SATELLINE-3AS and SATELLINE-3ASd
48
In great distances when the antennas are in optical positions a 6 dB power marginal is adequate.
Since 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 false
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 an attenuating 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 process.
Vertical polarized systems (antenna elements are in vertical position) are often used in radio
systems. In a system between a base station and sub-stations the vertical polarization is generally
recommendable. The antenna of the radio modem can not be mounted on the same level with
the other sub-station antennas in the same building. The best way to distinguish from the other
antennas situated in the neighbourhood, is by mounting the antennas as far 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.
A horizontal polarization can be used in data transmission between two points. With the
polarization attenuation more distinction is obtained in the vertical polarization interference. 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 polarized antennas
there must be a good attenuation of the back lobe. In addtion to this the interfering radiator
should be situated behind the antenna.
When the system does not demand the use of an omnidirectional antenna it is recommended to
use directional antennas e.g. two-element yagis in firm external installations. As the antenna
amplification increases the setting of the direction of the antenna demands for a greater care
The base stations in high places should be supplied with 4...6 degree band-pass filters. Please
note that the higher the antenna the larger the broadcast area. The disadvantages with a too
high antenna installation at the base station are that interferences from a larger area affect the
base station and that the base station occupies the channel of a too large area.
Therefore SATEL recommends the use of bandpass-filters with a high Q-value.
