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. Before the Acts of Union 1707, it was the postal system of the Kingdom of England, established by Charles II in 1660. ...

, and subsequently by its successor BT plc. 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 is a term commonly used to refer to a period of geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc. The term '' cold war'' is used because the ...

for its resilience against

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

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

optical fibre An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...

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 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 was ...

BBC television.

Experimental systems

London to Birmingham pre-war

In 1939 the Post Office placed a contract with EMI for an experiment in the relaying of television signals to Birmingham. In this case, the signals from

Alexandra Palace Alexandra Palace is a Grade II listed entertainment and sports venue in London, situated between Wood Green and Muswell Hill in the London Borough of Haringey. It is built on the site of Tottenham Wood and the later Tottenham Wood Farm. Origi ...

were to be received at Dunstable and transmitted over a radio link to Sharmans Hill, Charwelton, some 40 miles distant towards Birmingham; thus carrying the signal two-thirds of the way from London to Birmingham. World War II 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 with a deviation of 6 MHz per volt. Each relay station consisted essentially of back-to-back

rhombic antenna A rhombic antenna is made of four sections of wire suspended parallel to the ground in a diamond or "rhombus" shape. Each of the four sides is the same length – about a quarter-wavelength to one wavelength per section – converging but not to ...

s 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 Barnet may refer to: People *Barnet (surname) * Barnet (given name) Places United Kingdom *Chipping Barnet or High Barnet, commonly known as Barnet, one of three focal towns of the borough below. *East Barnet, a district of the borough below; an ...

, Hertfordshire *

Green Hailey Green Hailey is a hamlet in the parish A parish is a territorial entity in many Christian denominations, constituting a division within a diocese. A parish is under the pastoral care and clerical jurisdiction of a priest, often termed a p ...

, Buckinghamshire *

Widley Widley is an area of the Greater Portsmouth conurbation in the South East of England 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), tru ...

, Hampshire * Hook, Hampshire * Wotton-under-Edge, 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 South Wales and the West Country. It is situated close to the village of Wenvoe in the Vale of Glamorgan, Wales, i ...

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 to

. 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, Harrow Weald is formed from a leafy 1930s suburban development along with ancient woodland of Harrow Weald Common. It forms part of the London Boro ...

, Middlesex * Zouches Farm, Bedfordshire * Charwelton, Northamptonshire * Turner's Hill, 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 to the

Kirk o' Shotts transmitting station The Kirk o' Shotts transmitting station is a broadcasting and telecommunications site at The Hirst which lies just outside the village of Salsburgh which is near the town of Shotts in North Lanarkshire central Scotland. (Kirk o' Shotts means 'Chur ...

. 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, Newcastle and Edinburgh. The stations were at: * Manchester Telephone House *

Windy Hill Windy Hill may refer to: Places * Windy Hill, Essendon, an Australian rules football ground in the Melbourne area * Windy Hill Wind Farm, a wind power station near Ravenshoe, Queensland, Australia * Windy Hill (Pennines), a hill on the Pennines w ...

, Lancashire * Tinshill, Yorkshire *Arncliffe Wood, Yorkshire * Pontop Pike, 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 Loca ...

, Midlothian * Kirk o' Shotts, Lanarkshire

Backbone

The term ' backbone' 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 theoretical military conflict or prepared political strategy that deploys nuclear weaponry. Nuclear weapons are weapons of mass destruction; in contrast to conventional warfare, nuclear w ...

. 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, Staffordshire * Sutton Common, Cheshire *Saddleworth, Yorkshire *Hunters Stones, near Skipton, Yorkshire *Azerley, Yorkshire *

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

, 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. The village has a num ...

, Co. Durham *Cold Fell, Cumberland *

Lockerbie Lockerbie (, gd, Locarbaidh) is a small town in Dumfries and Galloway, south-western Scotland. It is about from Glasgow, and from the border with England. The United Kingdom Census 2001, 2001 Census recorded its population as 4,009. The town ...

, 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 (GCI) radar station near Ayr *

to Boulmer GCI station, Royal Observer Corps 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 in civil parish of Kelvedon Hatch, 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 ...

, Essex to RAF SOC, RAF bomber stations, RAF radar stations * West Malling, Kent to naval headquarters at Chatham and Dover, RAF radar and Fighter Command headquarters, continental communications * Upavon, Wiltshire to Army establishments on

Salisbury Plain Salisbury Plain is a chalk plateau in the south western part of central 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 wi ...

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

Box A box (plural: boxes) is a container used for the storage or transportation of its contents. Most boxes have flat, parallel, rectangular sides. Boxes can be very small (like a matchbox) or very large (like a shipping box for furniture), and can ...

, 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 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, and look rather odd as a result.