Introduction.
The masts of Ongar Radio Station were, for over 60 years, a familiar site to anyone travelling on the road from Ongar to North Weald. Throughout its life, it was essentially a radio telegraph transmitting
station, and in its busiest period provided simultaneous services to Europe, USA, Africa, India and Australia. It was widely used for several decades for rapid international transmission of newspaper pictures and text, before the evolution of modern satellite telephone networks made this commonplace. It saw the development of radio telegraph transmission, from single channel on/off keying of the carrier radio frequency to continuous carrier transmission with frequency shift keying (FSK), and - latterly - to single side band operation (SSB) capable of the simultaneous
transmission of multiple channels. Morse code was replaced by the
teleprinter 5-unit code, and, by the ingenious use of an error correcting multiplex known as ARQ, it was possible to achieve reliable telex operation worldwide, even under adverse atmospheric conditions.
The early years.
The site, which incorporates the Essex Redoubt built in the 1890's, was acquired by Marconi's Wireless Telegraph Company Ltd
in 1920, but due to legal and planning delays, the radio station was not completed before the summer of 1922. The separate receiving station at Pilgrims Hatch, Brentwood, was constructed at the same time. Completion was timely, as it reduced the problems caused by IRA sabotage of Clifden radio station on the west coast of Ireland in July 1922. Services previously based at Clifden were shared between Carnarvon, Towyn, Ongar and Brentwood.
In 1922, Ongar Radio Station represented the ultimate in wireless
communication. There were three separate transmitter buildings named "A", "B" and "C" stations, providing long wave telegraph services to Paris Berne and Madrid, with a capacity of up to 80 words per minute. Two transmitters of 3 and 6 kw in "A" station, for services to Paris and Berne respectively, were designed to work simultaneously, with the signals mixed and fed to a single aerial. A later commentator wrote " It is hoped that no one will rediscover this ingenious system, as we understand that the economy in masts and aerials was more than outweighed by the heavy responsibilities thrust on the engineering staff, whose duty it was to prevent these two frequencies becoming mixed up Inside the building".
The transmitters were worked on the independent drive or master
oscillator principle, with an anode voltage of 10,000 volts. Contemporary photographs show this exposed equipment fenced off with a simple rail.
Each station had its own separate aerial, consisting of two circular cages of four wires slung between a pair of 300 foot lattice masts. Lines of cables mounted 30 feet off the ground formed an earth screen to reduce power loss. The masts were ungalvanised, and required re-painting every three years. Messages for transmission originated at Radio House, Wilson Street, City of London and were encoded in Morse onto paper tape by a keyboard perforator, and then converted into electrical signals and sent directly by landline to the relevant transmitter at Ongar.
As main electricity was not available in the area power was generated on site by three sets of 50 kw Vickers-Petters oil engine generators (with hot bulb ignition) which remained in service until 1957. The hot bulbs had to be heated up with a paraffin blow lamp before starling up, and if this was done too enthusiastically the bulb blew out dramatically when the engine was turned over! There were eight motor alternator sets which generated single phase AC at 1000 volts and 350 cycles per second. Four of these (rated at 25 kw each) supplied power for the valves of the mam oscillation generators, the other four (rated at 15 kw each) fed the valves of the independent drives, as well as the valve filaments themselves after
transformation down to the appropriate voltage. It is possible that the power house incorporated one of the storage buildings of the Essex Redoubt. Cooling water for the engines was pumped from a special constructed pond just to the south of the engine house. At a later date, when fan assisted cooling was installed, the cooling pond was converted into a staff swimming pool by removal of the baffles!
At an early stage, and certainly by 1923, a row of semi-detached
bungalows was built along the Ongar road at the extreme NW corner of the site. These were intended to be for the use of overseas personnel on furlough, but one at least (number 6) was being permanently let to Mr Benjamin Cooper at £3-6-8d per calendar month in 1923.
