper_SC_Manual_
001-5008-000_Rev12e.docx |
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A radio modem requires a minimum amount of received RF signal to operate reliably and provide adequate data
throughput. This is the radio’s receiver sensitivity. In most cases, spectrum regulators will define or limit the amount of
signal that can be transmitted and it will be noted on the FCC license. This is the effective isotropic radiated power (EIRP
)
.
Transmitted power decays with distance and other factors as it moves away from the transmitting antenna.
2.3.2
TERRAIN AND SIGNAL STRENGTH
A line of sight path between stations is highly desirable and provides the most reliable communications link in all cases. A
line of sight path can often be achieved by mounting each station antenna on a tower or other elevated structure that
raises it high enough to clear surrounding terrain and other obstructions.
The requirement for a clear transmission path depends on the distance to be covered by the system. If the system is to
cover a limited distance, then some obstructions in the transmission path may be tolerable. For longer-range systems, any
obstruction could compromise the performance of the system, or block transmission entirely.
The signal strength (RSSI) at the receiver must exceed the receiver sensitivity by an amount known as the fade margin to
provide reliable operation under various conditions. Fade margin (expressed in dB) is the maximum tolerable reduction in
received signal strength, which still provides an acceptable signal quality. This compensates for reduced signal strength due
to multi-path, slight antenna movement or changing atmospheric conditions. CalAmp recommends a 20 dB fade margin for
most projects.
2.3.3
RADIO INTERFERENCE
Interference is possible in any radio system. However, since the Viper is designed for use in a licensed system, interference
is less likely because geographic location and existing operating frequencies are normally taken into account when
allocating frequencies.
The risk of interference can be further reduced through prudent system design and configuration. Allow adequate
separation between frequencies and radio systems. Keep the following points in mind when setting up your radio system.
1)
Systems installed in lightly populated areas are least likely to encounter interference, while those in urban and
suburban areas are more likely to be affected by other devices.
2)
Directional antennas should be used at the remote end of the link. They confine the transmission and reception
pattern to a comparatively narrow beam, which minimizes interference to and from stations located outside the
pattern.
3)
If interference is suspected from another system, it may be helpful to use antenna polarization opposite to the
interfering system’s antennas. An additional 20 dB (or more) of attenuation to interference can be achieved by
using opposite antenna polarization.
4)
Check with your CalAmp sales representative or CalAmp Technical Services for additional options. The Technical
Services group has qualified personnel to help resolve your RF issues.
2.3.4
SELECTING ANTENNA AND FEEDLINE
Viper can be used with a variety of antenna types. Viper has been tested and approved with antennas having a maximum
gain of 10 dBi. Refer to
Section 1.5
for a list of tested antenna recommendations. These antennas are FCC approved for use
with Viper. Similar antenna types from other manufacturers are equally acceptable. It is important to follow the
manufacturer’s recommended installation procedures and instructions when mounting any antenna.
