Fenny Stratford Repeater Station

What are Repeaters?

Overhead truck telephone routes, Denbigh Railway Bridge, Watling Street, Bletchley, circa 1900.

However, with the increase of telephone traffic at that time, particularly between London and the Midlands, the size and cost of these overhead open wire routes was becoming impractical. Long-distant underground cables started to be laid just before the turn of the century, notably the London-Birmingham No.1, laid between 1897 and 1898. These early cables used conductors of up to 2.5mm in diameter, and were primarily used for telegraph rather than speech. With the smaller conductor size, the attenuation of the speech was greater. Thus, there became a need to overcome this problem, if cables were to be used.

It was a British mathematician, Oliver Heaviside, who, between 1887 and 1890, worked on the theory that adding inductance to cable pairs would reduce the volume-loss of transmitted speech. In some cases, the loss could be reduced to as much as one quarter. However, it was Professor Pupin of America who put Heaviside's ideas into practice. This involved placing coils of wire, wound round soft-iron cores, in the circuits at regular, mathematically calculated distances along the cable. The first cable in Britain to be loaded along its entire length was laid between Manchester and Liverpool in 1910.

In the years running up to the outbreak of the First World War, attempts were made to amplify speech on long-distance telephone circuits. These early attempts were made using mechanically coupled telephone receivers and transmitters. These devices were made notably by S. G. Brown of England and Shreeve of America. The use of these were not very successful. With the development of the triode valve, investigations were carried out to discover if these devices could be used to amplify speech on long-distance telephone circuits. Repeaters employing valves were first used in Britain during 1916 at Liverpool and Birmingham.

An early experimental repeater using a soft valve

These early repeaters used soft valves, known as gas discharge relays, and tended to be unstable. This made their use limited. With the introduction of more stable repeaters in 1918, using hard valves, amplification of long-distance trunk circuits became a commercial proposition using smaller gauge loaded cables with conductors of 0.9mm and 1.27mm in diameter. Consequently, the Post Office proceeded with a network of dedicated repeater stations, initially at Fenny Stratford, Derby and Guildford, with further stations planned for the Northern and Western main trunk routes.