The British Telecom microwave network was a network of point-to-point microwave radio links in the United Kingdom, operated at first by the

General Post Office The General Post Office (GPO) was the state postal system and telecommunications carrier of the United Kingdom until 1969. Established in England in the 17th century, the GPO was a state monopoly covering the dispatch of items from a specific ...

, and subsequently by its successor

BT plc BT Group plc (formerly British Telecom) is a British multinational telecommunications holding company headquartered in London, England. It has operations in around 180 countries and is the largest provider of fixed-line, broadband and mobile ...

. From the late 1950s to the 1980s it provided a large part of BT's trunk communications capacity, and carried telephone, television and radar signals and digital data, both civil and military. Its use of line-of-sight microwave transmission was particularly important during the

Cold War The Cold War was a period of global Geopolitics, geopolitical rivalry between the United States (US) and the Soviet Union (USSR) and their respective allies, the capitalist Western Bloc and communist Eastern Bloc, which lasted from 1947 unt ...

for its resilience against

nuclear attack Nuclear warfare, also known as atomic warfare, is a military conflict or prepared political strategy that deploys nuclear weaponry. Nuclear weapons are weapons of mass destruction; in contrast to conventional warfare, nuclear warfare can prod ...

. It was rendered obsolete, at least for normal civilian purposes, by the installation of a national

optical fibre An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at ...

communication network with considerably higher reliability and vastly greater capacity. BT remains one of the largest owners of transmission and microwave towers in the UK. The most famous of these is the

BT Tower The BT Communications Tower, also known simply as the BT Tower, is a Listed building, grade II listed Radio masts and towers, communications tower in Fitzrovia, London, England, owned by BT Group. It has also been known as the GPO Tower, the P ...

in London, which was the tallest building in the UK from its construction in the 1960s until the early 1980s, and a major node in the BT microwave network.

Television links

The earliest operational GPO microwave links were provided for

405-line The 405-line monochrome analogue television broadcasting system was the first fully electronic television system to be used in regular broadcasting. The number of television lines influences the image resolution, or quality of the picture. It ...

BBC television.

Experimental systems

London to Birmingham pre-war

In 1939 the Post Office placed a contract with

EMI EMI Group Limited (formerly EMI Group plc until 2007; originally an initialism for Electric and Musical Industries, also referred to as EMI Records or simply EMI) was a British transnational conglomerate founded in March 1931 in London. At t ...

for an experiment in the relaying of television signals to

Birmingham Birmingham ( ) is a City status in the United Kingdom, city and metropolitan borough in the metropolitan county of West Midlands (county), West Midlands, within the wider West Midlands (region), West Midlands region, in England. It is the Lis ...

. In this case, the signals from

Alexandra Palace Alexandra Palace is an entertainment and sports venue in North London, situated between Wood Green and Muswell Hill in the London Borough of Haringey. A listed building, Grade II listed building, it is built on the site of Tottenham Wood and th ...

were to be received at

Dunstable Dunstable ( ) is a market town and civil parish in Bedfordshire, England, east of the Chiltern Hills, north of London. There are several steep chalk escarpments, most noticeable when approaching Dunstable from the north. Dunstable is the fou ...

and transmitted over a radio link to Sharmans Hill,

Charwelton Charwelton is a village and civil parish about south of Daventry in Northamptonshire, England. The 2011 Census recorded the parish's population (including Fawsley) as 220. The villages name means ' River Cherwell farm/settlement'. The prese ...

, some 40 miles distant towards Birmingham; thus carrying the signal two-thirds of the way from London to Birmingham.

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

intervened and this early experiment had to be abandoned.

