10
6193-3201
In certain circumstances, much longer distances can be achieved by reducing the trans-
mitter power and using higher gain antennas. Although the effective radiated power at the
transmitter end is the same, the additional antenna gain at the receiver gives increased
distance. This is only true for locations of low background noise as the antenna gain will
also increase the noise level. For example, in America where 4W ERP power is permit-
ted, a combination of 0.1W transmitter power and 16dB antenna gain (giving 4W ERP)
can give distances of more than 60 miles (100km). However antennas will need to be
elevated to give line-of-sight. This is a special installation and advice from WESTERMO
should be sought.
Where it is not possible to achieve reliable communications between two RM-90 mod-
ules, then a third RM-90 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.
An antenna should be connected to the module via 50 ohm coaxial cable (eg RG58,
RG213 or Cellfoil) 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. For use on unlicensed frequency channels,
there are several types of antennas suitable for use. It is important antenna are chosen
carefully to avoid contravening the maximum power limit on the unlicensed channel – if
in doubt refer to an authorized service provider.
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 permitted is:
Country
Max. gain (dB)
USA / Canada
6
Australia / New Zealand
0 for 1W transmit power
10 for 0.1W transmit power
The gains and losses of typical antennas are:
Antenna
Gain (dB)
Antenna
Gain (dB)
Dipole with integral 15’ cable
0
6 element Yagi
10
5dBi Collinear (3dBd)
5
9 element Yagi
12
8dBi Collinear (6dBd)
8
16 element Yagi
15
Cable type
Length (m)
Loss (dB)
CC10/900
10
3
CC20/900
20
6
The net gain of the antenna/cable configuration is determined by adding the antenna gain
and the cable loss. For example, a 6 element Yagi with 70 feet (20 metres) of Cellfoil has
a net gain of 4dB (10dB – 6dB).
Connections between the antenna and coaxial cable should be carefully taped to prevent
ingress of moisture. Moisture ingress in the coaxial cable is a common cause for prob-
lems with radio systems, as it greatly increases the radio losses. We recommend that the
connection be taped, firstly with a layer of PVC Tape, then with a vulcanizing tape such as
“3M 23 tape”, and finally with another layer of PVC UV Stabilized insulating tape. The first
layer of tape allows the joint to be easily inspected when trouble shooting as the vulcan-
izing seal can be easily removed.
