The following article is from the Trader magazine of 12th November 1949.
G.E.C. Stage Press Show in Co-operation with the G.P.O.
THE radio link between London and Birmingham for the transmission
of television programmes was demonstrated last week to the national and technical
Press by the General Electric Co., Ltd., who have designed and built the link
from first principles according to a stringent specification drawn up by the
General Post Office.
The demonstration was extremely satisfactory, and the link undoubtedly places Great Britain well in the forefront from the point of view of vision distribution. Nowhere else in the world does there exist a television relay system so technically advanced for continuous public service. The many new inventions embodied in the link and the information gained during its construction will be of the greatest value in designing radio links for other telecommunication projects.
The design and development of the link has been carried out under the direction of Dr. D. C. Espley, of the research laboratories of the G.E.C., with the assistance of teams of specialists in the various branches of the project.
The specification was issued at the end of 1946, and in May,
1947, the G.E.C. was awarded the contract by the G.P.O. Since then the design,
development and production teams engaged on it have been working literally day
and night in order to get the link ready in the time specified.
In its simplest terms the London-Birmingham television link may be described as a system of ultra-high-frequency radio transmitters and receivers by means of which the London programmes will be relayed to the Midlands. To begin with, only one-way traffic will be possible, but this will be reversible. During the next six months further equipment will be installed to enable signals to be sent in both directions.
The link comprises six stations — at the Museum Telephone Exchange (London), Harrow Weald, Dunstable, Blackdown (near Charwelton), Rowley Regis (near Birmingham) and finally Telephone House, Birmingham. Each repeater station receives the signal from the previous station, amplifies it and transmits it to the next one. The received signal at each station is about 1µW, and it is amplified and trans- mitted at a power of l0W.
Because U.H.F. waves follow straight lines each station has to be in sight
of those on either side of it; for this reason, they are usually situated on
hill tops, the aerials being mounted at the summit of a tower so as to obtain
extra height. The distance between stations can be anything up to 40 miles,
but owing to the natural contours of the land between London and Birmingham
some of the stations are no more than 10 or 20 miles apart.
In order to avoid the necessity for relay stations of unduly high power, the signals are directed between stations in the form of a narrow beam, on a wavelength of about 80 centimetres with frequency modulation. Only the vision programmes will be sent over the link; the accompanying sound will go over Post Office lines.
The highest aerial towers are at London and Birmingham and are respectively 167ft and 196ft above street level. The directional aerials which are mounted at the top of the towers consist of small rods, no more than 6 inches long, mounted within a paraboloid reflector formed from light alloy tubes. In order to secure a well-defined radio beam these re- flectors, which are 14ft across, have to be con- structed to an accuracy limit of half-an-inch. To prevent their reflecting properties being altered by the formation of ice, electric heating wires are provided inside the tubes.
An important feature of the link is that it is entirely automatic in operation.
All the repeater stations are capable of working almost indefinitely without
attention. Should a fault develop, duplicate equipment automatically comes into
service, and the control engineers in London and Birmingham are informed by
means of signal lights that a fault has occurred and that the stand-by apparatus
is in operation.
The masts so far in use (except at the terminal stations) are temporary ones; in the future self-supporting towers, carrying four paraboloid reflector aerials (two pairs for transmission and reception in both directions) will be erected, and at the top of them will be a 9ft square cabin housing the two receivers and two transmitters, with the same number of stand-by units, making four receivers and four transmitters in all.
It has been decided to place the receivers and transmitters eventually at the tops of the towers in order to obviate the unavoidable losses in receiver and transmitter feeders in each case. These losses are not serious in the 100 mile London to Birmingham run, but for greater distances they become appreciable. Part of the specification was that the link must be suitable for extension up to a distance of 400 miles; this will in fact be the case.
The Trader [magazine] saw the received picture at Telephone House, Birmingham, on November 3dr, when a special programme was trans- mitted by the B.B.C. from Alexandra Palace. The received picture was indistinguishable from that on a B.B.C. monitor in London, full detail being retained, with a complete absence of interference, streaking, reflections, or any other picture defects. It was, in fact, a most impressive demonstration, and congratulations are due to Dr. Espley and the G.E.C. team on a remarkable piece of U.H.F. engineering, bearing in mind also that it is fully automatic, and no engineers are needed at the intermediate stations.
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