London to Castleton 195 MHz

The GPO built an experimental chain of radio relay stations for television, which used the relatively low VHF frequency of 195 MHz and

frequency modulation Frequency modulation (FM) is a signal modulation technique used in electronic communication, originally for transmitting messages with a radio wave. In frequency modulation a carrier wave is varied in its instantaneous frequency in proporti ...

with a deviation of 6 MHz per volt. Each relay station consisted essentially of back-to-back rhombic antennas on opposite sides of a hilltop, connected via an amplifier. The frequency was not changed. The system was first tested on 24 March 1949. The stations were at: * Rowley Lodge, near Barnet, Hertfordshire *

Green Hailey Green Hailey is a hamlet in the parish of Great and Little Hampden, in Buckinghamshire Buckinghamshire (, abbreviated ''Bucks'') is a Ceremonial counties of England, ceremonial county in South East England and one of the home counties. ...

, Buckinghamshire *

Widley Widley is an area of the Greater Portsmouth conurbation in Hampshire, near Waterlooville and Purbrook. It is on the dip slope of the South Downs just north of the ridge called Portsdown Hill. Widley is served by the A3(T), trunk road which ru ...

, Hampshire *

Hook A hook is a tool consisting of a length of material, typically metal, that contains a portion that is curved/bent back or has a deeply grooved indentation, which serves to grab, latch or in any way attach itself onto another object. The hook's d ...

, Hampshire *

Wotton-under-Edge Wotton-under-Edge is a market town and civil parish in the Stroud district of Gloucestershire, England. Near the southern fringe of the Cotswolds The Cotswolds ( ) is a region of central South West England, along a range of rolling hills ...

, Gloucestershire * Post Office Radio Laboratory at Castleton, Monmouthshire

London to Castleton 4 GHz

The GPO built an experimental 4 GHz system, which was used operationally to feed TV pictures to the

Wenvoe transmitter The Wenvoe transmitting station, officially known as Arqiva Wenvoe, is the main facility for broadcasting and telecommunications for Cardiff and South Wales. It is situated close to the village of Wenvoe in the Vale of Glamorgan, Wales, in the Uni ...

during its first four months on air in late 1952, until a coaxial feed became available. Some of the equipment from this link was recovered, refurbished, modified and used to provide a permanent link from London to Rowridge, Isle of Wight in 1954.

London to Birmingham 900 MHz

A chain of stations was built between telephone exchanges in London and Birmingham to connect the

Sutton Coldfield transmitting station The Sutton Coldfield transmitting station is a broadcasting and telecommunications facility located in Sutton Coldfield, Birmingham, England. In terms of population covered, it is the third most important transmitter in the UK, after Crystal P ...

. The contract for this was placed with GEC in mid-1947. The stations were at: * London Museum exchange *

Harrow Weald Harrow Weald is a suburban district in Greater London, England. Located about north of Harrow, London, Harrow, Harrow Weald is formed from a leafy 1930s suburban development along with ancient woodland of Harrow Weald Common. It forms part of ...

, Middlesex * Zouches Farm, Bedfordshire *

, Northamptonshire *

Turner's Hill Turners Hill is the highest hill in the county of West Midlands, England at above sea level. The hill is in the Rowley Hills range, situated in Rowley Regis, near the boundary with Dudley. The hill can be seen from many miles away, and offers ...

, Staffordshire * Telephone House, Birmingham

Manchester to Kirk o'Shotts

The GPO placed a contract in July 1950 for a chain of microwave links to feed BBC television from

Manchester Manchester () is a city and the metropolitan borough of Greater Manchester, England. It had an estimated population of in . Greater Manchester is the third-most populous metropolitan area in the United Kingdom, with a population of 2.92&nbs ...

to the Kirk o' Shotts transmitting station. This was the first permanent GPO system to use the 4 GHz band. The chain was routed near the east coast in order to be close to

Leeds Leeds is a city in West Yorkshire, England. It is the largest settlement in Yorkshire and the administrative centre of the City of Leeds Metropolitan Borough, which is the second most populous district in the United Kingdom. It is built aro ...

