The GPO, Post Office Telephones and British Telecom (BT)

This page documents the Cold War infrastructure of the UK state telecommunications operator the GPO the predecessor of the now private limited company British Telecom. Descriptions of the services to support cold war communications may be found in other topics on this site.

The Company History

At the start of the Cold War the General Post Office (GPO) operated all UK telecommunications. The GPO was a Government department with its finances strictly controlled by the treasury. The Engineering Department was engaged in lots of 'non-core' business and research. These included development of NHS Hearing Aids, High reliability valves and then transistors as components for undersea cable amplifiers. Designing, building and exporting highly stable quartz crystal oscillators all over the world. Various computer projects and even baggage handling at London Heathrow.

In the same way as the GPO Engineers designed and built the Colossus code breaking machine in WW2, they continued to design and build or advise on many military and civilian projects during the cold war period. Examples being the nuclear early warning system and callout systems for firemen.

In October 1969 the Post Office ceased to be a Government department and became a pubic corporation. The letter and parcel business became Royal Mail and the telecommunications arm became Post Office Telephones.

In 1981 the British Telecommunications Act separated the services of the Post Office and Post Office Telephones became the public corporation British Telecom (BT). The Government privatised BT in November 1984 whilst still retaining a controlling share in the new company. The term BT is used throughout this site to mean British Telecom PLC and its predecessors.

In the 1960's and early 70's the UK telecommunications infrastructure was very different from today's modern networks. Telephones were a luxury item with less than 50% of households owning one. All telephones were fixed landlines with a handful of very costly vehicle based mobiles operating around major cities. The network was exclusively run by the Post Office. As a Government department the Post Office had been starved of money by the exchequer. There were long waiting lists and a shortage of lines forced many people to accept party lines. Two houses would share the same wires to the exchange in a party line. Although they had different numbers there was no privacy between parties. The lack of lines between exchanges meant users would encounter the engaged signal and there were even television adverts discouraging the use of the telephone.

Although some large companies had their own internal telephone systems they were forced to rent BT lines to connect between offices. These lines were known as a Private Wire ( PW ) or a Private Circuit in the U.K. or as a 'Leased Line' in some countries. This gave the renter exclusive use of these wires provided to their premises by BT. As a government department along with their monopoly, it was obvious the GPO / BT would provide all landline based Military and Civil Defence communications too. Television pictures were carried on BT cables or microwave links from studios to transmitter sites.

It was not until 1982 that other companies were allowed to provide telephone services. Mercury Communications were the first to provide an alternative. They provided the backbone for many of the emerging cable television companies who began to provide business and residential telephone lines in the area they cabled. The Mercury network only connected major centres in a figure of eight and was very reliant on BT to deliver the final leg to the customer. As this wasn't a truly independent network, the majority if not all the civil defence communications remained in BT's hands.

Post WWII - The Need for a Resilient Communications Network

With tensions increasing between East and West in the Cold War, the GPO made a multi-prong approach to improving the resilience of communications in the UK. At the same time there was a large growth in the use of the telephone and many military projects requiring telecommunications services.

Modernisation of the network was underway in the fifties, replacing manual operator switchboard with automatic Strowger type telephone exchanges, to handle long distance telephone calls. This project known as 'Trunk Mechanisation' was also design to move the switching away from London into provincial 'Zone' centres.

City centres were vulnerable to bombing, three deep level exchanges were built in Birmingham, London and Manchester. These and other cities such as Bristol had what was called 'Ring' schemes, diverting long distance telephone cables away from the centre and around the outskirts of the city. Some of these rings had specially hardened repeater stations.

Another proposal was to provide microwave radio as a backup to long distance cables known as 'Backbone'. In the event, the demand for long distance television circuits took precedence so it was many years later before it was implemented. The early plans in the National Archives have misled many authors into thinking it was the driver for the long haul microwave network.

