User guide

SATELLINE-3AS NMS / NMS Epic / NMS 869 / VHF
User Guide, Version 1.0
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and transmits too much data to the radio modem, the buffer will be cleared and the
transmission is restarted.
In the receiving mode, the data coming from the radio is also buffered thus evening out
differences in data transfer speeds at the serial ports.
If the terminal device transmits data to a radio modem in receiving mode, the data will go into
transmit buffer memory. Transmission will start immediately when the radio channel is free,
depending on the RX/TX priority setting of the radio modem:
a) TX-priority ON, immediately start transmitting the data sent by the DTE or
b) RX-priority ON, continue in receiving mode and buffer the data sent by the DTE. The
buffered data will be transmitted when the radio channel is free (when the modem is not
in the receiving state).
Terminal equipment can be also connected to the serial port of the repeater station. If the radio
modem is already receiving a message to be forwarded, the data coming simultaneously from
the serial port will be buffered until the store-and-forward procedure has been completed.
8.7
Factors affecting to the quality/distance of the radio
connection
o Power of the radio transmitter
o Sensitivity of the radio receiver
o Tolerance of spurious radiation’s of the radio modulating signal
o Gain of transmitting and receiving antennas
o Antenna cable attenuation
o Antenna height
o Natural obstacles
o Interference caused by other electrical equipment
The transmitter power of the base model of SATELLINE-3AS NMS is 1 W (maximum) and the
sensitivity of the receiver is typically better than -115 dBm. Thus in a flat area and in free space
with a 1/4 wave antenna (with antenna gain of 1dBi) and antenna height of 1 m communication
distances app. 3 to 4 km can be achieved. Distances may be considerably shorter in situations
where there are metallic walls or other material inhibiting the propagation of radio waves.
Over long distances, increasing antenna height can often solve problems caused by natural
obstacles. A ten-fold increase in distance can be achieved by using antennas with higher gain.
Frequent topographical variations over long distances may require that at least one of the
antennas be raised to a height of 10 to 20 m.
If the antenna cable is more than 10 meters long it is necessary to use a low loss cable (< 0.7
dB /10 m) in order not to waste the antenna gain. Adding a repeater station can also solve
problematic radio connections. In systems with many base stations the RSSI signal can be used
to assist in choosing the base station with the best signal. A communications network can also
be built with a combination of cables and radio data modems.