GPO / BT Long Haul Microwave Network

This topic tries to debunk some of the stories about The GPO / BT long haul microwave network.

Network Development

Conspiracy Theories

Over the years there has been much speculation about the purpose of the GPO / BT 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 but cannot be ruled out. 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 Cabinet Office paper in the National Archives file CAB 134/1207 dated 1956 proposing a microwave backbone bypassing major cities to safeguard vital communications at times of war, but that planning outline wasn't the reason for developing the main network.


During the late fifties and early sixties the demand for long distance television links outstripped the provision of the preferred option 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 coaxial cables did not exist and could not be provided in time to meet the broadcasters demands.
Once the surge in demand for television links were met, microwave systems were used to augment the telephony capacity. At the beginning of the nineteen sixties, work started building the strategic 'Backbone' network to safeguard important long distance circuits. Some authors have tried to unsuccessfully fit the concrete towers which were built for aesthetic reasons, not resilience against attack, into the outdated 1956 Cabinet Office plan.
Microwave Radio Stations at Copt Oak and Heysham
Lattice tower
Only in the sixties 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.

Insider's View

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 as head of planning he should know.

Design of Towers

Chiltern Design Concrete Tower : Drum Type Dish Aerials
Tower design
The early parts of the network used microwave dish aerials as seen on the top of Copt Oak tower. 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. Modern dish antenna are encased to protect them from the elements, these look like drums, as seen at Twycross above. With the introduction of digital microwave systems in the eighties on the 11 GHz band, the dish size could be reduced slightly.
Microwave 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 tower heights. 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. A stepped 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. The 1961-2 GPO Engineer-in-Chief's annual report Page 27 comments on the introduction of a standardised design of concrete tower. 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 designs of concrete tower were built in London and Birmingham city centres. A one-off design used at Tolsford Hill for the cross channel relay.
UK Mainland Microwave Network by the mid-Eighties
BT Microwave Network Diagram
By the time the network had reached maturity in the mid-eighties its tentacles has spread right across the mainland. The network diagram shows only the long haul common bearers. Branching off from these were links to customers sites, mainly television distribution but also a number of radar stations for the Linesman-Mediator system for the Royal Air Force and civilian air traffic control. The diagram omits the Scottish Highlands and Islands communications modernisation microwave links as these were for connections between local exchanges and not long haul traffic. This national network is sometimes called the microwave backbone as it indeed was, but should not be confused with 'Backbone' for important military and civilian circuits, which is explained in the next topic.

The End Game

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-2000s 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 in 2018. 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.

