1970 Coaxial Cable Page
Description of the STC 1.47” A Armour Cable
This cable is a coaxial design with an inner and outer conductor and was introduced into production in 1970. The centre of the cable comprises three layers of high tensile steel wires laid up in opposite directions around a central king wire. This central core provides the main tensile strength of the cable and the strand wires are laid up in opposite directions so that the final cable is torsion balanced and easy to handle. Around the central strand is a longitudinally wrapped ‘C’ tube of copper, which is welded along its overlapping seam. This copper tube forms the inner conductor. The copper tube is covered with a layer of Medium Density Polyethylene (MDPE), making the overall diameter of this section 1.47 inches (33.4mm). Around the MDPE is a longitudinally wrapped ‘C’ tube, the overlap of which is separated by a Polyvinyl Chloride (PVC) tape. This tube forms the outer conductor. In the picture the outer conductor is made of copper, but as a cost-saving measure, aluminium was used in later production. The outer conductor is covered in a layer of High Density Polyethylene (HDPE). This completes what is known as the Lightweight (LW) form of the cable.
LW was deployed in water depths between 1,000m and 8,000m, and in shallower water additional external protection was added to the cable in the form of steel wires wrapped helically around the LW. Different levels of protection were provided for different water depths. Typically, the deployment was as follows: From the beach to 50m water depth Double Armour (DA); from 50m down to 200m Single Armour Heavy (SAH); from 200m to 500m Single Armour Medium (SAM); and from 500m to 1,000m Single Armour Light (SAL). The level of protection was identified by the following designations, which specified the number and diameter of the armour wires: DA = AB, SAH = A, SAM = B and SAL = C. As water depths increased the diameter of the armour wires was reduced and the number of wires increased. The need for external protection diminished as the water depth increased and each step reduced the cable’s strength and its weight in water. Originally the armour wires were made of mild steel but, for later production, high tensile steel wires were introduced, increased tensile performance for the same weight in water.
The photograph shows an A-armour cable. A helically-lapped layer of hemp impregnated with a coal tar compound was applied over the HDPE sheath to provide a bedding layer for the armour wires. Over the hemp layer, twenty-four, 0.315 inch (8.0MM) armour wires were laid up helically, with a left hand lay to aid clockwise coiling in cable tanks, and with a lay length of 33 inches (84cm). The armour wires were coated with coal tar compound during the laying up, and the wires were then overlaid with two layers of coal tar impregnated hemp serving laid up in opposite directions. In later production, the hemp was replaced with polypropylene serving.
The History of Submarine Telephone Cable Manufacture on the Greenwich Peninsula.
Submarine telegraphy enjoyed a monopoly for long distance telecommunication during the reign of Queen Victoria and into the early part of the 20th century. After the First World War, submarine cable telegraphy came under threat from the new radio telegraph technology of Guglielmo Marconi (1874-1937), although submarine cables remained the medium of choice for business, governments and the military because of the greater level of security they provided. In the 1920s the submarine cables’ overall market share of international telegraph traffic continued to dwindle as cheap radio telegrams opened up the market to the general public. By this time there were only two manufacturers of submarine cables in the UK, the Telegraph Construction & Maintenance Company (Telcon) at Enderby Wharf, and Telcon’s long-standing competitor Siemens Brothers, with a factory downriver at Woolwich. The only foreign competition at that time was a French company, La Société Industrielle des Téléphones, which began cable production in Calais in 1891. With the decline in the market for new submarine telegraph cables, Telcon had diversified into other products, as had Siemens Brothers, so in 1935 they agreed to merge their submarine cable divisions and form a new joint venture company, Submarine Cables Limited (SCL). It was agreed that the Greenwich site of Telcon, rather than the Siemens factory at Woolwich, would become the manufacturing base and headquarters of SCL, which then became the sole British supplier of submarine cables.
Telcon had manufactured and installed its first submarine telephone cables in 1896, by which time it had closed the Morden Wharf factory and all production had been consolidated on the Enderby Wharf site. However, these cables were based on telegraph cable designs and were limited in transmission distance by the high capacitance of the gutta percha insulation. To overcome the capacitance problem, two technological breakthroughs were required. The first was the research of British physicist and engineer Oliver Heaviside (1850-1925) into the ‘skin effect’ of telegraph signals, leading to his patent of the coaxial cable in 1880.
Telcon was granted a patent for a subsea telegraph cable with a helically-wrapped copper outer conductor in 1895. However, this idea was not exploited until 1921, when Telcon made three coaxial cables and laid them between Havana, the capital of Cuba, and Key West in Florida.
