User's Manual

Chapter Two Installation
man_905_2.0.doc Page 11
Obstructions which are close to either aerial will have more of a blocking affect than obstructions in
the middle of the radio path. For example, a group of trees around the aerial is a large obstruction,
and the aerial should be raised above the trees. However if there is at least 100 metres of clear path
before a group of trees, the trees will have little affect on the radio path.
An aerial should be connected to the module via 50 ohm coaxial cable (eg RG58 or RG213)
terminated with a male BNC connector. The higher the aerial 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 aerials suitable for use. It is
important aerial are chosen carefully to avoid contravening the maximum power limit on the
unlicensed channel - if in doubt refer to an authorised service provider.
Connections between the aerial 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 aerials are mounted on elevated masts, the masts should be effectively earthed to avoid
lightning surges. Although the 905 module is fitted with surge protection, additional surge
suppression devices are recommended if lightning surge problems are experienced. If the aerial is
not already shielded from lightning strike by an adjacent earthed structure, a lightning rod may be
installed above the aerial to provide shielding.
2.2.1 Dipole aerial.
A unity gain dipole is the normal aerial for use on unlicensed channels. As it does not provide any
gain, then the power transmitted from the aerial will be the same as the power out of the module,
and hence will not exceed the permitted power of the unlicensed channel.
For marginal radio paths, the following lengths are the recommended maximum for the coaxial
cable to the dipole aerial. RG58 -10 metres RG213 - 20 metres. Note that this applies to
marginal paths only - if the radio path has a strong radio signal, then longer lengths of cable ( and
hence more cable loss) can be tolerated. If more than 20 metres of cable is required for a marginal
path installation, then a low loss cable such as 10D-FB, or a higher gain aerial should be used.
Dipole aerials should be mounted vertically, at least 1 metre away from a wall or mast.