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Section 5: Coaxial Cable: The Underground Connection

The first major installation of coaxial cable in Australia was in the early 1960s, when the first broadband link joining Sydney, Canberra and Melbourne was constructed. The link opened for traffic in April 1962.

The cable laid between these centres contained six coaxial tubes and 32 pairs of other wires, and had a potential capacity of thousands of simultaneous telephone calls, in addition to being able to relay television programs.

A coaxial tube consists of an outer conductor which is a hollow metal tube, usually copper, about a centimetre in diameter, with an inner conductor, usually a solid wire a few millimetres in diameter, running along inside it. The inner wire is kept centred in the outer tube by means of polythene discs spaced at regular intervals throughout the tube. Apart from the polythene discs, the inner conductor is kept insulated from the outer tube simply by air. Such tubes are called coaxial because the two conductors share the same centre, or axis.

Such a tube can be used to carry electrical signals in exactly the same way as an ordinary pair of wires, with equal but opposite currents flowing in the two conductors. The great advantage of coaxial construction in such a situation, however, is that the outer conductor shields the inner conductor to contain inductance effects. This both means that coaxial tubes can be packed closely together without fear of mutual interference, and that (particularly at high frequencies), there is very little attenuation or loss of signal strength over distance, as energy is not lost readily from the tube because of this shielding effect.

Prior to 1934, coaxial type construction had been used only for submarine cables. These used solid insulating material in order to withstand the great pressures encountered on the ocean floor and had a relatively narrow bandwidth transmission capability. At this time however, research at the Bell Telephone Laboratories showed that air insulated coaxial cables were capable of transmitting a much greater bandwidth such that hundreds of telephone channels could be carried over a single pair of coaxial tubes, using frequency-division multiplexing.

In coaxial systems using frequency-division multiplexing a pair of tubes is required to provide a complete "go and return" circuit, the equivalent of a group of four ordinary wires in a conventional system.

Many tubes can be packed together within one coaxial cable (up to 22 such tubes in some very large systems used overseas), but within Australia, four or six tubes are the most common, with limited use of twelve-tube cables. In the gaps between these tubes can be packed a number of simple wire conductors which can be used to provide communications to smaller towns along the route of the cable, or for control information.

Modern Six Tube Co-Axial Cable
The construction, manufacture and laying of underground cables has made great strides since the early days.
Cut-away view of modern six tube co-axial cable

Like a microwave system, a coaxial cable must have repeaters placed at certain intervals along its route to boost the signal strength. These repeaters must be spaced much more closely than those of microwave systems. In the 1960s, the equipment to pick up the weakened signal from the coaxial cable and boost it before re-transmission was valve-operated and located in small buildings along the route. But in the 1970s, the decreasing size and increasing reliability of electronic components following the invention of transistors enabled most of this equipment to be placed in small underground containers, accessible through a manhole.

In metropolitan areas, coaxial cables are installed in underground ducts. However, in country areas, and over long-distance routes, the cable is buried about a metre deep along its route. The path for the cable is first cleared, to allow access for machinery, and then the earth is broken up by digging equipment, or sometimes by blasting. Large boulders and rocks which could damage the cable are removed.

Care must be taken not to bend the cable too much, or the coaxial tubes could be distorted or pinched. Large bulldozers have been adapted to feed the cable from huge drums down into the trench as it is being ploughed open.

Rather larger excavations have to be made for the underground housings for the repeater equipment, into which the cable is led and jointed. However, some repeaters are still housed in small buildings and any communication lines that have to be split off from the main cable to service towns along the route are separated at such points.

Once the cable is laid, the coaxial tubes are pressurised, so that any hole or break in the tubes causes an escape of air. This both keeps out dust and allows the break to be detected a long distance away, due to the drop in pressure, which can be detected by special instruments.

Lengths of cable are usually planned so that the end of a length will fall at the point where a repeater has to be installed. But lengths still have to be spliced together in the field, and this is a slow, careful process. The exclusion of dust is a high priority, and the joint must be carefully tested.

Once the cable has been laid and buried, the ground along the path of the cable is smoothed over, and conditions returned as closely as possible to the way they were before. Once grass grows back, it is often very difficult to tell where the cable goes.

The extent of coaxial cable routes in Australia is less than the extent of microwave systems. Why is one kind of broadband system chosen over another? There are many factors which come into this decision.

More land has to be acquired for a coaxial cable route, though once work on laying the cable has been completed, land can often be returned to its original use; often cable can be laid alongside highways, on Crown land. However, terrain is an important factor in laying coaxial cable: it is difficult to lay cable over hilly or rough ground, whereas radio repeaters can take advantage of hills, and skip over difficult terrain. Coaxial cable is also vulnerable to being cut by digging or farm machinery.

These factors and many others, are taken into consideration when planning a new broadband link. Generally speaking coaxial cable is provided where terrain is not a problem and where a high capacity link, with frequent "drop-offs" to service intervening towns, is needed. Radio links are used where long distances must be covered with few intervening drop-off points. Of course, all other factors being equal, the relative costs of the systems is the determining factor.
You will notice that there is often the possibility of sending traffic over two alternative paths between two places. For example, between Adelaide and Melbourne via Mildura or via Bordertown; between Tasmania and the mainland via King island or via Flinders Island. There are many other examples.

This ability to route traffic over alternative geographical routes is a key factor in maintaining network security. For example, if the coaxial cable between Melbourne and Sydney was completely broken (perhaps by a bulldozer excavating a dam) then a lot of Melbourne - Sydney circuits and also circuits to Canberra, depending on the location of the break, would be lost. However, communications would still be able to be maintained, despite some likely congestion, because traffic would be routed over the microwave radio system.

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