Before the advent of coaxial cables, the problem of cable capacitance was overcome by a technique known as ‘loading’, which involved the inclusion in the cable construction of an alloy tape with special magnetic properties. Telcon supplied the first cross-channel telephone cable of this type in 1912. This same technique was also applied to telegraph cables. In 1924, Telcon supplied the Western Union Telegraph Company, the predominant US telegraph operator, with a transatlantic cable between New York and Horta in the Azores, capable of a transmission rate of 1,500 words a minute. Further developments involving the use of Mumetal as part of the cable structure increased this capability to 3,000 words a minute by 1928.
The second technological breakthrough came in 1933, when scientists at the Northwich, Cheshire, laboratories of Imperial Chemical Industries Ltd (ICI) discovered polyethylene, solving the problem of high capacitance in gutta percha insulated cables. This material had a lower dielectric constant than gutta percha; it was tougher, more easily processed, non-hygroscopic, and most importantly, cheaper to manufacture.
Polyethylene became available for experimental cable manufacture in 1938, but its use was restricted to military cables during the Second World War. The first coaxial submarine cable to use polyethylene was made by SCL and was laid across the English Channel between Cuckmere and Dieppe in 1945.
The French manufacturing capability in Calais, established in 1891, had been destroyed during the Second World War. The manufacture of submarine cables in Japan, which had started in 1915, had grown steadily, based on the needs of its domestic market. In 1941, a major factory had been opened by the Nippon Submarine Cable Company, but as a result of the Second World War, this facility was also out of action. The Americas had yet to fully enter the industry and so, once again, submarine cable manufacture was a British monopoly.
Low-loss polyethylene cables allowed submarine telephony over medium distances, but to cross oceans, amplification of the signal was essential. The idea of including housings in the submarine cable had first been patented in 1865, but the technology had not been forthcoming. To insert an amplifier into a subsea housing raised a number of major technical problems, such as how to enclose the amplifier in a water-tight casing but still get access to the transmission path, how to integrate the housing into the cable, how to provide power to the amplifier, and, because it had to be based on vacuum tubes (valves), how to dissipate the heat. Most importantly, all the amplifiers had to be reliable, so that they would not have to be recovered and replaced. These problems took time to resolve, but the first submerged amplifier, in a specially designed housing, was introduced into the already-laid Anglesey to Port Erin coaxial cable in 1943.
Recovered 1943 Repeater
By 1947 a new coaxial system containing a submerged amplifier had been laid between the United Kingdom and Germany, and shortly afterwards other systems followed to the Netherlands and Denmark. Because the North Sea was relatively shallow, the effects of hydrostatic pressure were not overly significant. However, to lay a system across the Atlantic presented far greater problems.
1947 SCL 1.7” Coaxial Cable Sample
In the USA, Bell Laboratories developed a flexible repeater housing that could be laid by passing it through the standard cable-laying machinery of the time, and which contained a unidirectional valve amplifier. The amplifier was powered by direct current sent down the cable. Design work was completed by 1941 but the Second World War intervened, and a full-scale trial of this repeater design was not conducted until 1950, when twin coaxial cables were laid between Havana and Key West. The cable was manufactured by the Simplex Wire and Cable Company in the United States, and this marked the start of significant US involvement in subsea cable manufacture. Each cable contained three flexible-housing amplifiers manufactured by Western Union, and between them the two cables provided 24 voice channels. The installation of this system is generally accepted as the event that marked the beginning of the submarine cable Telephone Era.
In parallel with submerged amplifier development, the ailing subsea telegraph industry was looking for ways to improve the performance of its cables. Western Union developed a submerged housing that contained circuitry to detect incoming telegraph signals and regenerate or ‘repeat’ them. The first of these regenerators was successfully inserted into the 1881 transatlantic American Telegraph cable in 1950. Over the next decade, several more of these devices were inserted into existing subsea telegraph systems. Western Union named these devices ‘submerged repeaters’, and although no new systems were ever installed that included these housings, for some reason, the name for the watertight housing survived the demise of the technology.
In Britain, from 1952 onwards the General Post Office (GPO) and Cable & Wireless led a design and development programme in collaboration with SCL and new entrant Standard Telephones & Cables (STC), which was based at North Woolwich next to the old W T Henley factory. This programme resulted in an in-line, rigid housing repeater design which had room inside it for filters that allowed bi-directional transmission over a single cable. The British repeater could also provide up to 60 x 4 kHz voice circuits, over twice the capacity of the United States design and, being bi-directional, it only required a single cable. However, because of the rigid housing it would not pass through the shipboard cable machinery used to deploy the cable.
Laying a Rigid Housing Repeater
(Courtesy of ASN)
The first transatlantic submarine telephone cable was TAT-1, laid in 1955-56, which was capable of 36 simultaneous phone calls, much more capacity than the restricted radio-telephone service could offer. Once again submarine cable technology was in the ascendancy. TAT-1 comprised two cables using the American-designed unidirectional valve amplifier repeaters and was laid between Oban in Scotland and Clarenville in Newfoundland, Canada. For this system, SCL made 7,739km of the cable at its Greenwich and Erith factories. Only 616km was made in the USA, by Simplex Wire and Cable Co. TAT-1 went into service in 1956, almost a century after the first successful transatlantic submarine telegraph cable.
