Written by Nick Catford on 01 August 1996.

Ongar Radio Transmitting Station occupied a site of 730 acres at North Weald in West Essex adjacent to the late 19th Century North Weald Redoubt, one of 13 London Mobilisation Centres.

It was originally built in 1920 and operated by Marconi’s Wireless Telegraph Company. In September 1929, control passed to Imperial and International Communications when the telegraphic communications of the Empire were placed in the hands of a single operating company. The name of the company was changed to Cable and Wireless in June 1934 and the company owned the radio station until the passing of the Commonwealth Telegraphs Act, 1949, whereby the United Kingdom radio services of the Post Office and Cable and Wireless Ltd. became integrated on April 1st 1950. Ongar Radio was one of three pairs of transmitting stations, each pair consisting of a transmitter and receiver station; Ongar being paired with Brentwood.

These dates have figured prominently in the stations affairs, nevertheless traffic continued to flow steadily despite the vicissitudes of transition and the changes in stationary.

The original scheme was for a short wave beam station with four transmitters housed in three buildings, two in ‘A’ station, one in ‘B’ station, one in ‘C’ station plus a power house with three Vickers-Petters diesel engines coupled to DC generators. The A B & C stations and the power house were separated from each other by up to half a mile.

The first radio-telegraphic services from Ongar in 1921 connected London with Paris and Berne using Morse code. The transmitters were designed to operate simultaneously from ‘A’ station and the signals were mixed and radiated from one aerial on two different frequencies.

The first short wave transmitters installed at Ongar were GLS and GLQ known as Gee London Sugar and Gee London Queenie, both rated at about 6kW. They were installed in 1924, adjacent to the ‘C’ station building and radiated from single harmonic aerials; just pieces of vertical wire of no specific length.. The ‘C’ station short wave transmitter hall was completed in 1931 when five SWB 1 (Short Wave Beam) transmitters were installed. The Morse signals came down the line from Radio House (the Marconi station in London) where the operators punched out the messages and amplifiers at Ongar stepped up the voltages until the full radiated power of the transmitters was chopped into dashes and dots. The beam wireless picturegram service opened between London and Melbourne in 1934 with the transmission of facsimiles being added to Ongar’s functions.

There was a programme of continual progression at Ongar, two new 120kW long wave transmitters were installed and the gaps in the short wave building were gradually filled with more transmitters. During 1939 and 1940 the original SWB 1’s were converted for cooling by water instead of oil to reduce the risk of fire by incendiary bombs.

With the closing down of the Tetney Transmitting Station in 1943 a further concentration of the Cable & Wireless services was brought about by re-erecting the Tetney masts and beam aerials at Ongar. These were fed from a new building known as ‘D’ station, three quarters of a mile from the main station. This was the first transmitting station in the country to run completely unattended except for visits from maintenance staff. Four 25kW short wave transmitters were installed to take over the services to Bombay, Melbourne and Moscow while a fifth 20kW transmitter known as ‘D1’, of an advanced design, communicated with Cairo and the Middle East.

With the completion of ‘D’ station in 1944 another building to be known as ‘E’ station was under construction for further expansion of the telegraphic services. This was located on the south side of the Epping - Ongar railway line and no aerials were allowed to cross the line. It was designed for six transmitters of the SWB 8/10 type feeding a self-contained group of rhombic aerials.

It was common practice for 26 of the 31 transmitters at Ongar to be radiating simultaneously during peak traffic periods in the afternoon and evening. This represented a load of over 800kW from the power supply and 280kW of radiated energy.

In 1971 Ongar Radio was refitted with transmitting equipment costing £1M. A total of 23 new self tuning high frequency radio transmitters made Ongar one of the most up-to-date radio stations of its kind in the world transmitting telephone, telegraph and facsimile services to Eastern Europe, the Middle East, Africa and South East Asia from the new rhombic aerials which replaced the now outdated beam arrays. Its new status was however short lived.