Annual Reports made by the GPO Engineer in Chief (E-i-C) shed light on the developments of the network and resilience schemes, but often lack the detail to be certain as to what they are referring to. The blue italic text from these annual reports are sourced from the BT Archives.

E-i-C Report 50-51

Page 56: Two standard buildings of a specialized character have been designed to give a high degree of protection to equipment housed therein from blast and external fire hazard. They will have gross superficial floor areas of approximately 19,000 and 22,000 sq. ft. The buildings, which are known as Types PR1 and PR2, differ only in the length of the apparatus rooms; they are semi buried and of windowless monolithic reinforced concrete construction. The report goes on the describe features of the air conditioning.

E-i-C Report 51-52

Page 62: Although some interior details of the PR1 and PR2 buildings, described in the last Report (page 56), have yet to be finally settled, the building of a number of each type has started. Plate 7 on Page 108 shows one under construction, this photograph and those mentioned in the next annual report are reproduced in the PR1 Construction gallery below.

E-i-C Report 52-53

Page 44: During 1952/53, contracts placed for defence works, including Rotor and Infrastructure, totalled £1.41m. and preponderated over those placed for development of the normal trunk network, which amounted to £1.05m. In addition, instructions were issued to Regions for the installation by direct labour, of audio equipment for 45 Rotor and defence stations and 87 normal works, and for 74 music amplifier installations including nine for the B.B.C. emergency scheme. . .

Page 45: Contracts have been placed for the initial equipment requirements of the Birmingham and Bristol Provincial Ring Scheme. . . . . . . During 1952/53, contacts were placed for the installation of repeater station power plant to a total estimated value of £325,000. This is less than half the value of plant ordered during 1951/52 (£777,000) because, that year, an abnormal proportion of large plants for the new stations in the Provincial Ring Schemes was ordered.

In this year, the BT Archives contain a number of photographs of the construction of London Kingsway exchange dated 30 January 1952 (TCB417/E17648 to E17671)

Page 67: Almost all the year's work was expended on the provision of buildings for defence schemes, including Rotor.

Seven new R5-type terminal repeater station buildings were completed, one being a Rotor requirement and the other six forming parts of the London and Provincial Ring Schemes.

Twenty-one intermediate carrier station buildings have been completed, all for carrier ring and 24-circuit conversion schemes required for Rotor and defence. Nine of the buildings are of the R4-type with accommodation for a small number of carrier terminals. These are situated at selected points where circuits may need to be extracted to feed Services installations, and terminal equipment is already being provided in one of the stations for this purpose. Eight of the remaining 12 buildings are of the R3-type to house pre-51-type carrier line equipment, and the final four are the first R3 A.C.-type buildings to be provided and will be used for the first route to have 51-type carrier line amplifiers (Edinburgh - Dundee).

Accommodation for Rotor and defence schemes, which was required too urgently to permit the erection of permanent buildings, has been provided in nine wooden huts. Seven of the huts will house transmission equipment, and two will contain power plant. They will serve as temporary repeater stations until permanent buildings can be made available. The huts, known from the manufacturer's name as "Bath" huts, are provided by the Ministry of Works and are constructed in sections 6 feet long and 20 feet wide, with a clear height of 12 feet. As delivered by the manufacturer, the huts are not lined but the contract for erection includes lining with foil-backed plaster board. As the huts would not be strong enough to withstand the lateral stresses imposed if the normal practice of tying transmission equipment racks to angle irons on the walls were adopted, Steel channels 3 in. x 1½ in., set in the concrete base, are erected vertically at intervals along the walls and used to support the horizontal 2 in. x 2 in. angle iron to which the rack tie-bars and cable rack and busbar supports are fixed.