C S Monarch (4) Loading TAT-1 at SCL, Erith
(Courtesy of ASN)
Some of the cable was made at Enderby Wharf and to accommodate the loading of this new coaxial design a gantry and cable hauler was installed on the jetty, across the river path from the sculptures. However, the majority of the cable was made at SCL’s new factory at Erith, several miles downriver from Greenwich.
The British joint development team continued to work with the rigid housing repeaters, and these were deployed on the first transatlantic cable between Britain and Canada, CANTAT, installed in 1961. The British in-line rigid repeater housing became, and still is, the de facto industry standard.
CANTAT was the first system to use Lightweight (LW) cable in deep water (>1000 m); this type of cable had the strength member in the centre of the cable structure and no external protection. Until then, the strength of the cable had always been provided by external armour wires, but due to the mostly benign nature of the seabed in very deep water, external protection was unnecessary. This design had first been proposed by the GPO in 1951 and it was included in the joint development programme. Prototype cables were successfully manufactured and trialled by SCL in 1956 and STC in 1958, STC having opened its own cable manufacturing facility in Southampton Docks in 1956. At the same time to accommodate this new LW cable design, SCL upgraded the cable loading equipment on the main Enderby Wharf Jetty, this included improvements to the gantry that supported the catenary of pullies which carried the cable from the factory to the cableship, moored in the river, and the cable hauler that pulled the cable out of the factory cable tanks. They can still be seen on the jetty today.
Cable Gantry and Hauler in 2000
(Courtesy of ASN)
The diameter of this coaxial cable design was 0.990 inches (25.1mm), and it was believed that supporting the weight of the repeater housing in the catenary to the seabed, in deep water, would cause excessive strain on the cable or cause cable runaway, so a method was needed to relieve this additional strain. The answer was to attach parachutes to the repeaters when they were deployed, the theory being that the parachute would open in the water column and bear some of the weight of the repeater during its descent to the seabed. These parachutes, made of silk, were successfully trialled in Loch Fyne in 1960, and were used on all British manufactured repeaters deployed in deep water until the development of the new, stronger, 1.47” (33.7mm) coaxial cable design, in 1968. After this design went into production in 1970, the parachutes were abandoned.
STC took over SCL in 1970, thus becoming the sole supplier of submarine systems in the UK. It established a subsidiary company, STC Submarine Systems, with its headquarters on the Enderby Wharf site. STC closed its North Woolwich factory in 1976, and during this same period the SCL factory at Erith was also shut down. After 118 years, production of submarine cable on the Enderby Wharf site ceased in 1975, and all cable manufacture was transferred to Southampton. The Greenwich site then focused on marketing, project management, engineering, and the manufacture of repeaters, power feed and transmission equipment. The last cableship to be loaded at Greenwich was the C S John W Mackay, which in 1976 installed the APNG cable system between Cairns, Australia and Port Moresby, Papua New Guinea.
CS John W Mackay moored off Enderby Wharf
From 1970, silicon transistors began to replace valves in repeaters, reducing line current and allowing the design of amplifiers with much wider bandwidth. The voice channel carrying capability of submarine telephone cables was gradually increased, and it peaked at 5,680 x 4kHz channels with the PENCAN 3 system between mainland Spain and Gran Canaria in the Canary Islands. This system was designed, manufactured and installed by STC Submarine Systems in 1977. Unfortunately, the increase in capacity could only be achieved by bringing the repeaters closer together, which significantly increased the cost of the systems. For PENCAN 3, repeater spacing had to be reduced to just 2.7nm (5.01km).
The launch of communications satellites Telstar and Relay in 1962 was the start of real competition for long-distance telephony. Then in 1963 the first successful geosynchronous communication satellite, Syncom II, was launched. Despite the fact that submarine cables offered a better quality of service, with no perceivable delay or echo and far better security, satellites offered more voice channel capacity and a cheaper service. This put pressure on the submarine cable industry, and by the mid-1970s satellite systems had become the dominant service for transoceanic transmission.
Despite competition from manufacturers in France, Japan and the USA, STC Submarine Systems remained the leading supplier of submarine cable systems during the telephone era and supplied the last coaxial telephone system to be installed, India to the United Arab Emirates, in 1986. The repeaters, power feed and transmission equipment were all manufactured on the Enderby Wharf site. This marked the end of the Telephone Era and the advent of the Optical Era.
For information about cable from the Telegraph Era see the tile to the left. (Click Here)
For information about cable from the Optical Era see the next tile to right. (Click Here)
A more detailed history of cable manufacturing on the Enderby Wharf site go to: