SATELLINE-2ASxE
User Guide, Version 2.0
41
When ordering antennas please note that the antennas are tuned to a certain frequency range.
Simple antennas and those made of stacked yagis have a relatively wide bandwidth. The frequency
range of an antenna becomes narrower the more elements there are in the antenna.
Keeping in mind the testing and service of a system it is generally more practical to use a long
antenna cable instead of installing the radio modems into a place which is difficult to access.
The antenna cable should be chosen according to the length bearing in mind the following
recommendations:
Length
Type
Attenuation
< 5 m
RG58
3.0 dB/10 m/450 MHz
5 ... 20 m
RG213
1.5 dB/10 m/450 MHz
> 20 m
Nokia RFX 1/2"-50
0.5 dB/10 m/450 MHz
> 20 m
0.8 dB/10 m/450 MHz *)
*) cable is partly air insulated, thus an absolutely air tight connection between the cable
and the connector is required.
If there is a line-of-sight path between the antennas a 6 dB power marginal is usually adequate.
Instead, 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 polarized antennas
(antenna elements are in vertical position) are often used in radio
systems. In a system between a base station and substations the vertical polarization is generally
recommendable. The antenna of the radio modem can not be mounted on the same level with 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 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.