Various experimental transmissions took place from Ongar Radio Station, which was conveniently placed for the Marconi works in Chelmsford in 1919, Marconi had proposed a service to South Africa, India and Australia using high power long wave transmitters, but this idea was turned down by the Norman Committee which favoured a line of stations every 2000 miles along these routes. In 1921, this recommendation was rejected as outmoded by the Imperial Communications Committee (chairman; Winston Churchill). In 1924, Marconi challenged the idea that long wave plus high power was best for long distance transmission, and had equipped
"C" station with short wave transmitters. Initially these used vertical
aerials suspended from portable 70 foot masts, and by 1927 Ongar was transmitting on the short wave to Argentina and Brazil. By this date, the original "A" station long wave aerial had been extended with a third 300 foot mast, and long wave transmissions were being made to France, Spain, Portugal, Switzerland, Austria, Yugoslavia and Russia. The growth of flying, and the proximity of North Weald airfield, necessitated the installation of warning lights on the masts. In April 1925 a formal agreement was signed by Marconi and the Secretary of State for Air, by which the latter agreed to reimburse the former for both the cost of power (at threepence per kw hour) and the maintenance costs. Doubtless they acted as a useful guide for the pilots who had no navigational systems to aid them. One early training flight did collide with an aerial, but landed safely without injury to pilot or trainer in a hedge within the radio station!
The development of the short wave beam aerial in 1926 was to have a marked effect on the development of Ongar Radio Station. But before that happened, control of the station passed, in September 1929, to Imperial and International Communications (renamed Cable and Wireless Ltd in 1934). Control of the transmitters passed to this company's headquarters at Electra House, Moorgate, City of London. By 1934 a large transmitter hall had been built adjacent to "C" station. This was equipped with a 40 kw short wave Marconi transmitter (capable of switching between 16, 21, 25 and 32 metres, which would now be termed 18.8, 14.3, 12.0 and 9.4 mHz) and using omni-directional aerials for New York and other distant
destinations. There were four 20 kw Marconi short wave transmitters using the beam aerial system for services to New York, Africa, Egypt, Turkey, Las Palmas and continental stations on various wavelengths between 14 and 40 metres (i.e. 21.5 to 7.5 mHz). When a frequency change was required, this had to be done manually on the transmitter itself, a process that took about 25 minutes. A smaller beam transmitter was used for Vienna and other European services. These transmitters were at floor level, with the associated thermionic valves and mercury vapour rectifiers in the gallery above. Long wave transmitters occupied a separate part of the hall - a 110 kw Marconi, principally used on the trans-Atlantic
service on 9630 metres (i.e. 31 kHz), and four smaller Marconi
transmitters of 5, 10, 15 and 25 kw for European services on 3950, 2950, 4196 and 5309 metres (i.e. 82, 102, 72 and 57 kHz). These larger transmitters had water cooled valves. Cooling water was provided from a purpose built pond Just to the south of "C" station.
New aerial systems had also been installed. One, of the eight wire type for the 110 kw long wave transmitter, ran NE from "C" station supported by four 285 foot masts and was nearly a mile long, with a tuned earth screen at each end. There were also four new beam aerials for short wave transmission, two of the characteristic Marconi "uniform" type and two of the compact "series phase" type. There was a unique system of copper coaxial feeders, which is believed to have been developed at Ongar. Two copper tubes, one inside the other, were separated by ceramic insulators.
To allow for thermal movement, the outer tubes were connected at
intervals by flexible bellows, while the inner tubes had a sliding
connection which in use proved to be rather prone to failure.
By 1934, mains power was supplied by the County of London at 11,000 volts, transformed down on site to 3400 volts to run two 250 kw units which generated 240 volts DC for driving a series of generators for the voltages required by the various transmitters. For stand-by purposes, the original three 50 kw Vickers-Petters oil engine generators were supplemented by four ABC 120 kw generators, as well as two high tension DC machines each of 150 kw at 12,000 volts for the big long wave transmitter.
In 1938, Ongar was transmitting to Belgrade (Beograd), Berne, Las
Palmas, Lisbon, Madrid, Melbourne (including facsimile), Moscow,
Nairobi, New York, Paris, Salisbury (Harare), Shanghai, Sofia (Sofiya),
Stamboul (Istanbul), Teheran, Tokio (facsimile only), and Vienna.
To be continued