WSN Manual
Page 32 of 34
Rev. G
– 4 March 2007
7.4.6
Antenna Mounting
Location:
The antenna should be mounted in a clear area, as far away as possible from
obstructions such as metal constructions, buildings and foliage.
Height:
The WSN operates in the UHF band, which requires near line of sight
communication. Hence, for extended ranges the height of the antenna is
important.
7.4.7
Polarisation
A Yagi or corner reflector antenna can be mounted for vertical or horizontal polarisation.
Scanning systems employing a vertically polarised antenna will necessitate the outstation
antennas to be of the same orientation. In vertical polarisation the elements are perpendicular to
the ground. By mixing polarisation within systems, unwanted signals can be reduced by as
much as 18dB. However, such systems require detailed planning.
7.4.8
Alignment
If a directional antenna is to be used, it will need alignment with the scanner or communicating
station. A map and compass can be used, but the final adjustment should be performed by
measuring the receive signal strength (RSSI) from the scanner, as outlined in the operations
section.
7.4.9
Antenna Coax Feeder:
As with the antenna, the use of the wrong coax feeder can seriously affect the performance of
the system. Hence, the coax cable should be selected to give a low loss over the distance
required. For outstations in the local vicinity of the scanner/ base station, the loss is not very
important but for distant stations the loss is very important. As a rule of thumb, never operate a
system with a loss of more than 3dB.
To illustrate the point, a 3dB loss in the feeder will result in a 50% loss in transmitted RF power
and a 50% reduction in the received signal strength. Therefore, double the received signal
strength will be required for the same bit error rate. Although increasing the RF power will
compensate for the loss in transmitted power, there is no effective way to improve the received
signal strength.
Coax cable should be installed in accordance with the manufacturers’ instructions, with cable
runs kept as short as possible. Sharp bends, kinks and cable strain must be avoided at all
costs. If long term reliability is required, the cable must be securely mounted to avoid excessive
movement and longitudinal strain, due to high winds, rain and snow.
7.4.10 Signal Loss v. Cable Length at 500MHz
Cable Type
Attenuation per 100ft
Attenuation per 100m
RG58
13.0dB
37.0dB
RG213
6.0dB
17.5dB
LDF2-50 3/8inch Foam Heliax
2.44dB
8.0dB
LDF4-50 1/2inch Foam Heliax
1.60dB
5.26dB
LDF5-50 7/8inch Foam Heliax
0.883dB
2.9dB
LDF6-50 1-1/4inch Foam Heliax
0.654dB
2.15dB
LDF7-50 1-5/8inch Foam Heliax
0.547dB
1.79dB
