Westermo RM-505U-K - Manual (Page 20)

505K Radio Telemetry Module User Manual
    ELPRO Technologies Pty Ltd 2008 Page 20
3.2 Antenna Installation
The 505K module will operate reliably up to 5km. The distance that may be reliably achieved
will vary with each application - depending on the type and location of antennas, the degree of
radio interference, and obstructions (such as hills or trees) to the radio path. Where it is not
possible to achieve reliable communications, a 105U 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 input/output (I/O) signals connected to it and form part of the I/O network.
An antenna must be connected to each 505K module using the SMA connector at the top of
the enclosure. It is important to weatherproof this connection by wrapping with
waterproofing tape (for example 3M 23 tape).
To achieve the maximum transmission distance, the antennas should be raised above
intermediate obstructions so the radio path is true “line of sight”. Because of the curvature of
the earth, each antenna will need to be elevated at least 5 metres above ground for paths of 5
km (3 miles). For shorter distances, the modules will operate reliably with some obstruction
of the radio path. Obstructions which are close to either antenna will have more of a blocking
affect than obstructions in the middle of the radio path. For example, a group of trees around
the antenna is a larger obstruction than a group of trees 100 metres from the antenna.
An antenna should be connected to the module via 50 ohm coaxial cable (eg RG58, RG213 or
Cellfoil) terminated with a male 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. It is important antenna are chosen carefully to avoid contravening the maximum
power limit on the unlicensed channel - normally the net gain of the antenna/cable
configuration should be no more than 2dB.
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 gains and losses of typical antennas are
Antenna Gain (dB)
Dipole with integral 5m cable 0
5dBi Collinear (3dBd) 5
8dBi Collinear (6dBd) 8
3 element Yagi 5
6 element Yagi 10
Cable type Loss (dB per 10 m)
RG58 -5
RG213 -2.5
Cellfoil -3
The net gain of the antenna/cable configuration is determined by adding the antenna gain and
the cable loss. For example, a 3 element Yagi with 10 metres of Cellfoil cable has a net gain
of 2dB (5dB – 3dB).
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 problems
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 vulcanising tape such as