 Metre- and Centimetre-Wave Radio Links between London and Calais
by M.J.L. Pulling, M.A, M.I.E.E. (BBC Television Service) |
It was not very long ago that the use of the BBC's television OB units was limited to
an area within about 25 miles radius of the receiving point at Highgate [near Alexandra Palace, north London],
this being the limiting range of the only two mobile vision transmitters then available. These
transmitters have a radiated power of 1kW and work on a frequency of about 65 MHz.
Their chief drawback is size and weight and the considerable demands which they make
on manpower.
More recently experimental work has been undertaken with transmitters of
much lighter weight and lower power and working on much higher frequencies, with the
object of using two or more in tandem and so extending. the outside broadcast
"catchment area." The particular frequencies used have been in the neighbourhood of
200 MHz and in bands near 5,000MHz and 7,000MHz. Some success was achieved earlier
this year in the use of these bands for outside broadcasts from more distant points;
notably from Southend on May 26th and 29th, and from Trent Bridge, Nottingham, from July
20th to 25th. |
It was decided to mark the centenary of the laying of the first cable across the
Straits of Dover by a television programme from French soil. Calais was an obvious
choice both from the point of view of a programme as well as from its nearness to
England, and the date decided on for the programme was August 27th. (A second
programme was also taken from Calais on August 30th.) This project was clearly more
ambitious than any of the previous ones, and in practice it turned out that four radio
links were needed for the first programme, and a fifth was added for the second
programme. |
As the map indicates, the first link was from Calais to Swingate, on the
cliffs near Dover. At Calais, a microwave transmitter with its paraboloid was installed
at the top of the tower of the Hotel de Ville, operating on a frequency of 4,700 MHz.
At Swingate, the receiving paraboloid was set up on one of the masts of the RAF
radar station at a height of 350 feet above sea level.
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| The First Relay:
The paraboloid for the microwave transmitter mounted on the upper platform of the masts
at the RAF radar station near Dover. The transmitter, working on 6,800MHz, is housed
in the canister at the back of the reflector.
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The output from this was fed in turn to another microwave transmitter, immediately
adjacent, working on 6,800 MHz. The second relay point was established at Warren Street,
near Lenham, where the receiving paraboloid was mounted on the top of a water-tower. For
the third link it had been intended to use a transmitter working on 187 MHz, but trouble
developed on this link a day or two before the first programme and at the last moment
one of the higher power 65 MHz transmitters had to be sent down to take over this
particular link. This was a disappointment, because it had been hoped to demonstrate
a range of this kind (Calais to London) could be spanned by a series of lightweight
transmitters and receivers in tandem.
The third relay point was established at Harvel, near Wrotham, and here again the top of
a water-tower proved a most convenient location for a receiving aerial and for the 4,750
MHz transmitter which was to cover the final link to London.
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| The Third Relay:
Interior of the transmitting van used at Harvel, near Wrotham. The technician is shown adjusting the gear associated with the STC microwave transmitter.
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The receiving point in London was on the top the tower of the London University, Senate
House, in Bloomsbury. This receiving point had been used for previous tests and had been
found to be very satisfactory: so it proved also on this occasion.
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At this point the picture signals were fed to the GPO, Museum Exchange, little more than half a mile away, a normal telephone circuit being used. At Museum Exchange the signals were fed
over the normal route to Broadcasting House and thence to Alexandra Palace. Over the
whole of this part of the route a 1-inch diameter coaxial cable is used.
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At this time all telephone and broadcasting land lines were laid and operated by
the General Post Office (GPO). Later, under privatisation, this operation would be separated
from the Post Office and become British
Telecom (BT).
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The chief novelty and technical interest in these two programmes lies in the linkage by which the picture signals were transmitted from the mobile control room in Calais to the central control room at Alexandra Palace. The total distance is about 100 miles and, at
various stages on their journey the signals were conveyed by almost every means at
present known in the television art - coaxial cable, a normal telephone pair, a radio
link using a frequency near the television broadcast band, and, of course, microwaves.
To this already impressive list was added a further local link in Calais for the second
programme on a frequency of 187 MHz.
A complete television OB unit and its staff were sent over to Calais and good pictures
were obtained with Marconi camera equipment using image orthicon pickup tubes. The
television radio link equipment was supplied by three British companies - Standard
Telephones and Cables (STC), Marconi's, and Pye.
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 | End-of-tube picture taken during the first television transmission from the Continent. |
The communication transmitter-receivers,
which are indispensable for maintaining communication between adjacent stations, were
supplied by Mullard Electronic Products. They were frequency modulated and operated
on 72 MHz. The sound signals were carried by Post Office lines from Calais to London.
The success which attended this enterprise was in large measure due to the quite
remarkable degree of help and co-operation which was received. In France, the civic
authorities in Calais, and officials of the French PTT and of the French Television
Service went out of their way to put at the BBC's disposal every facility that was
needed. On this side of the Channel, the same can be said of the Post Office and the
radio industry, on many members of which abnormal demands were made, often at short notice. |
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