CalAmp Viper 100 User Manual page 26

― Omni Directional Antenna. In general, an Omni directional antenna should be used at a master station and Relay
Points. This allows equal coverage to all of the remote locations. Omni directional antennas are designed to radiate the
RF signal in a 360-degree pattern around the antenna. Short range antennas such as folded dipoles and ground
independent whips are used to radiate the signal in a ball shaped pattern while high gain Omni antennas, such as a
collinear antenna, compress the RF radiation sphere into the horizontal plane to provide a relatively flat disc shaped
pattern that travels further because more of the energy is radiated in the horizontal plane.
― Yagi Antenna. At remote locations (not used as a Relay Point), a directional Yagi is generally recommended to minimize
interference to and from other users.
― Vertical Dipoles. Vertical dipoles are very often mounted in pairs, or sometimes groups of 3 or 4, to achieve even
coverage and to increase gain. The vertical collinear antenna usually consists of several elements stacked one above
the other to achieve similar results.
2.3.4.1 ANTENNA GAIN
Antenna gain is usually measured in comparison to a dipole. A dipole acts much like the filament of a flashlight bulb: it
radiates energy in almost all directions. One bulb like this would provide very dim room lighting. Add a reflector capable of
concentrating all the energy into a narrow angle of radiation and you have a flashlight. Within that bright spot on the wall,
the light might be a thousand times greater than it would be without the reflector. The resulting bulb-reflector combination
has a gain of 1000, or 30 dB, compared to the bulb alone. Gain can be achieved by concentrating the energy both vertically
and horizontally, as in the case of the flashlight and Yagi antenna. Gain can be also be achieved by reducing the vertical
angle of radiation, leaving the horizontal alone. In this case, the antenna will radiate equally in all horizontal directions, but
will take energy that otherwise would have gone skywards and use it to increase the horizontal radiation.
The required antenna impedance is 50 ohms. To reduce potential radio interference, the antenna type and its gain should
be chosen to ensure the effective isotropic radiated power (EIRP) is not more than required for successful communication.
2.3.4.2 FEEDLINE
The choice of feedline should be carefully considered. Poor quality coaxial cables should be avoided, as they will degrade
system performance for both transmission and reception. The cable should be kept as short as possible to minimize signal
loss. See Table 8 for feedline recommendations.
Table 8 – Transmission Loss (per 100 Feet)
Outside cable connections should have a weather kit applied to each connection to prevent moisture. Feedline connections
should be routinely inspected to minimize signal loss through the connection. A 3 dB loss in signal strength due to cable loss
and/or bad connections represents a 50% reduction in signal strength.
Frequency Range
Cable Type VHF
LMR-400 1.5 dB
1/2" Heliax 0.68 dB
7/8" Heliax 0.37 dB
1 5/8" Heliax 0.22 dB
UHF 900 MHz
2.7 dB 3.9 dB
1.51 dB 2.09 dB
0.83 dB 1.18 dB
0.51 dB 0.69 dB
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