The following article is from the Trader magazine of 24th December 1949, which included a major section on the opening of the first provincial television transmitter.
Technical Details of the Station and Auxiliary Equipment
THE Sutton Coldfield television transmitting station is the most powerful of its kind in the world. The vision transmitter operates on a carrier frequency of 61.75Mc/s and its normal peak power output is 35 kilowatts with positive amplitude modulation. The sound transmitter, which is amplitude-modulated, operates on a carrier frequency of 58.25 Mc/s and has a power of 12 kilowatts.
The station will transmit the same programme as the London television station, and the first-class service area of the station is expected to have a radius of about 50 miles. It is estimated that six million potential viewers will be served.
The station is on a 24-acre site adjoining the main Birmingham-Lichfield road, some 10 miles north of Birmingham and 2½ miles north of Sutton Coldfield. The altitude of the station is 550 feet, which, with the 750-foot mast, bring the transmitting aerial to a height of 1,300 feet above sea level.
The mast is 750-feet high and has an all-up weight of 140 tons. It is the tallest mast yet erected at any B.B.C. station.
The base is located by a steel ball in a socket, which forms a pivot to allow angular movement of the mast in high winds. Up to the 610-foot level the cross-section is triangular, each face being 9 feet across. Between 610 feet and 710 feet the cross-section is circular. The eight tiers of four slots in the surface of this part will form an aerial for V.H.F. sound broadcasting, should this system be adopted at Sutton Coldfield in the future. Above the circular section is a short square-section topmast, which supports the television aerial.
The mast is supported by four sets of stays. The factor of safety is four under the most severe conditions, which include a coating of ice ½-in thick throughout, simultaneously with a wind pressure approaching 60 lb/sq ft at the mast-head. A lift inside the mast runs between the ground and the 610-foot level.
Rear view of modulated output stage
The vision transmitter is the most powerful yet built anywhere in the world. It will work at a peak output power of 35kW, which is more than double the power of the vision transmitter at Alexandra Palace.
The transmitter has an overall length of 38 feet and is built in ten cubicles placed side by side. Viewed from the front, the modulator stages are arranged in order of increasing power from left to right, and the radio-frequency stages from right to left. Thus the modulator output stage is next to the final radio-frequency amplifier.
The modulator has four stages, a pre-amplifier, sub-sub-modulator, sub-modulator
and modulator output stage. The radio- frequency section of the transmitter
has a drive unit, crystal controlled, three low-power stages, a driver stage
and the final modulated output stage, consisting of two CAT21 triode valves,
coupled to the feeder through a balance-to-unbalance band-pass circuit.
Vestigial sideband filter, made of
lengths of concentric feeder.
This is connected between the output of the vision transmitter and the feeder
to the aerial. Its purpose is to give the transmission the asymmetric sideband
characteristic that is being adopted for all future B.B.C. television transmitting
stations operating in the band 41Mc/s to 68Mc/s. The filter comprises a high-pass
and a low-pass section, and is constructed of lengths of concentric feeder mounted
on the wall behind the transmitter. The low-pass section is terminated by the
feeder leading to the aerial, and the high-pass section by a water-cooled constant-resistance
absorber load. The lower-frequency sideband is transmitted fully, but the upper
sideband is increasingly attenuated for vision frequencies above 0.75 Mc/s.
High-voltage supplies are obtained from hot-cathode mercury-vapour rectifiers. The phases of the 415-volt A.C. supply to the transmitter are stabilized and, in addition, the high-voltage anode supplies (for which constancy of output is important) are also provided with stabilizers.
The filaments of all the valves are A.C. heated, except those of the CAT21 valves in the modulated output stage, which are supplied with D.C. from a motor-generator.
The 12kW television sound transmitter.
The sound transmitter has an average carrier power of 12 kW and employs high-power
class-B modulation. The drive unit is similar to that for the vision transmitter.
The first three radio-frequency stages are push-pull amplifiers, and the output
stage consists of a single valve in an earthed-grid coaxial-type circuit.
This stage is anode modulated, and its output is coupled to a second concentric
feeder leading to the aerial.
Both transmitters are operated from a single control desk in the control room. Windows between the control room and the transmitter hall afford the engineer on duty a clear view of both transmitters.
The supplies for the vision transmitter are controlled through two pairs of push-buttons, one pair being for the modulator and the other for the radio-frequency stages. Of each pair, one push-button controls the H.T. and bias supiplies, and the other valve cooling equipment and the supplies to thevalve filaments. By operating the push-buttons, power is supplied to each stage of the transmitter in the correct sequence. Facing the desk is a mimic diagram having some 200 lamps that light up in turn as the supplies to each stage are established.
