User manual

Antenna Feeders :

The basic rule of thumb is the greater the length of cable, the greater the loss. Always attempt to keep

cable runs to a minimum and whenever possible, use the lowest loss cable available. Both cables and

terminations should always be of 50 Ohm impedance. Cable bends should not be too tight i.e. the radius of

the bend should be greater than 10 x the diameter of the cable.

It is important to remember that coaxial cables have losses proportional to their length and quality. The

following table gives loss figures from typical popular 50 Ohm coaxial cables :-

DB Loss per 10m length

Cable Type Dia 100MHz 1000MHZ

RG58 5.0 2.0 7.6

URM67 10.3 0.68 2.52

URM76 5.0 1.6 5.3

Fig. 4 : Cable types

Our modems are normally supplied with N-type terminators (sockets). Other terminations (BNC or TNC)

can be supplied to customer specific orders. Generally, we find the N-Type to be the most reliable, robust

and readily available that can accommodate essentially all cables including low loss URM67.

Lightning Surge Protection :

A direct lightning strike can never be totally protected against or predicted. Currents exceeding 10,000A

can flow vaporising antenna, feeders, towers and other such structures. Lightning conductors will give a

degree of protection to the building but not to the electronic apparatus within.

Generally, the probability of a direct strike is very small, but a nearby strike, for example, within a 1km

radius can be quite a regular occurrence in many locations. Nearby strikes or “strokes” can lead to the

creation of large EM waves that can induce large voltages into antenna, feeders, signal wires and PSUs.

Data cable Coaxial feeder

Systems Ground

Sur

rresto

Data

Terminal

PSU

RTcom

Modem

The best form of protection is to use a surge arrestor. The arrestor connected in series between the

antenna and the modem is intended to safely limit the induced voltage. However, a surge arrestor can only

be effective if the impedance of the cable connecting it to ground is lower or equal to that of the modem

and the other connected apparatus. Secondly, they are only effective if all connections are to a common

Earth point.

Note: Failure to connect the data terminal to the same Earth point compromises the protection of the

apparatus. If, however, is not practical to implement, then a second surge protection devise should be

connected in series with the data and/or power connections, again bonded to the common Earth point.