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Where it is not possible to achieve reliable communications between two RM-240 mod-
ules, then a third RM-240 module may be used to receive the message and re-transmit it.
This module is referred to as a repeater. This module may also have a host device con-
nected to it.
The RM-240 unit has two antenna connections at the top of the module, allowing two
antennas to be fitted to the unit. The left connector (looking at the front) labeled “RX”
is connected only to the internal wireless receiver. The right connector labeled TX/RX is
connected to both the transmitter and receiver.
Note: when only one antenna is used, it must be connected to the right TX/RX connector.
Plant and factory installations
Most installations in industrial plants and factories use a single omni-directional antennas.
Installations can suffer from “multi-path fading” effects where multiple reflected radio
signals adversely affect the signal strength. This can be checked by moving the antenna
a short distance (10 cm or 4 inches) – if the signal increases significantly then there are
multi-path effects.
In a “static” installation, where the radio path is not changing, moving an antenna to the
position of maximum signal solves this problem. However where the radio path changes
because the RM-240 is mounted on moving equipment, or if there is moving equipment
in the area, then the solution is to use two antennas. Because the two connectors are
separated, the RF signal at each connector will be different in the presence of multi-path
fading. The RM-240 unit will automatically select the higher RF signal.
Note that directional antennas are not normally used in plant and factory installations.
Line-of-sight installations
In longer line-of-sight installations, the range may be increased by using a high gain
antenna on the TX/RX connector. However the gain should not cause the effective radi-
ated power (ERP) to exceed the permitted value. A second higher gain antenna can be
connected to the RX connector without affecting ERP – this will increase the operating
range provided the background noise in the area is low.
Antennas
Antennas can be either connected directly to the module connectors or connected via
50 ohm coaxial cable (eg RG58 Cellfoil or RG213) terminated with a male SMA coaxial
connector. The higher the antenna is mounted, the greater the transmission range will be,
however as the length of coaxial cable increases so do cable losses.
The net gain of an antenna/cable configuration is the gain of the antenna (in dBi) less the
loss in the coaxial cable (in dB). The maximum net gain of the antenna/cable configuration
connected to the TX/RX connector is 0dB in Europe (100mW ERP). In USA, Canada
and Australia (4W ERP), the maximum gain is 12dB for the RM-240-300 or 16dB for the
RM-240-100. There is no gain restriction for antennas connected to the RX connector.