The history of Rugby Radio Station
Produced by Radio Engineering Services of British Telecom in September 1995,
Customer liason
Events leading to the building of Rugby Radio
To fully appreciate the history of Rugby Radio, one must go back in time of the year 1910, for it was in that year that the newly formed Marconi Company approached the Colonial Office for licenses for the construction of eighteen wireless stations throughout the British Empire.
While the idea proved of great interest to the Government, the general scheme was found to be unacceptable as it was felt that an undertaking of this magnitude should be in the hands of the Government and not a private concern.
After more than two years deliberation an agreement was concluded with the Marconi Company for the construction of six high power stations. The details of this agreement however, were severely criticised in Parliament and a Selected Committee was appointed to report on it.
the Selected Committee in turn recommended the appointment of a committee of experts to examine the question from a technical point of view. This committee recommended modifications to the proposals, and a revised agreement was concluded in July 1913, ratified by the House of Commons and subsequently by the Dominion Governments concerned.
In December 1914, the Cabinet questioned how the Imperial stations would be affected by the war. As a result, they instructed the Postmaster General to terminate the contract with the Marconi Company as the Government had decided not to proceed with the Imperial wireless Chain.
At that date the erection of the masts at the English and Egyptian stations had practically been completed and the mast material for the Indian station delivered.
At the end of 1919, the Committee was again sitting to reconsider the issues. After further wrangling with the Dominions, who by then had definite thoughts of their own on how the scheme should operate, an agreement for an Empire Chain of wireless stations was finally reached in 192. Meanwhile, a makeshift service using arc transmitters was in operation between Leafeld in Oxfordshire and Abu Zabul in Egypt.
This was the position when the Coalition Government went out of office but the situation was reconsidered in 1923 by the new Conservative Government, which agreed to throw open the Imperial Communications to private enterprise. At the same time they decided to build a Government station equal in power to any in the world, partly for strategic purpose and partly to prevent an absolute monopoly by the Marconi Company.
This decision was announced by Mr. Bonar Law in the House of Commons on 5th, March 1923, and with the purchase of about 920 acres of land at Hillmorton in the same year, (a Royal Naval Air station), the Post Office embarked upon what was possibly its greatest single project up to that time, the building of Rugby Radio.
The Building of Rugby Radio
the site, approximately 340 feet above sea level and with a maximum variation in level throughout of 30 feet, was chosen to accommodate sixteen masts each placed one quarter of a mile apart, although only 12 masts were subsequently erected. The aerial system was designed within the limit of structural possibilities to give the best attainable radiation effiency, the principal requirements being high effective height, high capacitance to earth and low earth resistance.
The aerial system consisted of 820 feet masts (considerably higher than any masts previously constructed), spaced at quarter mile intervals to form an irregular octagon with two extensions to the north. These masts supported cage aerials 12 feet in diameter. The aerial was constructed in two sections consisting of one large octagonal cage aerial two miles long supported on eight masts and another shorter cage aerial one and a quarter miles long supported on six masts, two of the masts used in common with both sections of the aerial. In all, about 27 miles of copper cable were used in forming the aerial. The arrangement was such that the two sections could be connected together to form one large aerial for extreme power.
The masts, stayed in three directions and at five levels, are of braced steel construction with a uniform triangular section with 10 feet sides. The base is in the form of an inverted tripod, the apex of which is supported by a dome and a hemispherical joint, insulated from earth by means of porcelain insulators and a block of Swedish granite. Each mast complete with its stays, weighs 200 tons and can sway as much as 10 feet at the top. All the masts are fitted with electrically driven winches for raising and lowering the aerial and also for operating an internal lift capable of carrying three persons.
When completed the aerial had a capacity of 45 nF with an inductance of 0.358 mH and a resistance of 0.4 Ohm. The resistance to earth was 8 Mega ohm. With all masts insulated the aerial had an effective height of 180 metres and an effiency of about 29% at 16 kHz.
The normal working voltage of the aerial was 165000V r.m.s. and the current at the base of the aerial was about 750 amps.
The earth system consisted of an open network containing about 120 miles of copper wire buried a few inches in the ground. This occupied a strip 1600 feet wide under the entire length of the aerial.
The aerial system remained substantially unmodified over a period of 30 years, and only in 1956 it was necessary to renew the stays, a striking confirmation of judgement in the design.