The Backbone Microwave Network

The 'Backbone' microwave network provided a secure alternative routing for long distance important military private circuits and the Emergency Manual Switching System (EMSS) by bypassing centres of population and other possible targets for nuclear weapons. This was known as Back Bone Radio (BBR) in government emergency planning documents. Utilising towers provided for the GPO Long-Haul network gave the required clearance from targets with the exception of Pontop Pike, so a Backbone only tower was constructed at Muggleswick linking between Arncliffe Wood and Corby's Crags.
Backbone mainly used channels in the 2 GHz band freeing the 4 and 6 GHz bands for civilian use requiring a larger bandwidth channel. Unlike their civilian counterparts the Backbone radio repeaters and terminal equipment were powered from a no-break supply (what we now call a U.P.S.) to bridge the gap until the diesel generator starts. Civilian equipment fails for 10-15 seconds until the generator takes over.
The GPO Engineer-in-Chief's Annual Report for 1960-61 Page 68 marks the completion of some sections of Backbone. Hunters Stones - Craigowl Link. This link for 960 telephone circuits is the northern section of the backbone route. It comprises four baseband terminal stations and three repeaters. The terminal stations being Hunterstones YTHS, Arncliffe Wood YACW, Corby's Crag YCBC and Craigowl YCRG. The three repeater stations at Muggleswick YMUG, Blackcastle Hill YBCL and East Lomond YELM. The report continues with: Copt Oak - Hunters Stones Link. This link for 960 telephone circuits is part of the southern section of the backbone route. It comprises three baseband terminal stations and two repeaters. The remainder of the route has been delayed, pending planning clearance of the Kiln Farm site in the Chiltern Hills (see page 86). The three terminal stations being Copt Oak YCOP, Sutton Common YSCN and Hunterstones YHTS. This section included repeater stations at Windy Hill YWIL and Pye Green YPGN.
Linking in from the extract above, Page 86 of the same report : Kiln Farm. Stokenchurch. Radio Station. A public inquiry into a Post Office proposal to establish a radio station at Kiln Farm, Stokenchurch, in the Chiltern Hills was held on 17 and 18 January 1961. The radio station is required to form part of a network to inter connect the Midlands, the West Country, East Anglia, Southern England and London. The inquiry was the culmination of five years protracted negotiations, which failed to secure a technically suitable site which was acceptable to the planning authorities and the various amenity bodies. The Inspector of the Ministry of Housing and Local Government has reported that he finds in favour of the Post Office proposal. Stokenchurch wasn't only required for Backbone but for telephony and television.
It wasn't until the 1964-65 Annual Report (Page 34) that backbone appeared to be fully complete. A 960-channel telephony system between Stokenchurch and Copt Oak. This forms the first part of a system of links extending to Dundee and the remainder is now under test.
Emergency Microwave Backbone Network
Defence Backbone Network Diagram
A terminal station is the only point where a microwave link connects to the cable network, allowing circuits to be added and removed. The cable interface may either be coaxial cable or carrier cable. Terminal stations are marked on the map with a green dot (  ). The remaining sites are intermediate repeaters required to bridge the gap between terminals.
In the network diagram above, the Ferro-Concrete towers were located at (North to South) Sutton Common YSCN, Pye Green YPGN, Morborne Hill YMOR, Charwelton YBFM, Stokenchurch YSTK and Wotton-under-Edge YWPE. It may be seen that these are interspersed with standard steel lattice towers, indicating they were built for asthetic purposes and not for their resilience to nuclear blast. Other Ferro-Concrete towers at Tinshill (Leeds), Heaton Park (Manchester) Birmingham, Purdown (Bristol) and BT Tower in London did not form part of the 'Backbone' network although they played an important part of the television and telephony networks.
At the western end of the Backbone network, Fiveways tower at Corsham (YCSM) is connected by coaxial cable into the Central Government War Headquarters (CGWHQ) below ground in the nearby quarries. As a contingency, a coaxial cable linked the CGWHQ to the Wotton-under-Edge tower, with a manual change-over facility to bypass the radio link. This would safeguard against the destruction of the Fiveways tower. The Backbone link continues on from Wotton-U-E to Sparsholt Firs.
Fiveways to Wotton Changeover Arrangements
The file HO 322 / 1116 at the National Archives in Kew - 'British Telecoms Civil Defence Planning', records that by the eighties, parts of the backbone equipment was becoming unserviceable. BT suggested moving from a dedicated platform to their commercial microwave network, with the sacrifice of a no-break for a short break power supply. This was to be phased over five years. Circuit advices at Worcester EMSS show rearrangements taking place on one of the Worcester - Reading EMSS circuits in April 1985 for the closure of the Albrighton - Copt Oak section. Some Perth circuits to Worcester were rearranged to meet the closure of the Craigowl - Hunterstones radio system in November 1989.

Closure of Backbone Radio

Cabinet Papers from 10th June 1993, held in the National Archives at Kew, give a closure date. This decision was prompted by the Government Review of Emergency Communications (GREC) report. The report was further scrutinised by the Post Review Working Party (PRWP).
CAB 134 / 5766 extract . . . Two separate, BT provided, EMSS transmission systems, the 'Backbone Radio' (BBR) and 'All Underground' (AUG) schemes, had also been reviewed by GREC. In accordance with its recommendations, the BBR was closed in January 1989, . . .
The cabinet paper does not quite match the facts. Perhaps the government funding ceased in January 1989, but the Craigowl - Hunterstones section did not close until sometime after November 1989. There is no information to hand about closure of other sections of Backbone.
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