User's Manual

SATELLINE-3AS
User Guide, Version 2.2
89
The type of the antenna cable is dependent on the length of the antenna cable, and the
following table can be used to select a suitable type:
Length Type Attenuation 10m/450MHz
<5m RG58 3.0dB
0…20m RG213 1.5dB
>20m NK Cable RFF 1/2"-50 0.7dB
>20m AirCom+ 0.8dB*
*) AirCom+ cable is partially air insulated, so the use of this cable requires that the connection
between the cable and the connectors are fully airtight.
If there is a line-of-sight path between the antennas a 6 dB power marginal is usually adequate.
However, if the connection is built on the reflection and/or the knife-edge diffraction the path
loss can vary even 20 dB depending on the weather conditions. In this case a short test can give
a too positive result of the quality of the connection. Thus the height of the antennas and
topographical obstacles must be surveyed with great care. From time to time a marginal
connection can be used if the data transmission protocol is well prepared for this and the data
transmission that occasionally slows down does not cause any problems to the system.
Vertical polarised antennas (antenna elements are in vertical position) are often used in radio
systems. In a system between a base station and substations vertical polarisation is
recommended. The radio modem antenna can not be mounted on the same level as the other
substation antennas in the same building. The best way to distinguish from the other antennas
situated in the neighbourhood is to mount the antennas as far a part as possible from each
other on the altitude level. The best result is generally obtained when all the antennas are in the
same mast. With an extra ground plane between the antennas more distinction can be obtained
between the antennas in the mast.
Horizontal polarisation can be used in data transmission between two points. With the
polarisation attenuation more distinction is obtained to vertical polarised systems. The influence
of the directional patterns of the antennas must, however, be taken into consideration. If a
distinction to another interfering antenna is wanted with the horizontal polarised antennas there
must be a good attenuation of the back lobe. In addition to this the interfering radiator should
be situated behind the antenna.
When a system does not demand the use of an omni-directional antenna it is recommended that
directional antennas are used e.g. two-element yagis in permanent external installations. Please
note that as antenna amplification increases the setting of the direction of the antenna requires
more attention.
Base stations in high places should be supplied with high-Q cavity filters. Please note that the
higher the base station antenna, the larger the coverage area and in turn that the risk of
interference is also increased.
SATEL recommends the use of a bandpass filter with a large Q in the antenna cable of the base
station.