The tuning coils of the transmitter were wound with Litzendraht cables, consisting of 6561 strands of No 36 SWG copper wire, each strand insulated with enamel, and one covering of cotton or silk. The formers were hexagonal spiders of American white-wood, the external side being 7ft 9in in the largest former, and the turns were 6in apart. Five spiders of eight turns each formed the aerial coil, three spiders of four turns each the primary, and one spider of two turns the coupling coil.
The transmitter was designed to operate at a frequency of 16 kHz, the primary source being obtained from a valve maintained tuning fork vibrating at a frequency one-ninth of the radiated frequency. The output of the tuning fork stage was fed via a 50 kW exciter stage to the final power amplifier units, of which five were provided, each containing eighteen 10 kW water cooled valves in parallel.
The five amplifier units could be used in parallel to give one 500 kW transmission with two spare units, or two separate 300 kW transmissions with one spare unit.
The High Tension Anode supply to the power units had to be capable of withstanding a short circuit with impunity and comprised of three sets of machines each consisting of a 3 phase 416 V motor driving two d.c. generators. Each generator could develop 250 kW at 3000 volts and all generators could be put in series giving 1500 kW at 18000V.
The effiency of the transmitter was about 72% excluding filament power or 65% including filament power. The transmitter was opened for traffic on January 1st, 1926, under the call sign GBR, and having worldwide coverage, was an unqualified success.
Meanwhile a second transmitter was being installed by Western Electric (later to become the ST & C Company in Britain) and experiments were carried out in radiotelephony between the United Kingdom and America. The outcome of these tests was the first two-way telephone conservation across the Atlantic on 60 kHz (GBT) and the inauguration of the Transatlantic Telephone Services in 1927.
The transmitter had a power of 300kW, and operated on single sideband, with the carrier suppressed, probably the first use in this country of a system now universally in use in carrier working on telephone lines.
The tuning coils were similar to those in use of GBR and part of the original aerial system was adapted for the transmission, which could also be radiated on 68 kHz (GBY).
A fourth d.c. generator was installed to furnish the EHT supply to this transmitter and was of similar design to the previous except that it had an output of 12kV using two 6kV generators in series.
The main capacitators for both transmitters (GBT/GBY and GBR) consisted of serial/ paralell combination using mica insulation, suspended in oil filled steel tanks.
A third transmitter of 50kW output was also built and utilised for telegraph broadcasts to Europe, on a frequency of 78 kHz. Again water-cooled valves were used, and the transmitter was frequency controlled by means of a tuning fork vibrating at a seventeenth of the radiated frequency. Use was made of the two masts nearest the building to support a "T" type cage aerial similar to those in use on the other transmitters.
The use of so many water-cooled valves necessitated the building of two reservoirs, each with a capacity of about one quarter of a million gallons, and a heat exchange system to cool the distilled water flowing through the valves.
The original receiving station was at Wroughton in Wiltshire, and employed a Beverage aerial, five miles long. Later, in order to reduce noise on the circuit a receiving station was installed much further north, at Cupar in Fifeshire. Again a Beverage aerial was used but it was subsequently replaced by loops extending over an area of several square miles.
The development of high frequencies
At this time the potential of short wave communication was being realised and experiments carried out in an old farmhouse on the site, paved the way for the opening of a second telephony channel to America in August 1928, using wavelengths between 16 and 32 metres. At such wavelengths, construction of highly directional aerial arrays became practicable and radiated power could be concentrated in the direction of the receiving station. This resulted in a considerable reduction in the cost of establishing radiotelephony circuits.
After the establishment of short wave radiotelephone communication from Rugby Radio, a second building(A building) was set up half a mile from the site of the original to cater for the installation of further shortwave transmitters.
Rapid expansion of short wave radiotelephone services followed the equipment of the A building in 1929 and plans for building further low frequency transmitters were dropped.
By then the transmitters were frequency controlled by means of a temperature stabilised quartz crystal oscillator, followed by frequency multipliers and amplifiers up to a power of about 60 kW. They were modulated by speech in the low power stages. This remained the basic design for a number of years.
The filament, grid bias and intermediate HT supplies were obtained from dc. Generators driven by a.c. synchronous motors, while the final HT voltage was from thermionic valve rectifiers.
Although, at this time, the merits of single sideband operation had been well proven, it was not possible to control the frequencies of HF oscillators with the necessary precision, and double sideband operation was therefore used.
The aerials were in general, various patterns of arrays of half wave elements and reflectors working on particular frequencies in fixed directions. They were suspended from self-supporting steel towers 200 feet in height.
