Site Records

SiteName: Rugby Radio Station

Rugby
Warwickshire
OS Grid Ref: ('C' Building) SP553747

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 and 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 in technique and in the extent to which it economised in manpower. The HS51 type transmitters were manufactured by "Marconi" and were rated at 30 kW peak power output capable of transmitting virtually all types of telephone and telegraph signal with continuous coverage in the band 4-30MHz. The final and penultimate stages used grounded grid technique and negative feed back could be applied as required.

'B' Building newly completed in 1954

A two-fold plan was followed in designing the building. First the low power equipment, the high power equipment and the aerial switches were segregated in different parts of the building and secondly, a large measure of automatic control and monitoring was introduced centred at a Central Control Position making the station easy to operate and of pleasing appearance.

One large room houses the drive equipment for telephony and telegraphy, the carrier oscillators, the automatic monitors and the landline equipment. The transmitters are air cooled and installed in three halls converging on the Central Control Position, from where, by remote control, any transmitter can be operated on any one of six
predetermined frequencies with the appropriate aerial. Hitherto in HF radio 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. balanced coaxial feeders to carry the power from the transmitters to remotely controlled motor driven switches in one or the other of two aerial switch rooms.

'B' Building - North Transmitter Wing - 1954

From this point each transmitter has immediate access to any one of six outgoing aerial feeders. This is quite sufficient for normal operations but by manually altering connections any aerial feeder can be made available to any transmitter. From aerial switch room the aerial feeders pass first via balanced feeders inside the station, to a periodic exponential line transformers and then via open wire feeders outside the station to some 70 aerials around the periphery of the site.

Most of the aerials are three wire rhombics, many of which are 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 signal to be reversed or dephased by remote control from the Central Control Position. The monitoring is so arranged that continuous comparison is made between the incoming and outgoing telegraph signals, and when on telephony any variation of greater than 3db in carrier power level will indicate a fault. Reflectometers are situated in the aerial feeders and constantly measure the voltage standing wave ratio at the output of the transmitter. Should this at any time exceed a predetermined ratio of 2:1 the transmitter will automatically be shut down to prevent damage and an alarm signal will be given at the Central Control Position.

The new station or ("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 and arrangements were made to scrap nine of the oldest units and replace them by twelve modern more powerful air-cooled transmitters. They were of a different make and design, type DS13, manufactured by "ST&C". They provided similar facilities to those already installed in the "B" Building and allowed a fair measure of automation and Central Control to be introduced into the "A" Building also.

The one real point of difference in the transmitters installed in the "A " Building was that they have an unbalanced 75. output and so are 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, are less critical as to frequency and have 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 two 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 1,646kW. The standby plant therefore has a combined capacity of 2, 796kW and the time was the largest prime mover installation in use by the Post Office.

The new engines are English Electric type 12SV with 12 hour rating of 1,350 b.h.p. and a 30 second rating of 1630 b.h.p. at 750 r.p.m. The short time rating and the special heavy flywheels fitted were required to deal with the peak loads imposed when GBR is being keyed. The complete station is fed via a ring main by an 11kV supply from the Public Grid and the emergency supplies can be paralleled and synchronised in and out as required. Two further English Electric type 12SVA engines were installed to replace the "Fraser & Chalmers" in 1967

Suitable equipment was installed in the main Building and included three high- grade 100khz crystal oscillators to provide signals accurate to within two parts in 108 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 ST&C ES4A 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 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 were later replaced by the Essen Ring type suspended on silken cords and giving a higher degree of stability. The carrier and modulation frequencies were all derived from the crystal oscillator which was regulated with reference to a caesium resonator at the National Physical Laboratory (NPL)

At the introduction of the service the signals were radiated continuously on a 24 hour program using carrier 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 predominates the HF signals are received after reflection in the ionosphere.

Variations in ionospheric conditions can cause significant fluctuations in the received carrier frequencies at HF, thus limiting the accuracy of the MSF emissions on short waves. This problem, together with a reduction in usage, lead to the HF services ceasing in 1988. LF and VLF transmissions are not subject to these same limitations and so the 60 kHz transmission was soon found to be in great demand and coverage was extended to 24 hours daily. This high stability of transmission possible in the LF and VLF ranges began to assume great importance in other fields also 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 the 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 windings of 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 been carried out on 68 kHz (GBY) using various methods of keying the outcome of which was that the transmitter was used for a 24-hour teleprinter service. To make the transmitter available for this service the Medium Power LF transmitter was retuned from 78kHz to 60kHz and took over the MSF transmissions on LF.