Newcastle Newcastle usually refers to: *Newcastle upon Tyne, a city and metropolitan borough in Tyne and Wear, England, United Kingdom *Newcastle-under-Lyme, a town in Staffordshire, England, United Kingdom *Newcastle, New South Wales, a metropolitan area ...

and

Edinburgh Edinburgh is the capital city of Scotland and one of its 32 Council areas of Scotland, council areas. The city is located in southeast Scotland and is bounded to the north by the Firth of Forth and to the south by the Pentland Hills. Edinburgh ...

. The stations were at: * Manchester Telephone House * Windy Hill, Lancashire * Tinshill, Yorkshire *Arncliffe Wood, Yorkshire *

Pontop Pike The Pontop Pike transmitting station is a facility for telecommunications and broadcasting situated on a 312-metre (1,024-ft) high hill of the same name between Stanley and Consett, County Durham, near the village of Dipton, England. The mast ...

, County Durham *Corby's Crags, Northumberland *Blackcastle Hill, East Lothian *

Blackford Hill Blackford Hill is a hill in Edinburgh, the capital city of Scotland. It is in the area of Blackford, between Morningside, and the Braid Hills. Together with the Hermitage of Braid, it comprises the Hermitage of Braid and Blackford Hill ...

, Midlothian * Kirk o' Shotts, Lanarkshire

Backbone

The term '

backbone The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

' is often applied to the core of a communications network, i.e. the part that provides high-capacity links over long distances between major nodes. In the early 1950s, the term was used by the General Post Office (BT's predecessor) to describe a chain of microwave links designed to provide resilient communications in the event of

nuclear war Nuclear warfare, also known as atomic warfare, is a War, military conflict or prepared Policy, political strategy that deploys nuclear weaponry. Nuclear weapons are Weapon of mass destruction, weapons of mass destruction; in contrast to conven ...

. It was originally designed as a chain of stations between south-east England and Scotland. The exact location of the Backbone sites changed as the project developed, but in July 1956 there were 14 planned sites at (from south to north): *Tring, Hertfordshire *Charwelton, Northamptonshire *Coalville (Copt Oak), Leicestershire *

Pye Green BT Tower Pye Green BT Tower is a tall telecommunication tower built of reinforced concrete at Pye Green, Staffordshire, England. Standing on the far southern edge of Cannock Chase, it is one of 14 telecommunication towers in the United Kingdom built of ...

, Staffordshire *

Sutton Common Sutton Common is the name of former common land and a district and neighbourhood located in Sutton, London. The area is mostly located within the London Borough of Sutton, with some of the streets to the north and west of Sutton Common Park ...

, Cheshire *Saddleworth, Yorkshire *Hunters Stones, at Norwood near Menwith Hill which is between Harrogate (6 miles) and Skipton (14 miles), Yorkshire *Azerley, Yorkshire *

Richmond Richmond most often refers to: * Richmond, British Columbia, a city in Canada * Richmond, California, a city in the United States * Richmond, London, a town in the London Borough of Richmond upon Thames, England * Richmond, North Yorkshire, a town ...

, Yorkshire *

Muggleswick Muggleswick is a village and civil parishes in England, civil parish in County Durham, England. It is situated a few miles to the west of Consett. the population was 130 at the 2001 Census reducing to 113 at the 2011 Census. Geography A signifi ...

, Co. Durham *Cold Fell, Cumberland *

Lockerbie Lockerbie (, ) is a town in Dumfries and Galloway, located in south-western Scotland. The 2001 Census recorded its population as 4,009. The town had an estimated population of in . The town came to international attention in December 1988 when ...

, Dumfriesshire *Green Lowther, Dumfriesshire * Kirk o' Shotts, Lanarkshire (GPO site near BBC site) Two additional 'backbone spur' sites were planned for Shrewsbury and Grantham ( Carlton Scroop), which connected to the main Backbone spine at Pye Green and Coalville (Copt Oak) respectively.