The original high power thermionic valve rectifiers were soon replaced by much more efficient mercury vapour and later mercury arc types. Demountable valves were also introduced in the final high power amplifiers.
The demountable valve, as its name may suggest, could be dismantled into its separate parts of that any component, for example filament, grid, or screen grid could be replaced when a failure occurred. The components mentioned were rigidly mounted on the valve head which in turn was fastened to the unit, the complete assembly being carefully sealed at the joints, using bitumen for the more permanent joints and plasticine treated with a special grease for the others. The assembled valve was continuously evacuated by means of two oil condensation pumps in cascade, backed by an oil immersed rotary pump evacuating to atmosphere. The pumping equipment was capable of creating a vacuum pressure of 10 6 mm Hg.
In this type of valve water-cooling was necessary on the filament and grid seals as well as on the anode. The oil condensation pumps were also water-cooled.
A valve built of this principle, using 18 grid filament assemblies set in a single anode jacket, and devised for use on the longwave transmitter GBR, operated successfully for some time, taking the place of a power panel.
By 1937, sufficient advances had been made in the design of oscillators to permit the use of single sideband working on high frequencies, and this type of communication was introduced on the high frequency TAT service in 1938
The War Years
With the advent of war in September 1939, the radio telephone overseas services (with the exception of one or two particular services), were suspended and the transmitters were mostly converted for telegraph working and put to the armed forces. GBR in particular, became of vital importance to the Navy and other shipping interests.
During the war years the station suffered two major setbacks, the first being at the end of January 1940, when during a severe prolonged ice storm, the aerial became heavily overloaded with the weight of the ice forming on it. Normally the release gear at the mastheads would have catered for this eventually by automatically lowering the aerial and thereby easing the strain. Unfortunately the automatic slipping gear had frozen so solidly, it failed to operate, and the aerial finally gave way under the strain. Very low fog persisted for some days frustrating efforts to investigate and carry out repairs, and when at last it was possible to see the aerial it was found that the extent of the damage was great. Many of the spreaders were distorted or broken beyond repair, insulators were shattered and almost a complete aerial rebuild was necessary.
The second disaster was of greater magnitude and occurred one evening in March 1943, when without warning, the woodwork of the roof of the main station housing for VLF transmitters ignited due to the radiation effect of GBR. Within a few moments, the whole roof was ablaze and despite the efforts of the staff and many fire brigades, the transmitting room was almost gutted. Although very high winds prevailed at the time it was possible to insulate the fire to the main transmitting room, and the major damage was suffered by the transmitters, the power equipment remaining untouched.
For some time, uneasiness had been felt concerning the vulnerability of the station to damage from enemy action, and to off-set this, another radio station housing a counterpart to GBR had been set up in North Wales. This station was nearing completion at the time, and by salvaging some equipment from GBR it was possible to resume the VLF transmissions from this site within a few days of the fire.
The results achieved by the new station were found to be far below those of GBR and the Admiralty expressed great dissatisfaction. The rebuilding of GBR was therefore given top priority and six months later the familiar call sign was again being broadcast around the world.
The Long Wave TAT Transmitter was also rebuilt and modernised. The Low Power Transmitter GBV, which had suffered relatively little damage, was also restored to service after slight modification.
During the rebuilding and re-equipping of the Main Building, the opportunity was taken to install a standby plant to supply power in the event of a total failure in incoming supplies. This was a directly coupled 1150kW a.c. generator driven by a six cylinder blast injection diesel engine running at 300rpm.
Further development on HF and Power Supplies
At the end of the war the station was reconverted to cope with the rapidly increasing demand for overseas telephone circuits, but it was soon found that the demand for those circuits was outstripping the available plant. Accordingly, arrangements were made to purchase a further seven hundred acres of land adjoining the site and work was commenced on the construction of a new building to house twenty eight transmitters of the most modern type.
The new station, probably the biggest ever built as a single project, was well in advance of any other in existence at that time, both in technique and in the extent to which it economised on manpower.
The transmitters were of 30 kW peak power output, capable of transmitting virtually all types of telephone and telegraph signals with continuous coverage in the band 4 30 MHz. The final and penultimate stages used grounded grid techniques and negative feed-back could be applied as required.
A two-fold plan was followed in designing the building. Firstly, the low power equipment, the high power equipment and the aerial switches were segregated in different parts of the building. Secondly, a large measure of automatic control and monitoring was introduced, located at Central Control Position, making the station equipment for telephony and telegraphy, the carrier oscillators, the automatic monitors and the landline equipment.