The construction of a number of the PRl and PR2 buildings, referred to in the 1951/52 Report (page 62) has now reached an advanced stage. A building is illustrated in Plate 12, Figs. 2 and 3. The main part of this building is approximately 190 feet long by 50 feet wide and protrudes about 14 feet above ground level. Drying out those large concrete structures, particularly during the wet weather of the past year, has proved to be a bigger problem than had been anticipated. One Region overcame this problem very successfully by using an agricultural grain dryer, and arrangements have been made for other Regions to employ this method where they find it necessary.

Major building extensions at five terminal repeater stations and extensions of four temporary stations have been completed.

Much work has been done on the planning and allocation of accommodation in basements of buildings for which special protective measures are to be provided.

E-i-C Report 53-54

Page 17: Special measures have been taken to provide security for trunk communications throughout the country. Schemes have been prepared for giving protection to trunk switching equipment and telephone repeater equipment, and the use of several different cable routes for the groups of trunk circuits between main centres has minimised the general effect of any interruption on individual trunk cables. Note: Trunk = Long Distance

Page 40: Reorganisation of the Control and Reporting System continued throughout the year. Cabling for 25 new stations has been planned and 17 of the are completed; the remainder are proceeding satisfactorily. Rearrangement and provision of a large number of private circuits, in preparation for the opening of these new stations, was put in hand and, for four of the 25 completed. . . . The requirements of an additional 13 stations for a later stage of the Control and Reporting System have been advised by the Air Ministry and preliminary planning has actively pursued. Control and reporting sounds like part of the ROTOR plan.

Page 52: In connexion with the ring schemes, equipment has been ordered for the diversion of the following carrier cables. Bristol - Salisbury, Patchway - Warmley, Bristol - Oxford, Felton - Warmley, Lyndon - Queslett, Selly Oak - Queslett, Selly Oak - Lyndon, Bristol - Gloucester, Oldham - Swinton, Stockport - Swinton, Oldham - Stockport, Moortown - Rothwell Haigh, Gildersome - Moortown.

Those familiar with Hardened Repeater Stations will recognise Lyndon and Queslett (Birmingham ring), Warmley (Bristol ring,) Swinton and Stockport (Manchester ring), Uddingston (Glasgow ring) and Rothwell Haigh (Leeds ring). On Page 72, there is a report of the completion of 6 PR1 and 1 PR2 type of protected repeater buildings.

E-i-C Report 54-55

Page 13: During the year, the final stages in the present trunk mechanisation plans for London were completed with the opening of London Trunk Kingsway and the incoming section of London Trunk Faraday on 30 October 1954 and 8 January 1955 respectively.Note: Kingsway is the underground exchange. Also during this year, the Speaking Clock rolled out to 34 centres outside the major cities, to play its part 10 years later as the bearer for HANDEL.

Page 40: The first stage of the Control and Reporting System has continued throughout the year and cabling has been completed. Rearrangement and provision of a large number of private wires has resulted in 19 stations being prepared for service. The Air Ministry has opened 17 of these and decided to abandon two. Abandoning these two stations marks the demise of ROTOR.

Page 41: At the end of 1954 it became known that the A.A. requirements were totally reorganised, which would mean the cessation of most of the A.A. network. . . . . Instructions to cease some 600 circuits were received. This refers to the stand down of the Anti Aircraft batteries in the Gun Defended Areas (GDA) and the A.A. Operations Rooms (A.A.O.R.) that were reused for other purposes, Lansdown ROC Sector HQ, Ullenwood RGHQ 7.1 and Local Authority Emergency Centres like Mistley, Essex.

Glasgow Ring Scheme Page 54: The Glasgow - Oban coaxial route will terminate at the new Glasgow / Uddingston repeater station, and 24 circuit carrier line amplifier equipment has been ordered for this station to enable the existing Glasgow - Perth - Dundee and Carlisle - Glasgow routes to be be intercepted. This will associate the new Uddingston station with the main carrier network. There is a reference to another new repeater station at Kirkintillock. This was in Washington Rd G66 1DP but appears to have been demolished and replaced by flats, so its not known if it were a protected repeater PR building.

