SATELLINE-3AS
User Guide, Version 2.3
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11.2 Radio field strength
Radio signal strength must be good enough for successful data transfer. Where field strength is
above a certain level, the operational results are very good. Below this level, a few dB marginal
areas occur in which errors begin to be generated by noise and interference that will eventually
lead to loss of connection.
The field strength is at its optimum level in open space, although increasing distance will still
reduce it. It must also be remembered that one open space has different environmental and
external factors to another, and that the effects on transmission quality must be taken into
account when planning the system.
Ground, ground contours and buildings cause attenuation (loss of energy through absorption)
and reflection of radio waves. Buildings reflect radio waves and therefore the affects of
attenuation are not as acute when transmission is over a short distance.
However, the reflected waves will often be a bit delayed, and when they combine with the direct
radio waves they interact in either a weakening or a strengthening way. This causes the fading
effect in mobile systems. In reality very sharp signal drops spaced about 35 cm apart may occur.
The attenuation may even reach 40 dB, normally less.
11.3 Remarks concerning the 869 MHz frequency band
According to a recommendation of ETSI, the frequencies range 869.4…869.65 MHz is reserved
for use of license free radio appliances. The application of this recommendation varies in each
country, and for this reason local regulations concerning this frequency range must always be
checked.
In frequency range 869.4…869.65 MHz, the maximum allowed radiated power of an antenna
is 500 mW erp (+27dBm). When calculating the power antenna cable attenuation and antenna
gain must be taken into account. For example, if the gain of the antenna is 10 dBd and the
attenuation of the antenna cable used is –3dB, the maximum allowed transmission power is 100
mW (+20 dBm). It should be noted that by increasing the gain of the antennas, connection
distance could be increased. This is due to the fact that the output power of the transmission
remains constant, but the added antenna gain of the receiving end will enable reception of
weaker signals than otherwise possible.
System designers must also take into account that at the frequency range of 869.4…869.65
MHz the transmitter is allowed to be ON only 10% of the time. Whether this limit is exceeded or
not, depends on the protocol used. At a frequency range of 869 MHz there are in addition to
the frequency channel 869.4…869.65 MHz also other ranges, but at these ranges the
maximum allowed radiated power is 25 mW and the transmitter is allowed to be ON only 1 %
or 0.1 % of the time.
