From the Droitwich transmitting station of the British Broadcasting Corporation there are radiated simultaneously two separate programmes—National and Midland Regional, on long and medium wavelengths respectively. The long-wave National transmitter serves nearly the whole of the British Isles
WHEN A VALVE BURNS OUT in the long-wave transmitter, the replacement is expedited by mounting the new valve in a rubber tyred trolley which protects the delicate apparatus from damage in transit. Entrance to the inside of a transmitter unit to change a valve would automatically cut off the power if this had not previously been done.
STANDING alone in the wide fields of Worcestershire, Droitwich broadcasting station appears to be aloof from the busy centres of population. That appearance is deceptive, for the site of the station and its chief technical characteristics were determined by the distribution and density of population of the British Isles.
Among the transmitting stations of the British Broadcasting Corporation Droitwich holds a place of unique importance. The B.B.C. planned to serve every potential listener with a choice of two distinct programmes. The main broadcasting stations were therefore designed to be of the twin-wave type, having separate transmitters to radiate respectively the National and the Regional programmes. Except at Droitwich the Regional is the more important “twin”, with the better wavelength and using superior power.
At Droitwich, however, the relative positions are reversed. Its central position qualifies Droitwich to be the truly National station, holding the key position, and housing a National transmitter superior to all Regional standards.
This superiority is twofold. Droitwich National transmitter has about three times as much power as any of the Regional transmitters, and it works on a “long” wavelength (1,500 metres), which gives it a much greater range for reliable reception than can be achieved by any of the Regionals.
The various stages in the working of Droitwich, or of any broadcasting station, are best understood by contrast with those of a radio receiver. Attached to the receiver is some form of aerial wire, in which innumerable electric currents are always flowing.
These currents are of a special type, discovered only within the last few decades. One of their characteristics is that they flow up and down the receiving aerial at amazingly high frequencies, a different frequency being used for each broadcasting station. (The ordinary alternating current used for domestic lighting has a frequency of about fifty cycles a second, but in the radio aerial a frequency of more than one million cycles a second is not unusual.)
Another feature of the currents in the radio receiver’s aerial is that they are extremely weak, and can be measured only by expensive and delicate instruments. Yet, weak as they are, each one continually changes its strength in conformity with a certain complicated “pattern” imposed upon it. This is a pattern of sound, bearing all the electrical equivalents of speech or music.
In the receiving aerial we have these innumerable high-frequency currents from different broadcasting stations, each modulated with its own distinctive pattern of sounds. Tuning the receiver enables it to select one of these high-frequency transmissions and
to discard the others. A point of great importance is that each broadcasting station must adhere rigorously to its allotted frequency; any alteration to this at the transmitter would throw all the receivers out of tune.
THE LONG-WAVE TRANSMITTER at Droitwich is mounted along one side of the transmitting hall, facing the Midland Regional transmitter, on the opposite side of the hall. The arrangement of the units on a gallery enables the filament-current machines to be housed immediately below their respective valve units.
After selection by tuning, the received current is amplified until it is perhaps millions of times stronger than in the receiving aerial. This high magnification must be achieved without altering the electrical characteristics of the received current, which remains of the same type as the original current in the aerial. The process is known as high-frequency amplification. When sufficient high-frequency amplification has been obtained in the receiving set, the detector comes into action. Its task is to isolate the sound pattern.
Sound is not present acoustically, but is in the form of low-frequency variations of the high-frequency current. It is the detector’s function to suppress the high-frequency current, and to pas? on only the low-frequency variations. They are the electrical equivalents of the sounds that it is desired to receive.
After further strengthening, known this time as low-frequency amplification, they are able to operate a loudspeaker. The diaphragm of the loudspeaker is caused to vibrate, and to set up complex movements of the air that correspond exactly with the low-frequency currents. The listener hears sounds, and says he is receiving them from the selected broadcasting station.
He is — in one sense. For the radio engineer has enabled the. sounds in a distant studio to be recreated in replica instantaneously, in millions of homes. In that modern miracle the major part is played by the broadcasting station. Although its power is high, the outstanding feature of a broadcasting station is the delicacy of its control. The aerial power of Droitwich National is 150 kilowatts, but it can be controlled by a whisper. Its tens of thousands of volts, that would be fatal to any unwary intruder, are held in leash by the notes of a piano, or follow faithfully in every particular the movements of the strings of a violin.