Further down the page: The Margaretting - Tunbridge Wells carrier route will complete a new carrier ring cable system on the South side of London, interconnecting at Guildford with the Northern carrier ring previously provided and at Margaretting with the existing London - Norwich carrier route.

E-i-C Report 55-56

Page 58: Reports that carrier equipment was ordered for a number of routes and these include Anchor - Queslett and Anchor - Lyndon. Anchor being the Birmingham underground exchange, Queslett and Lyndon are two protected repeaters on the Birmingham Ring Scheme.

E-i-C Report 58-59

Page 65: The installation of standby generating plant at the Birmingham Anchor and Manchester Guardian schemes has been completed. Each installation consists of three three-cylinder two stroke oil engines each developing 400 b.h.p. at 600 rev/min and directly coupled to 300 kVa alternators.

Secret Underground Exchanges

Three underground exchanges were built in the nineteen fifties that were classified secret until the mid sixties. These were designed to withstand a Hiroshima type blast but as atomic weapons became more powerful they were rendered useless. The exchanges were used until the early 1980ís but now only the cable infrastructure remains.

The largest was 'London Kingsway' which BT placed for sale in 2008. A good account of London Kingsway following its history from WW2 to present day may be found at that is linked from this site.

The 'Manchester Guardian' exchange came to the public notice in 29 March 2004 when a fire in the cable network severely disrupted the telephone network around the Manchester area.

For a time the 'Birmingham Anchor' exchange and cable tunnels had been declared unsafe to work in due to water seepage. But BT refurbished them (circa 2010) to make them safe again as they still carry underground cables across the city.

Anchor Exchange

Click here to watch the 1998 BBC video of Birmingham Anchor on YouTube

Birmingham Anchor Exchange (BM/AN) is underneath Telephone House in Newhall Street. The exchange Strowger switching equipment and repeater station were in two large tunnels on two levels, other large tunnels contained the standby generators, fuel tanks and air conditioning plant. All these large tunnels are joined together with smaller diameter tunnels, a detailed layout is amongst the photographs of the excavation and fitting out work, in the image gallery.

As part of the Anchor Project a cable tunnel runs South and connects with Midland Exchange ending near the corner of Essex St and Bromsgrove St. This had access from the BT Essex St stores complex, now redeveloped as flats. Before the tunnels became unsafe some of my planning colleagues would walk through them between Midland exchange and Telephone House to avoid getting wet on a rainy day. A Northward cable tunnel was planned to connect to a point at the rear of Hockley Sorting Office, however this apparently was never completed.

A 7 foot diameter cable tunnel, is not part of Anchor but constructed around 1961/62 at the time the City Council were creating the inner ring road, approaches Aston Exchange and the University but ends outside the 'W i l l i a m   B o o t h   C e n t r e' in Shadwell Street. The other end of this tunnel passing Telephone House finishes at the city end of Broad Street. This is shown on the map in green and connects to Telephone House in Newhall St, which is above Anchor.

Other sources on the web falsely suggest it was possible to walk to Smallbrook and Aston Cross exchanges. A series of deep level cable tunnels do exist around the city but do not give access to those exchanges. Another falsehood on the web, is the assertion the basement of Aston University was connected into the cable tunnels.

A BBC video on YouTube shows plenty of detail in a quick tour of the Anchor tunnel system but wrongly suggests it was the Midlands Regional Seat of Government, that was actually RGHQ 9.2 located at Drakelow. Nevertheless Anchor was an important place for telecommunications with circuits extending all over the Midlands area.

There are a few visible signs of the Anchor tunnels on the surface. Air Vents and Lift Access points can be seen next to the Telephone House in Fleet Street and also Lionel Street goods lift access. The Church Street lift is hidden from view at street level but can be distinguished on the aerial photograph. The staff entrance lift is in Telephone House. At the time of the construction of underground Anchor Strowger Exchange, above ground Telephone House contained a local telephone exchange known as 'Central' and two trunk exchanges 'Newhall' and 'Colmore Lodge' all using Strowger switching equipment. Anchor's Strowger switches were removed in the early eighties. By 1995 the Digital Network had completely replaced all Strowger and electronic / transistorised analogue switches throughout the country.