In the centre of the desk is a waveform monitor, and facing the desk is a high-grade picture monitor, on which can be displayed the picture at the modulator input and output and also the picture as radiated.
The control system for the sound transmitter is conventional. The supplies for the various stages are sequence interlocked and switched on and off from the desk.
The vision signal is transmitted from Museum Exchange, London, to Telephone House, Birmingham, over a U.H.F. radio link specially provided for the purpose. A coaxial cable is also being installed so that ultimately it will be possible to send the vision signals either by coaxial cable or by the radio link. In either case the signals are transmitted from Alexandra Palace to Museum Exchange and from Telephone House to the transmitting station at Sutton Coldfield by coaxial cable. Over the coaxial cable the signal is carrier-borne; and it is demodulated by G.P.O. equipment in the line termination room at the transmitting station before being passed to the modulator via the control room.
The lines for the accompanying sound programme and for communication purposes also terminate in this room.
The radio relay link, which beams the visual signal from London to Birmingham on a wavelength of about 33 centimetres, consists of two terminal and four repeater stations, and has unique technical features. It can be reversed to work in the opposite direction so that London could receive television broadcasts from Birmingham, and eventually will be capable of operating in both directions simultaneously.
The repeater stations, each of which amplifies the power of the received signal ten million times, are fully automatic and could be left unattended for months. If a fault should develop, duplicate equipment automatically comes into service and the engineers in London and Birmingham are informed by signal lights of what has happened.
The present reversible system uses two frequencies of transmission: 870 and 890Mc/s. A station which receives on 870 Mc/s transmits on 890 Mc/s and vice versa. These two frequencies are used for either direction of transmission with the reversible link, and will eventually be used for one direction of the two-way link, 917 and 937 Mc/s being used in similar fashion for the other direction.
This room contains a film scanner, by means of which cinematograph films can be televised in the event of a breakdown in the radio or cable link from Alexandra Palace.
A single array radiates the sound and vision signals. It consists of eight vertical folded dipoles arranged in two identical groups placed one above the other and separated by a distance of approximately one wavelength. Each of the four dipoles in the two groups is mounted on one face of the topmast, the dipoles on opposite faces being approximately two-fifths of a wavelength apart.
The dipoles are constructed of galvanized steel strip, 10 inches wide, and incorporate 7½-kW heaters to prevent ice formation.
Concentric feeders having an outer diameter of 5 inches and a characteristic impedance of 51 ohms carry the radio-frequency outputs of the sound and vision transmitters to the top of the cylindrical section of the mast. The feeders are built up from 12-foot sections, with an expansion joint every 150 feet to accommodate changes in length resulting from temperature variations. Dry air is blown continuously through the feeders to prevent condensation.
The feeders terminate in a device known as a "diplexer," which combines the sound and vision radio-frequency signals. The combined signals from the output of the diplexer are taken via a concentric feeder to two unbalance-to-balance transformers at the foot of the topmast, and thence by two pairs of concentric feeders to the aerial itself. One of these pairs of feeders is connected to the north-south dipoles in each tier, and the other to the east-west dipoles.
A special method of feeding the eight dipoles with the vision and sound signals not only increases the power gain of the aerial, but also results in a more constant input impedance over the frequency band. Furthermore, it permits a single aerial to be used for sound and vision with the minimum interaction.
The station draws its supply from the British Electricity Authority at 11kV, 3-phase, 50c/s, over duplicate feeders, which terminate on switchgear in the sub-station. This supply is transformed down to 415 volts and distributed from the low-voltage switchroom in the main building to the transmitters and auxiliary services.
The range within which consistently good reception of the programmes broadcast by the Sutton Coldfield station can be expected depends upon many factors. Chief among these are the presence or absence of high ground between the transmitting station and the receiving point, the height of the receiving aerial, and the amount of electrical interference in the vicinity of the receiver. As all these factors vary widely from place to place, no hard and fast figure can be given for the range, but in general terms it is expected to be about fifty miles.
The vision signals radiated from Sutton Coldfield conform to the standards adopted for the British Television Service. The quality of the pictures transmitted is in no way impaired by the radio and cable links between Alexandra Palace and Sutton Coldfield, and viewers in the Midlands can count upon entertainment in every respect equal to that enjoyed by viewers in the London area.
Civil Engineering and Building
Programme Links (Contractors to G.P.O.)
|The transmitter hall. The sound and vision
room is behind the curved window on the left.
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22nd September 2004