RTcom Radio Modems - Manual (Page 6)

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If there is doubt over the signal reaching the receiver a path survey should be conducted. The outback
modem includes a test mode that places the modems transmitter into constant transmit mode. This
permits the measurement of signal strength at the receiver. Normally our modems will work
satisfactorily with a signal level below 1uV (-107dBm).
INSTALLATION
Power Supplies:
As with any radio communications system, the RTcom modem should be connected to a clean and
stable supply. Switch mode power supplies are rich in RF harmonics and should not be used. Please
note that in many cases as CE sticker on a power supply is insufficient protection against potentially
damaging harmonics. The reality is that the EMC pass limits are set at only -57dBm (316uV), whereas
a radio is quite capable of operation down to an beyond -115 dBm (0.5uV). Hence, a power supply is
capable of legally generating harmonics quite capable of blocking the modems operating channel. As
a word of warning the frequency of the offending harmonics can shift with both ambient temperature,
time and power supply load. Please do not be fooled into thinking that a system fine only to find that
later it fails due to blocking from its own power supply. Hence as a rule of thumb we only recommend
the use of linear power supplies. Similarly where supply interruption is to be protected against, we
recommend the use of float charged (12V or 24V dc) batteries and not switching UPS's.
Location:
For indoor installations we recommend that the modem is located away from sources of heat and
electrical apparatus such as inverters. Care has to be taken to minimise cable lengths both with
respect to the antenna location and the attached terminal equipment. Generally RS232 should be
used for short distance links < 30ft (10m) and RS422 / RS485 for long links of up to 1000ft (300m). In
practice the maximum distance achievable will depend upon the combination of the drive capability of
both the modem and data terminal.
The IP65 rating of the modem enclosure would to many imply that it could be operated outdoors in all
weather conditions. Experience has shown this to be true, but operation can be jeopardised through
long term exposure to rain, frost, direct sunlight, chemicals and blown sand. For these reasons we
also recommend that where possible the modem should be mounted within a second enclosure of a
similar IP rating and where the climate is variable a thermostatically controlled anti-condensation
heater used. This practice has been used by our own field service engineers for many years and has
proven very successful.