In view of the great importance attached to the VLF transmissions on GBR it was decided to rebuild the transmitter using more modern equipment and with increased power output. The new transmitter, completed in 1967, was designed and constructed by the Post Office Engineering Dept.

The new transmitter has retained the original aerial, tuning and power equipment, while the water cooled valves previously employed have been replaced by three vapour-cooled amplifier valves for use singly or in combination. The modulator circuitry has been redesigned so that frequency shift as well as C.W. signals can be generated at speeds up to 72 baud.

Also in 1967 the Essen Ring oscillators were superseded by the installation of Rubidium Vapour Standards to permit frequencies accurate to 1 part in 100,000,000,000 (1 second in 3,000 years) to be radiated.

The 24 hour 60kHz MSF standard frequency service that commenced in 1966 was originally on a new 50kW "Marconi" type H1211 transmitter, however, in 1974 it moved back to the GBY transmitter which had been completely modernised by the Post Office Engineering Dept. in 1972.

This was also the same year in which the first time code was radiated. On 18th December 1998 the service moved yet again to a new solid state "Telefunken" transmitter. This substantially improved the electrical efficiency and also enabled maintenance outages to be reduced from monthly to quarterly.

The MSF 60kHz transmitter radiates a 24-hour time signal service and is the principle means of disseminating the UK national standards for time and frequency.
The transmission type is A1 negative modulation, the carrier being modulated by both 100Hz BCD and 1Hz BCD time code. The 100Hz code was first introduced in September 1974 and gave just hours and minutes, this being extended to give calendar information in November 1976. The 1Hz 'slow code', as used today, was introduced 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 original 100Hz time code ceased on 7th October 1998. Today the station has a suite of three standards, two caesium beam and one rubidium vapour. The transmitted frequency and time code are derived from these atomic standards which enable the
transmissions to have a day-to-day stability of better than 2 parts in 10/12 . (Better than 1 second in 10,000 years) 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.

UP TO DATE
High Frequency transmissions have decreased with the transfer of overseas telephone traffic to submarine cable and satellite routes as these systems offer a more reliable and higher grade worldwide service. This does not mean that all of the H.F. transmitters have become redundant. Some maritime traffic is still transmitted as well as communication transmissions for private concerns.

The move from point-to-point services to long range maritime/mobile operations on High
Frequency entailed a phased replacement of the transmitters and aerial systems at the 'B' Building between 1987 - 1992. The now ageing high power HS51 transmitters were replaced with lower power 10kW units. Two main types are now in use, twelve, "ST&C" type QT3A4 transmitters recovered from Ongar Radio Station in Essex and thirteen, new "Marconi" Fast Tune (MFT) type H1141 transmitters.

Most of the rhombic aerials were replaced with "spiracone" inverted cone and stack quads omni-directional type, plus three, motor driven, steerable log periodic aerials. These long-range services are controlled by operators at Burnham-on-Sea (well known by most mariners as "Portishead Radio"). The receivers for the service being located at Somerton Radio Station in Somerset. These services, using Morse, teleprinter/telex or voice, are capable of connecting a ship anywhere in the world directly back to the UK for onward connection, if required, to any other country in the world.

A reduction in maritime traffic allowed all the services to be concentrated in Rugby 'B'. Therefore the 'A' Building closed as a transmitting station in the early nineties and BT Group Records and Information Services (GRIMS) converted it for use as a document storage facility.

The HF Maritime service at Rugby 'B' Building finally ceased at the end of April 2000.

The special applications found possible due to the particular propagation properties of the LF transmissions, seem to indicate that the future of some of the long wave transmitters, such as the MSF 60kHz "Time Signal" service, are assured for many years to come.

Rugby Radio Station is at the heart of BT Radio Services design, maintenance and support facility providing a wealth of expertise in an ever-changing radio environment.
During seventy-five years of changing radio techniques the twelve tall masts at Rugby have dominated the local landscape; a monument to a job well done. Whether you love them or hate them, to many Rugby residents returning from a long trip the call of "Look, there are the aerials!" has provided a warm feeling of being nearly home.

Have you noticed the new, brighter, aircraft warning lights installed in 1999? For most people, the 820ft masts have always been there and there is, at least, a few more years of radio history to be written before these engineering masterpieces cease to grace the surrounding countryside.

1st Edition - M. Johnstone - 1st January 1976
1st Revision - J. Richardson/C. Matthews - for IEE "100 years of Radio" Exhibition - Sept 1995.
2nd Revision - M.J.Hancock - for Rugby's New Museum - December 1999.
Updated - M.J.Hancock 15th January 2002.

© British Telecommunications plc.

Home Page