After 1971 the national high frequency rationalisation scheme then being implemented concentrated the maritime telegraph service at Rugby and Ongar and this eventually became Ongar’s main function. As Ongar was refitted the station at Slip End near Baldock closed with its traffic being passed to Post Office radio stations at Bearley in Warwickshire and Somerton in Somerset. Due to the spread of satellite, cable and fibre optic communications the existing stations were gradually closed over the next 20 years. The buildings at Ongar were put to other uses; one building on the main site being used as a BT engine generator test centre while some of the more distant buildings were put to farm use.

By the mid 1990’s all the aerials had gone and the site had been abandoned and vandalised. Most of the buildings were demolished after a fire in 1997 and the site has now been sold to a property developer. North Weald Redoubt is a listed building and will escape any redevelopment of the site. Work started in November 2002 on a new pay and play golf course that will occupy 160 acres of the site. The 18 hole par 71 course is expected to open in the summer of 2005.

Sources:

Post Office Engineering Journal

Written by Phil Boggis on 17 October 2026.

I was a Technical Officer (TO) in the Post Office and trained at Ongar Radio Station. I have compiled some personal memoirs of my time there whilst training and operating the transmitters. Would you like to incorporate the attached file for the public record? Because confusion exists today on Internet forums about the current status of this site and its surviving buildings and what they were used for. I believe it is important and to differentiate these surviving building from the late Victorian North Weald Redoubt.

Kindest Regards, Phil Boggis (Former BT Technical Officer)

ENGINEERING MEMOIRS: ONGAR (NORTH WEALD) RADIO STATION (1965–1970)

(First-Hand Technical Witness of the Transmitting Era)

RECORDED BY: An Ongar Maintenance Engineer.

PRESERVED: October 2026

These memoirs centre on maintenance work at Ongar Radio Station during 1965–1970. They also include earlier practices described by senior engineers, the transmitter replacement programme extending into 1971, and background on the site’s origins and later history. The technical recollections and wider historical account are presented together, but do not all describe events witnessed during the stated period.

1. THE MARCONI SWB ERA AND THE CAT1 & CAT3 VALVES

The Transmitters: Maintenance work covered the legendary, early-design Marconi SWB 1, SWB 8 and SWB 10 (Short Wave Beam) transmitters. These large, pre-war transmitters formed the foundational backbone of global long-distance shortwave communication.
The Valves: The Transmitters were driven by the famous CAT1 and CAT3 (Cooled Anode Triode) valves, the world’s first practical water-cooled transmitting valves. The heavy copper anode formed part of the outer wall, sitting directly inside a plumbing jacket.
High Voltage Isolation: Implemented via large, coiled rubber hoses carrying pure distilled water. The sheer length of the coiled rubber hose created a sufficiently high column of water resistance to completely isolate the 7500V DC anode potential from the grounded water pumps.
Construction: Originally the transmitters were constructed without enclosures. For operator safety they were subsequently enclosed with aluminium or brass sheeting.

2. THE REALITY OF THE LIQUID PARAFFIN MYTH

Historical account: Senior Cable & Wireless engineers described the use of paraffin cooling in the 1920s and early 1930s, before the SWB 8/10 era. In their account, the coolant was described as paraffin (kerosene) or liquid paraffin oil and was used for its electrical insulating, or dielectric, properties to eliminate high-voltage leakage currents.
The Danger & Removal: Because pumping a flammable petroleum derivative next to high-temperature anodes and violent RF arcs was a severe hazard, these systems were completely stripped out and replaced by pure distilled water closed loops just before the outbreak of WWII.

3. HIGH-POWER RF MISMATCHES & PI TANK CIRCUITS

Power Dissipation: When a mismatch occurred between the transmitter and the antenna feed or a mistune, the massive reflected power was violently thrown backward. Rather than destroying the robust CAT1/3 valves, this energy was absorbed and dissipated directly inside the output Pi tank tuning circuits.
Structural Burnouts: The sheer voltage standing waves created localized plasma arcs right at the junction points where the spring-loaded copper pickup “fingers” tapped into the bare turns of the main copper-tubing tank coils.
The Metal Workshop: These continuous RF arcs physically melted and burnt holes straight through the thick copper tubing. Maintenance routinely required removing the entire heavy tuning coil assembly and taking it down to the onsite station workshop (the ‘Power House’) to physically weld and patch the copper holes.
Emergency Interventions: In these early pre-war transmitter designs, there were no automatic microsecond electronic trips. If an engineering crisis occurred, the only way to save the station from catastrophic fire was to walk up to the transmitter, grab a U link, and physically pull it out of the RF drive chain to instantly kill the excitation to the final stage.