The transmitters were air cooled and installed in three halls converging on the Central Control Position, from where, by remote control, any transmitter could be operated on any one of six predetermined frequencies with the appropriate aerial. Previously in HF communications, aerial switching had always been a problem and was usually performed manually. In the new building the problem was solved very satisfactorily by using 200 Ohm balanced coaxial feeders to carry the power from the transmitters to remotely controlled, motor driven switches in one of the aerial switch rooms.
From this point each transmitter had immediate access to any one of six outgoing aerial feeders. This was quite sufficient for normal operations, but by manually altering connections, any aerial feeder could be made available to any transmitter.
From the aerial switch room, the aerial feeders passed first via balanced feeders inside the station to a periodic exponential line transformer and then via open wire feeders outside the station to some 70 aerials around the periphery of the site.
Most of the aerials were of the three wire rhombic type, many of them mounted in pairs, one above the other, 150 feet and 75 feet above the ground on light lattice masts of the Post Office No 1 type. Four Koomans arrays were set up on three 325 feet masts especially for use on the New Zealand service.
In some cases switches were fitted at the base of the aerials to permit the direction of transmitted signals to be reversed or dephased by remote control from Central Control Position.
The monitoring was so arranged that continuous comparison was made between the incoming and outgoing telegraph signals, and when on telephony, any variation of greater then 3db in carrier power level would indicate a fault. Reflectometers were situated in the aerial feeders and constantly measured the voltage standing wave ratio at the output of the transmitter. If this exceed a predetermined ratio of 2:1, the transmitter would automatically shut down to prevent damage, and an alarm signal would be given at the Central Control Position.
The new station (B Building), was put into operation in 1953 and after its completion, attention was given to the refurbishing of the original Short Wave A Building. The majority of the transmitters in this building were relatively inefficient and somewhat limited in performance. Arrangements were therefore made to scrap one of the oldest units and replace them by twelve modern, more powerful air cooled transmitters. Although of a different make and design, they were, with a single exception, similar in scope to those already installed in the B building, and allowed a fair measure of automation and central control to be introduced. The one real difference in the transmitters installed in the A Building, was that they had an unbalanced 75 Ohm output and so were not able to make use of exponential line matching.
To match the 75 Ohm unbalanced output to the 600 ohm balanced transmission line to the aerial, a balun was devised incorporating a 1 : 1 balance to unbalance transformer, a 4 : 1 balanced impedance transformer and two tandem stub compensated quarter wave transformer lines, mounted in a fibre glass container.
The half wave element aerials originally associated with the A Building were mostly dismantled and replaced by the more versatile rhombic which, while not having the high gain of the array, were less critical as to frequency and had good directional properties.
By this time, the overall power load of the station had increased greatly beyond the capacity of the existing standby plant and arrangements were made to install additional power generating equipment to offset the increased demand. The large d.c. generators supplying the EHT supplies to the high power VLF transmitters had now begun to require more frequent attention and were being regarded as a possible maintenance liability. It was decided therefore that the d.c. machines should be scrapped and replaced by three six-phase mercury arc rectifier units which were of much smaller physical dimension.
This permitted the installation of two turbo supercharged engine alternator sets with a combined output of 1646 kW. The standby plant therefore had a combined capacity of 2796 kW, and at the time was the largest prime mover installation in use by the Post Office.
The new engines were English Electric type 12sv with 12hour rating of 1350bhp and a 30 second rating of 1630bhp at 750rpm. The short time rating and the special heavy flywheels fitted were required to deal with the peak loads imposed when GBR was being keyed. The complete station was fed via a ring main unit by an 11 kV supply from the public grid and the emergency supplies could be paralleled and synchronised in and out as required.
Development in VLF and LF
Almost since its inception Rugby has broadcast time signals, originally these twice daily on 16 kHz. In 1951, Rugby was given an additional commitment for the transmission of reference Modulated Standard Frequencies.
Suitable equipment was installed in the main building, which included three high grade 100 kHz crystal oscillators to provide signals accurate to within parts in 10 000 000 of the assigned frequency. By means of frequency dividers and multipliers carrier frequencies were obtained at 60 kHz for the Long Wave Telephony Transmitter (the GBT/GBY TAT service by now having ceased) and 2.5, 5, 10, 15 or 20 MHz for three of a set of four HF transmitters which were newly installed for the purpose.