It is this delicacy of control that makes the broadcasting station so noteworthy an achievement of electrical engineering. In contrast to an ordinary power station providing electric light and heat, Droitwich has an output insignificant in quantity, but of incomparable quality. Instead of producing unfaltering power at a single frequency, it gives us a sound-controlled output, corresponding to every frequency known to the musician. Instead of conveying simple sensations, such as light or heat, it radiates all the complex effects of sound. Not merely words are radiated, but every tone and inflection; not merely musical notes, but every conceivable effect of harmony. In the broadcasting station, engineering has been perfectly wedded to art.
When Droitwich National was put into service, in October 1934, it was a pioneer station, embodying several features of design and layout that differed widely from those of its Regional predecessors. Using three times as much power as the 50-kW Regionals, it had a much longer wavelength, necessitating special precautions in design to obtain high-quality reproduction comparable with that on medium wavelengths.
Another unusual feature of Droitwich National is that its power house generates alternating current, instead of the usual direct current. On the high-tension side, its supply is normally obtained from mercury arc steel-tank rectifiers.
The Droitwich masts are 700 feet in height, being 200 feet higher than any previously used by the B.B.C. Inside the station building the unusual height of the great transmitting units made it convenient for the transmitter hall to be built with a gallery, the units being built through the floor of this gallery.
In the interests of good quality reproduction at the higher audible frequencies a novel piece of apparatus called the “transducer” was placed between the long-wave transmitter and the feeder lines to the aerial. The Droitwich switchgear, too, is novel, in that the various power supplies to the transmitters can be operated automatically from a control table in the transmitter hall. Seated at this table, the engineers on duty have a clear view of both transmitters and of most of the motor generators.
THE FILAMENTS of some of the longwave transmitting valves work at 10,000 volts above earth potential, and rise at times to 20,000 volts. Unusual precautions have to be taken in the insulation of machines supplying these filaments, as shown by the large porcelain insulators on which the apparatus is mounted,
Droitwich, in common with most of the B.B.C. stations, is independent of the public electricity supply mains. Its power is generated on the spot by diesel generating sets, and at the rear of the building are two tanks for fuel oil storage. Each tank has a capacity of 150 tons. The tanks contain sufficient fuel to enable the station to radiate two normal programmes on full power for a period of three months.
Precautions Against Vibration
The equipment of the power house comprises four six-cylinder diesel generator sets of 750 brake horse-power, each set being directly coupled to a 470-kW. three-phase alternator. The output voltage is 415, and the diesel sets run at a speed of 375 revolutions a minute.
In this power house there is a hint of the delicate electrical operations to be carried out by the transmitter, for the most thorough precautions have been taken to guard against vibration. The four diesel engines are mounted on a huge block of reinforced concrete, which “floats” on a bed of thick cork. The block measures 54 feet by 30 feet by 9 feet deep, and weighs 800 tons.
The power house is designed to have a maximum capacity of 1,880 kW, but the normal load when the station is working full power on both transmitters is about 1,100 kW; this is equivalent to running three of the oil engines on three-quarters load; thus one engine is always idle, and can be overhauled. In the event of the breakdown of one engine during the overhaul of the “spare”, the whole load of the station could be carried on the remaining two engines, with only a slight reduction of power to the transmitters.
Another example of preparation for emergency is afforded in the high-tension room. The design of the Droitwich station was more difficult than usual because the long-wave transmitter requires a high-tension supply of about 30 amps, at 20,000 volts, and the high-tension supply for the adjacent Midland Regional transmitter is 15 amps, at only 12,000 volts. Two mercury arc rectifiers have been installed for the 20,000-volts supply, one acting as a stand-by to the other. For the 12,000-volts supply required by the Midland Regional three high-tension motor generators have been installed. Only one is used at a time, the others being spare; but the switchgear is so arranged that two generators can be run in series for the supply of high-tension current (20,000 volts) to the long-wave transmitter, if required.
When Droitwich is not transmitting and the engines are not running it is necessary to provide light, and sometimes power for auxiliary apparatus; these requirements are then fulfilled from the battery room, which houses a 220-volts battery of 1,500-ampere-hours capacity.