4. HIGHLY INNOVATIVE COAXIAL SWITCHING

Aerial Switching Matrix: There was a unique system of copper coaxial feeders developed at Ongar by Marconi engineers. 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 tube had a sliding connection which proved to be rather prone to failure.
The ‘C’ Station aerial switching matrix: This was housed at the end of the transmitter hall which comprised of 50 ohm coaxial copper feeders whose inners were adjustable to enable them to be moved thus enabling connection to other transmitter feeders. This was done manually by sliding back the coaxial sheath with a special tool. The potentially ‘live’ inner could then be re-routed. There was extreme danger if the technician accidently selected the wrong feeder which was connected to a live transmitter with the potential to cause severe RF burns that are particularly deep and long lasting. To make sure this was not the case, the outer ‘earthed’ sheath was shorted to the inner copper tube conductor with the special ‘sheath tool’. No one told the technician exactly what would happen if a live transmitter was actually connected whilst doing this short circuit test but welding goggles would have been advised!

5. THE 1967 TRANSITION TO ST&C AIR-COOLED TECHNOLOGY

The Handover: The replacement programme ran from 1967 to 1971. During that period, the historic water-cooled transmitters reached the end of their service lives and were decommissioned. Their replacements were modern, high-efficiency, forced-air-cooled QT3 and QT8 transmitters manufactured by Standard Telephones and Cables (ST&C).
Space and Environment: Smaller transmitter cabinets did not remove the need for substantial cooling space. The modern ST&C cabinets occupied much less floor area than the room-sized Marconi installations, but their auxiliary equipment included industrial air blowers, specialised intake filters and heavy-gauge galvanised overhead ducting. This equipment required considerable space and carried the intense exhaust heat out through the station’s side vents.

6. HISTORICAL CONTEXT OF THE ONGAR STATION (NORTH WEALD)

Origin: Located at North Weald, Essex, the site originally served as the Victorian-era North Weald Redoubt (built c. 1880–1890) as an ammunition store. Marconi’s Wireless Telegraph Company bought the 730-acre site in 1919 and ingeniously converted the subterranean military brick earthworks into their original power generator rooms. Eventually these became the coppersmith’s workshops and store rooms.
Operations: Functioning exclusively as a High-Power Telegraph Transmitter Station (paired with the Brentwood receiving site), Ongar grew into a vital imperial hub. During its history there were four operating stations designated A through E. It handled international telegraphs, maritime safety traffic, and cutting-edge “Picturegram” facsimile services, instantly beaming print photos across the Commonwealth.
Post-Office & Legacy: Control transitioned through Cable & Wireless (1934) and was later nationalized under the General Post Office (GPO / British Telecom) in 1950. The historic site continued operating under modern guises until it was closed on the 31st October 1985 and eventually sold off during 1992, concluding nearly three-quarters of a century of frontline wireless history.
Powerhouse and Workshop: The Ongar Radio Station powerhouse and workshop were derelict circa 2020. During the 1950s and 1960s, a large engineering lathe stood near the entrance and a smaller lathe was installed to the right. On the left was a power hacksaw metal cutter driven by an overhead belt and pulley system, together with a grinder. To the left of the entrance was a brazing workstation for, amongst other things, the repair and manufacture of reflectometers. Two large workbenches stood in the centre of the workshop.
Site Layout: The powerhouse stood beside the main road leading to North Weald Village Hall at the bottom of the hill. The workshop was set back to the left of the main powerhouse. Buildings immediately to the left housed the local Amateur Radio Society, set up by staff members. The entrance to the Station Stores was at the front of the powerhouse.
Confusion with the North Weald (Ongar) Redoubt: It has been noted that contrary to some official records, the powerhouse and associated radio station buildings were not demolished with the establishment of the golf course between 2002 and 2005. They have been preserved to this day, albeit in a derelict and vandalised state. The whole area was fenced off to establish the boundaries of the historic redoubt and consequently these Radio Station buildings have therefore been preserved along with the Redoubt.