Hardened Repeater Stations

The connection between the home telephone and the exchange is carried on a single pair of wires. Over the relatively short distances involved, it is economical to bundle these together in large cables, some containing as many as 2000 wires. Over long distances, the use of large cables is not economic. A system of multiplexing 24 analogue speech channels into two pairs of wires was developed. Further developments increased this capacity. By the 1970's a 4MHz co-axial cable could carry 960 channels on two coax tubes, one for each direction of speech. Today 40GB optical fibres have 10,000 times this capacity.

When telephone calls are sent down a long pair of wires between exchanges, they must be amplified at regular intervals. The amplifying point is called a Telephone Repeater Station (TRS). Most large Automatic Telephone Exchange (ATE) buildings incorporated a TRS area too. In small exchanges a special area wasn't warranted for the TRS in this case the amplifier racks were placed alongside the switching apparatus. On long distance routes, the multiplex equipment at each end was housed in the repeater station rather than the exchange part of the building. Coax cables carrying television or multiplexed speech channels required amplifying every couple of miles. Where no suitable exchange building existed, a repeater station in a small brick building the size of a garden shed was built.

At the same time that the three underground exchanges were being built a small number of hardened repeater stations were constructed to house the repeater equipment, six PR1 types and two larger PR2 type. At Warmley (XEX) near Bristol once handled circuits from the Central Government War Headquarters (CGWHQ) bunker in Corsham, but this PR2 building has long since been demolished and is now Springley Court BS15 9RA. The OS Map Ref given on Sub-Brit is wrong, it should be ST 7726 7355. The Swinton RS (MR/F) building in Lawnswood Drive, Swinton M27 5NH, has been demolished and used for housing becoming the northerly side extension to Lawnswood Drive.

A two storey semi-sunken hardened repeater station was constructed in 1954 on the Coventry Road, Sheldon a suburb on the East side of Birmingham. Map reference SP141842. Known as Lyndon Green (BM/D), it was contemporary with the secret underground exchange in Birmingham City centre. At the end of its cold war use the building served as the Regional repair workshop during the 1980's - 90's. In the gallery there are pictures of the property taken in March 2009. The hardened concrete core has brick built offices attached. A similar hardened repeater station was built at Whitecrest Road, Queslett (BM/H) a Northern suburb of Birmingham. Map reference SP051943 Postcode B43 6EE and now houses the 'Beacon' telephone exchange.

The other hardened repeater stations are :- Glasgow, Uddingston (GW/J) located in Holmbrae Rd, Uddingston, Glasgow G71 6AN can be seen on Google Street View [October 2016]. At Manchester, Stockport (MR/G) the hardened repeater station is colocated with the exchange at Brundret St, Stockport SK1 4LW and visible on Street View [October 2014]. The Portsmouth, Portsdown (PT/C) repeater station in New Down Lane, Cosham is clearly visible on Street View [May 2011]. The former Leeds, Rothwell Haigh (LS/D) repeater station at Sharpe Lane, Wakefield, WF3 3AW, served a now disused RAF communications centre, has been sold off and houses built in its grounds, the PR2 building can be seen on Google Street View [June 2008] and hidden by trees [May 2015]

The GPO / BT Long Haul Microwave Network

Copt Oak 1965

Copt Oak Microwave Station

This topic has been revised during 2016, thanks to Dan Glover sharing his information with me as he researched into the BT Archives during the preparation of his website. [link to be added when it is live] Over the years there has been much speculation of the purpose of the microwave network. It certainly was used for carrying Television, Telephone and Private circuits over long distances. However some authors have suggested devious military or spying purposes which seem very unlikely. One rather stupid idea put forward was the microwave beams being intercepted at Regional Government Headquarters at times of war.

