Elpro Technologies 905U-D User Manual download pdf (Page 12)

905U-D Radio Modem User Manual

Man_905D_1.10.doc Page 12

tolerated for shorter distances. For very short distances, it is possible to mount the antennas

inside buildings. An obstructed path requires testing to determine if the path will be reliable -

refer the section 6 of this manual.

Where it is not possible to achieve reliable communications between two 905U modules, then a

third 905U 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 connected 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 authorised 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

The gains and losses of typical antennas are

Antenna Gain (dB)

Dipole with integral 15’ cable 0

5dBi Collinear (3dBd) 5

8dBi Collinear (6dBd) 8

6 element Yagi 10

9 element Yagi 12

16 element Yagi 15

Cable type Loss (dB per 30 ft / 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 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 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.

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Elpro Technologies 905U-D User Manual Page 12