Radio standby to line

The 1956 plan also described a series of links called 'radio standby to line'. These were spur links between the GPO backbone sites and defence 'customer' sites. They were designed to carry between 25 and 150 'private wire' (a.k.a. leased line) circuits each, by radio. The paper contains a list of sites and a network map, showing the following radio standby to line links: * Kirk o' Shotts to Gailes

ground-controlled interception Ground-controlled interception (GCI) is an air defence tactic whereby one or more radar stations or other observational stations are linked to a command communications centre which guides interceptor aircraft to an airborne target. This tactic wa ...

(GCI) radar station near Ayr *

to Boulmer GCI station,

Royal Observer Corps The Royal Observer Corps (ROC) was a civil defence organisation intended for the visual detection, identification, tracking and reporting of aircraft over Great Britain. It operated in the United Kingdom between 29 October 1925 and 31 Decembe ...

and regional communications, Seaton Snook GCI station *Hunters Stones to Forest Moor Admiralty radio receiving station, Shipton RAF Sector Operations Centre (SOC), Preston SOC, Regional Commissioner's HQ and Admiralty radio transmitting station *Grantham to RAF bomber bases and US Air Force bases *Norwich to RAF SOC (Bawburgh), US Air Force bases, GCI stations, naval headquarters, continental communications *

Kelvedon Hatch Kelvedon Hatch is a village and civil parish in the Borough of Brentwood in south Essex, England. It is situated just north of Pilgrims Hatch, approximately to the north of Brentwood and is surrounded by Metropolitan Green Belt. In 2019 the ...

, Essex to RAF SOC, RAF bomber stations, RAF radar stations *

West Malling West Malling ( , historically Town Malling) is a market town in the Tonbridge and Malling district of Kent, England. It has a population of 2,590. Landmarks West Malling contains several historic buildings, including St Leonard's Tower, West ...

, Kent to naval headquarters at Chatham and Dover, RAF radar and

Fighter Command RAF Fighter Command was one of the commands of the Royal Air Force. It was formed in 1936 to allow more specialised control of fighter aircraft. It operated throughout the Second World War, winning fame during the Battle of Britain in 1940. The ...

headquarters, continental communications *

Upavon Upavon is a rural village and civil parish in the county of Wiltshire, England. As its name suggests, it is on the upper portion of the River Avon which runs from north to south through the village. It is on the north edge of Salisbury Plain ...

, Wiltshire to Army establishments on

Salisbury Plain Salisbury Plain is a chalk plateau in southern England covering . It is part of a system of chalk downlands throughout eastern and southern England formed by the rocks of the Chalk Group and largely lies within the county of Wiltshire, but st ...

* Sopley and Portsmouth to naval headquarters at Portsmouth and naval radio stations at Horsea and Flowerdown *

Box A box (plural: boxes) is a container with rigid sides used for the storage or transportation of its contents. Most boxes have flat, parallel, rectangular sides (typically rectangular prisms). Boxes can be very small (like a matchbox) or v ...

, Wiltshire to Admiralty establishment at Bath, RAF SOC and Signals centre, Army signals centres at Cheltenham and Droitwich and Army radio stations, Foreign Office

GCHQ Government Communications Headquarters (GCHQ) is an intelligence and security organisation responsible for providing signals intelligence (SIGINT) and information assurance (IA) to the government and armed forces of the United Kingdom. Primar ...

and radio stations, important radio stations and miscellaneous radar stations in south-west England, South Wales and the Border Counties.

Aerials and towers

Various types of aerial have been used in the network's history. At first, prime-focus parabolic reflectors were used. In about 1960, dual-band horn aerials started to be used widely, and a few of these survive to the present day. They began to go out of fashion at the end of the 1960s, when types of parabolic antenna with an improved performance became available. Many of the towers were designed with particular types of aerial in mind. Often they were designed to carry horn aerials but no longer do so.