The equipment also provided for continuous intercomparison and recording of the frequencies of the master oscillators in pairs, so that any divergence from normal on the part of any one of the oscillator chains might be detected and corrected. Continuity of operation being an essential feature of a frequency standard of this kind, the master oscillators were fully protected against any interruption of the power supplies.
In the first instance, the crystals were quartz resonators cut in the GT mode, but these were later replaced by the Essen Ring type suspended on silken cords and giving a higher degree of stability. The carrier and modulator frequencies were all derived from the crystal oscillator which was regulated with reference to a caesium resonator.
At the introduction of the service the signals were radiated continuously on a 24 hour programme using frequencies of 2.5, 5 and 10 MHz and for one hour on 60 kHz. Except in a small region adjacent to Rugby, where the ground wave predorminates, the HF signals would be received after reflection from the ionosphere.
Variations in ionospheric conditions can cause significant fluctuations in received carrier frequencies at HF, thus limiting the accuracy of the MSF emissions on short waves.
LF and VLF transmissions are not subject to these same limitations and the 60 kHz transmission was soon found to be in great demand and coverage was extended to 24 hours daily in 1966. This high stability of transmission in the LF and VLF ranges began to assume great importance in other fields, and soon new uses were found for the three original transmitters which at that time seemed largely to have outlived their usefulness.
The MSF standard was arranged to generate the carrier frequency of GBR transmitter, but it was found that there was phase shift inherent in the transmitter, due to the magnetic attraction between the aerial tuning coils, creating movement of the inductances when the transmitter was being keyed. This movement was overcome by enclosing the inductances in paxolin sheaths and automatic phase correction was also introduced. The transmitter was then restored to service with greatly increased stability of transmission.
At about this time, tests had also carried out on 68 kHz (GBY) using various methods of keying, the outcome of which was that the transmitter was requisitioned for a 24 hour teleprinter service. To make the transmitter available for this service, the medium power LF transmitter was retuned from 78 kHz to 60 kHz and took over the MSF transmissions on LF.
In view of the great importance attached by the user of the VLF transmissions on GBR, it was decided to rebuild the transmitter using more modern equipment and increased power output. This was completed in 1967.
The new transmitter retained the original aerial, tuning and power equipment, but the water cooled valves previously used were replaced by three vapour cooled amplifier valves for use singly or in combination. The modulator circuitry was redesigned so that frequency shift, as well as CW signals, could be generated at speeds up to 72 baud.
Also in 1967, the Essen Ring oscillators were superseded by the installation of a rubidium vapour standard to permit frequencies accurate to 1 part in 100 000 000 000 ( 1 second in 3000 years) to be radiated.
The MSF transmitter radiates a 24 hour time signal service and is the principal means of disseminating the UK national standards for time and frequency.
The transmission type is A1 negative modulation, the carrier being modulated by both a 100 Hz BCD and a 1 Hz BCD time code. The 100 Hz was first introduced in September 1974 and gave just hours and minutes, this being extended to give calendar information in November 1976. The present more widely used 1 Hz code was first transmitted in June 1977, and relays in addition the year, GMT/BST status and DUT1 data (this being the difference between astronomic and atomic time).
The transmitted frequency and time code are derived from atomic standards which enable the transmission to have a day-to-day stability of better than 3 parts in 1 000 000 000 000. The station has a suite of three standards, two caesium beam and one rubidium vapour. Each standard feeds its own programmable clock and drive train. Three chains are required not only for reliability of service, but for triangular intercomparison.
Readings are taken daily to compare the accuracy of the Rugby standards with those at the NPL at Teddington.
Future Prospects
The special applications now found possible due to the particular propagation properties of the LF and VLF transmissions, seem to indicate that the future of the long wave transmitters are assured for some years to come.
High frequency transmissions have decreased with the transfer of overseas telephone traffic to submarine cables and satellite routes, as these systems offer a more reliable and higher grade worldwide service. This does not mean that all of the HF transmitters have become redundant. Some maritime traffic is still transmitted as well as communication transmissions for private concerns.
Rugby Radio station is the heart of BT Radio Engineering Services design, maintenance and support facility providing a wealth of expertise in ever changing radio environment.
During almost seventy years of changing techniques, the twelve tall masts at Rugby have dominated the local landscape,; a monument to a job well done; and it is certain that more years of radio history will be written before these engineering masterpieces cease to grace the surrounding countryside.