Guardian Manual 001-5006-000 Rev 0
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2.3.2
SITE SURVEY
A Site Survey is an RF propagation study of the RF path between two points or between one point and multiple
points. UHF radio signals travel primarily by line of sight and obstructions between the sending and receiving
stations will affect system performance. Signal propagation is also affected by attenuation from obstructions such
as terrain, foliage, or buildings in the transmission path. A Site Survey is recommended for most projects to
determine the optimal RF paths for each link. This is especially true when more than one RF coverage area is
required. A Site Survey will determine the best unit location for the Relay Points.
2.4
SELECTING ANTENNA AND FEEDLINE
The Guardian can be used with a variety of antenna types. The exact style used depends on the physical size and
layout of a system. The Guardian device has been tested and approved with antennas having a maximum gain of
10 dBi.
2.4.1
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.
See
Table 1-7
for a list of tested antenna recommendations. These antennas are FCC approved for use with the
Guardian. 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.
2.4.2
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.