Because of its unique two-storied design, the long transmitting hall of the Droitwich station presents the visitor with an unusually vivid impression of controlled power. Entering the front of the building and emerging into the transmitter hall at gallery level, he has before him two complete high-powered broadcasting transmitters, one on either hand. To the right is the National (long-wave) transmitter; to the left is Midland Regional, working on a wavelength of 296.2 metres.
At the far end of the long hall, at gallery level, is the power control table. Two engineers sit at this table, one monitoring the National transmitter and the other the Midland Regional transmitter. The controls that they operate regulate the various power supplies by automatic switchgear. Serious mistakes in switching — such as the connexion of two machines to the same load —are prevented by interlocked controls which refuse to act if the wrong thing is demanded of them.
Below and to the right, standing on the ground floor immediately under the long-wave transmitter units, are the motor generators which supply them with filament current. Each machine is immediately adjacent to the valve or unit which it supplies.
HIGH-TENSION SUPPLY for the long-wave transmitter at Droitwich is required at 20,003 volts 30 amperes. Two mercury arc rectifiers are installed, one acting as a stand-by to the other. To the right of the rectifier shown is the high-tension transformer. All the associated apparatus is contained in a screened enclosure.
To the left are the filament-current generators, high-tension generators and grid-bias generators for the early stages of the Midland Regional transmitter the units of which are installed immediately above them. The grid-bias machines and high-tension generators for the National transmitter are also on this side of the transmitter hall.
Despite the spaciousness of the transmitting hall, it seems extraordinary that two powerful radio stations, sending out entirely different programmes, should face each other in the same room without giving rise to unwanted interactions. Yet the power supplies to both transmitters are regulated from the single power-control table placed between them. The incoming speech and
music are conveyed to the station by four cable telephone circuits connected to the Birmingham control room. In the event of a line break on the normal route other high-grade telephone circuits can be interrupted at Droitwich, to give connexion to Great Britain’s simultaneous-broadcasting network A further safeguard is a radio receiver to pick up, if necessary, another B.B.C. station’s programme.
5,000 Gallons of Water a Day
Two stayed lattice-steel masts, 600 feet apart and 700 feet high, support the Droitwich aerials. Each mast weighs 100 tons, and stands on a porcelain insulator. The weight on each of these base insulators is increased by the tension of the stays to 150 tons. The masts are of triangular section, each being provided with an electrically operated lift inside it. To conform to Air Ministry requirements the top of each mast is lighted by a red-light fitting of special design, and there is a similar fitting on a bracket, halfway up the mast.
The aerial for the National transmitter consists of two wire cages, each 20 feet in diameter and 84 feet long. The single-wire down-lead, 600 feet long, is connected to the centre of the top portion between the two cages and goes straight down to the aerial-transformer house, situated midway between the two masts. Open-wire feeders, supported on 13-feet poles, connect the aerial-transformer house to the transmitter.
The aerial for the Midland Regional transmitter consists of a single vertical wire, 450 feet long, supported by a stay slung from the top of the more northerly mast. Near it is a reflector to improve the Midland Regional’s radiation in a northerly direction.
The long-wave earth system consists of seventy-two stout copper wires buried at a depth of approximately 9 in. For earthing purposes a series of furrows was ploughed, radiating from the aerial-transformer house , each wire was placed in a furrow and buried.
Another prominent external feature of the station is the water storage. A reliable and sufficient water supply is of primary importance, for about 5,000 gallons of water a day are required for cooling purposes. A reservoir capable of holding 300,000 gallons is provided as a safeguard against restrictions or interruptions of the supply, which is obtained from the water mains.
The site of the station, 2½ miles north of Droitwich and 170 feet above sea-level, has an area of 55 acres. Before the site was chosen the most searching examinations had to be made.
One unusual difficulty in the Droitwich area is the existence of an underground brine stream, which flows through the district at a depth of about 190 feet. Its precise position had not been located, but it was essential that a site above it should not be chosen for the erection of the broadcasting station.
To make sure and to gain better knowledge of the subsoil, the engineers bored into it for a depth of 300 feet. As the masts are 700 feet high, the operations at Droitwich extended altogether for 1,000 feet, above or below the surface. These unusually wide limits exemplify the far-reaching thoroughness which characterizes every detail of the Droitwich station.
IN THE AERIAL-TRANSFORMER HOUSE below the long-wave aerial, huge coupling coils transfer the high-frequency currents brought from the transmitter by feeder line to the aerial circuit. The long-wave aerial is supported between two 700-feet masts.
