805U Radio Modem Module
User Manual
Page
12
©
November 2004
The modules provide test diagnostics to test the radio path and display radio signal strength.
An antenna should be connected to the module via 50 ohm coaxial cable (eg RG58, Cellfoil
or RG213) terminated with a male SMA 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 antennas are chosen carefully to avoid contravening the
maximum power limit on the unlicensed channel - if in doubt refer to an authorized service
provider.
The maximum net gain of the antenna/cable configuration permitted is 0dB.
The gains and losses of typical antennas are
Antenna
Gain (dB)
3dB Collinear
3
6dB Collinear
6
6 element Yagi
10
9 element Yagi
12
16 element Yagi
15
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 6 element Yagi with 20 metres of RG58 has a net gain of 0 dB
(10dB – 10dB).
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 with a layer of PVC insulating tape, then a layer of vulcanizing tape
such as “3M 23 tape”, with a final layer of PVC insulating tape.
Where antennas are mounted on elevated masts, the masts should be effectively earthed to
avoid lightning surges. Although the 805U module is fitted with surge protection, additional
surge suppression devices are recommended if lightning surge problems are experienced. 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
3dB/6dB Collinear antenna.
A collinear antenna transmits the same amount of radio power in all directions - as such that
are easy to install and use.
For marginal radio paths, the following lengths are the
recommended maximum for the coaxial cable to the antenna. 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
