Churchill Nano Link IP67, Техническое руководство, Страница: 7 / 42

Page 5
2.2 Network Communications
2.2.1 Radio
Both
Micro_Link
and
Nano_Link
can be equipped with synthesised VHF or UHF transceivers which are approved to
ETSI standard EN 300 220-1 and can thus be used so can be used in all the European Community member states
(subject to national requirements). In the UK the de-regulated bands are designated MPT1328 (VHF) or MPT1329
(UHF) bands. Any of 32 channels can be selected to avoid conflict with other users.
The UHF MPT1329 band is the most popular, since it allows for transmit powers of up to 500mW ERP (Effective
Radiated Power), but doesn’t require a licence. There are 32 channels at 12.5KHz spacing, in the band 458.500MHz to
458.925MHz.
The radio range depends on the aerials used and the topography of the area, but will typically be up to 8Km in urban
environments and up to 25Km with elevated aerials.
There are two adjustments on the radio modules, Modulation Level and Transmit Power, both of which are factory set:
The Modulation Level trimmer sets the transmitter frequency deviation, and should never be changed.
The Transmit Power trimmer sets the output power in the range 20mW to 1W, and is pre-set to 500mW
. It can only be
changed by qualified personnel equipped with a calibrated UHF power level meter
. The output level can be set below
500mW to minimise power consumption and reduce potential interference with other users. It must be set below
500mW if the aerial has gain. It can only be set above 500mW if the aerial and/or aerial feeder has an overall gain of
less than unity, and then only to compensate for the losses.
The ERP must never be set to more that 500mW when
using MPT1329.
Other versions can be supplied to operate within the regulated MPT1411 band of 457.5MHz to 458.5MHz and
463.0MHz to 464MHz at power levels up to 5W, or the unregulated MPT1328 band of 173.20MHz to 173.35MHz at a
power level of 10mW.
2.2.2 Leased Line
Nano_Link
can alternatively be equipped with modems for use on leased telephone lines supplied by national network
providers such as BT and Mercury. The modems are approved for use through the European Union.
The modems are capable of operating with an end-to-end loss of up to 27dB, and provide correct impedance matching
in accordance with the regulations. Normal telephone wires have a loss of about 1.5dB per Km, giving a range of
typically 18Km. However, if more than one outstation is used, the line becomes mismatched, thus increasing the loss
and effectively reducing the maximum line length. This can be overcome by adding line amplification and/or
impedance matching pads. The network provider will normally arrange this.
2.2.3 Private Wire
Private wires are cable pairs similar to leased lines, but owned by the user. The user is thus responsible for
maintenance of the cable, but does not have to pay rental charges.
Over short distance the characteristics of the cable are unimportant. However, for distances greater than around 2Km
the cable should be matched to the terminating impedance of the modems, namely 600

. The modems are capable of
operating with an end-to-end loss of up to 27dB. Normal telephone wires have a loss of about 1.5dB per Km, giving a
range of typically 18Km. However, if more than one outstation is used, the line would become mismatched, due to the
additional load impedance. This increases the loss and effectively reduces the maximum allowable line length. A high
impedance version of the modem is available to overcome this and allow large numbers of outstations to be connected
across a single pair of wires, with appropriate terminations at each end of the cable.