Long Haul Microwave Network

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

Network Development

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]

Conspiracy Theories

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 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 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, but that wasn't the reason for developing the network.

Reality

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 of 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 was met, it was used to augment the telephony capacity. A Microwave Backbone network was provided for important long distance circuits. Some authors have tried to fit the 1956 plan to the concrete towers unsuccessfully, as the concrete towers were built for aesthetic reasons but not resilience against attack.

Microwave Radio Stations at Copt Oak and Heysham
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
Towers

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, that now look like drums as seen at Twycross. 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 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 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.

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-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 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.

Microwave Backbone

The 'Backbone' microwave network provided an alternative routing for long distance important military private circuits and the Emergency Manual Switching System (EMSS). Using channels in the 2 GHz band, the radio repeaters and terminal equipment were powered from a no-break supply.

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 Crags YCBC and Craigowl YCRG. And goes on 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.

The Terminal Station is the only point where the microwave link connects to the cable network to add and remove circuits. The cable interface may be coaxial cable with a granularity of 960 circuits or carrier cable with a granularity of 12 circuits.

Page 86 of this 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.

Microwave Backbone Network
Backbone

The file HO 322/1116 at National Archives in Kew - British Telecoms Civil Defence Planning mentions that by the eighties backbone equipment was becoming unservicable and BT suggested moving from a dedicated platform to their commercial microwave network with the sacrifice of no-break power supply to short break (while the standby generator started) 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. The Perth - Leeds section of the route to Worcester was rearranged to meet the closure of the radio system in November 1989.

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