105 Radio/Serial Telemetry Module
User Manual
Page
10
©
January 2004
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 or RG213)
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. 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 - 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 3m cable
0
Dipole without cable
2
5dBi Collinear(3dBd)
5
8dBi Collinear (6dBd)
8
3 element Yagi
5
6 element Yagi
10
Cable type
Loss (dB per 10 m)
400-500MHz
869MHz
RG58
-3
-5
RG213
-1.5
-2.5
Cellfoil
-1.5
-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 15 metres of RG58 has a net gain of 0.5dB (5dB –
4.5dB) at 450MHz.
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 “3M 23 tape”, and finally with another
layer of PVC UV Stabilised insulating tape. The first layer of tape allows the joint to be easily
inspected when trouble shooting as the vulcanising seal can be easily removed.
Where antennas are mounted on elevated masts, the masts should be effectively earthed to avoid
lightning surges. The 220MHz and 400 – 500MHz radios are fitted with surge protection, however
the 868MHz radio does not. For high lightning risk areas, additional surge suppression devices are
recommended. If the antenna is not already shielded from lightning strike by an adjacent earthed
structure, a lightning rod may be installed above the antenna to provide shielding.
2.2.1
Dipole antenna.
A unity gain dipole is the normal antenna for use on unlicensed channels. As it does not provide