The conspiracy theories have been fuelled by a GPO paper in the National Archives [CAB 134/1207] dated 1956 proposing a microwave backbone bypassing major cities to safeguard vital communications at times of war. However, during the late fifties and early sixties the demand for long distance television links outstripped the provision of coaxial cables, the early growth of the microwave links was for television not the cold war. This wasn't a network but separate point to point microwave links patched together where the preferred option of coaxial cables did not exist and could not be provided in time to meet the broadcasters demands.

Heysham 1968

Heysham Microwave Station

Only later on did a long haul network form. Modernisation of the highlands and islands telephone system in the seventies relied on microwaves to provide connections to local exchanges, but this wasn't part of the national long haul network characterised by 3 or 4 metre dishes on tall towers. The national network continued to grow for a number of years reaching maturity in the early eighties.

When Optical Fibre made its way into the BT network providing huge capacity for telephony and especially the growing data market. The relatively low bandwidth long haul microwave network became redundant. During the mid-2000's the long haul microwave dishes were removed from the BT microwave towers leaving some looking very bare. Only those serving the highlands of Scotland still remain. Microwaves are still used to feed some medium bandwidth customers premises, but with the drive to provide fibre to the customer, their life expectancy is rather short.

Mr L.R.N. Mills former head of PO Inland Radio Planning, writing for Connected Earth - Birth of the BT Tower sums up the position in the early sixties : . . . Thus we had a number of disjointed radio links connected together by cable with no apparent plan for a national radio network. Then came Backbone, a system planned to provide secure communications between strategic Government locations avoiding cables which, by the very nature of the network, passed through important towns and cities. The Backbone network deliberately avoided built-up areas and thus did not provide any basis for a national city to city radio network. Top management in the Radio Branch were somewhat irritated by this. . .
. . . Then came, BBC2, 625-line colour television. . . plans were at last laid for a national city to city microwave radio network. . .
Next question, if a national radio network were established for TV, would adding telephony channels to radio links be cheaper than laying more cables? [Yes] . . .
. . . The conclusion from all this is that we have mainly to thank the coming of 625-line colour television for the existence of BT's microwave radio network and the [London] BT Tower.
and he should know.

Concrete Tower

Wotton Under Edge Microwave Station

Twycross 2005

Twycross Microwave Station

The early parts of the network used microwave dish aerials. Later on microwave horn aerials became the norm as they could provide dual polarisation and dual band working for increased capacity. Developments in dish aerials design allowed their use to become widespread and eventually most of the bulky and heavy horn aerials were replaced by dishes, that now looked like drums. With the introduction of digital microwave systems in the eighties on the 11 GHz band, the dish size could be reduced slightly.

Microwaves links are limited in length by the curvature of the earth as the aerials must be in line of sight of each other. This dictates the spacing of microwave stations and the height of towers. The signals are affected by weather conditions and reflections from water if they cross the sea, causing fading. To overcome fading two dishes spaced a distance apart so they are affected differently by the changing condition are connected to the receiver which selects the strongest signal at any instance of time.

The original fifties microwave systems were just a dish on a straight vertical tower. The introduction of horn antenna during the sixties required a different design of tower, tapering towards the top such as Copt Oak. An Eiffelised design, like this at Heysham became widespread. Towers of this type, were not well received in some parts of the country, so ferro-concrete designs were introduced to meet planning requirements. Wotton under Edge shown here, was one built to the 'Chiltern' design along with Charwelton, Heaton Park, Pye Green, Stokenchurch, Sutton Common and Tinshill. Morborne Hill and Purdown were modified forms of the Chiltern with more aerial galleries. Two bespoke design of concrete tower were built in London and Birmingham city centres. A one-off design used at Tolsford Hill for the cross channel relay.

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