History

The London - Bristol SHF route became one of the most important sections in the UK network since it provided telephone and television links to Wales and the south west including the Earth Station at Goonhilly. Its capacity was restricted, however, due to a shared section between London and Bagshot and initial development was subject to a number of delays.

A 1959 map shows the Bristol route as "programmed" and running from "Museum" exchange via Stokenchurch, "Swindon" and Wotton-under-Edge. In this version there would have been five routes leading from London however it appears 1960s practice required approximately 90 degree separation between routes where horn antennas were used. This led to the Bristol and Southampton links sharing a common first "hop" between London and Bagshot.

Contract 23505 was awarded to GEC in January 1962 for a L6 GHz system - this seemed to be the preferred band for the links developed in the early 1960s. A cost of £111,205 was agreed, with the contractor expecting to meet the "Ready for Service" date of 1 September 1963 ("subject to access given to buildings and towers by 1 May 1963"). STC offered a 4 GHz system at higher cost and was unable to meet the RFS date, Marconi did not bid. This route may be an anomaly since it is listed amongst sections of the "Backbone" network and pre-dates Bagshot and the Post Office Tower. It is possible "Bristol" represents the Central Government Wartime Headquarters (CGWHQ) at Corsham which isn't listed with the other sections and "London" corresponds to Stokenchurch where Backbone runs north, with a spur running around London.

Taking "London - Bristol" at face value, and bearing in mind most of the intermediate sites would be the same as Corsham - Stokenchurch, the situation in 1962 was as follows:

• The terminal at Bristol was already in operation, forming part of an earlier television link, however alterations to the tower would be required.
• The "Swindon" site was to be an existing Air Ministry tower at Sparsholt Firs.
• Two new sites were required at Stokenchurch and Wotton-under-Edge, planed as concrete towers
• Bagshot was a new site.
• An existing tower on the roof of "Museum" exchange was the terminal for three routes which had started as television links
• Plans were in hand for what would become the Post Office tower

Photos show the tower at Stokenchurch was structurally complete by 1963 however antenna installation was still in progress during 1966. A temporary link to central London may have been feasible but there is no evidence this was attempted. Also the main Backbone links had been specified to use the L6 GHz band.

Details of the plans for London - Southampton suggest extensive use of L6 GHz, the first contract was awarded to STC in November 1962 and called for a "2 + 1" arrangement to replace the existing BBC and ITA links. Further channels were added, potentially creating a "5 + 1" or even "5 + 2" system including BBC2 and two telephony links. There are only eight channels within the L6 GHz band, leaving at most two available between Bagshot and Bristol.

Contract 23551 was awarded to STC in September 1963, for a 4 GHz link between London and Bristol. Valued at £200,000 this was a non-competitive award with RFS date 1 April 1965 for one channel (plus protection channel) towards Bristol and completion of the "Entire work" by 1 October 1965. The link was to be "capable of 625 line colour or 960 Tp circuits" - from the timing and other comments, this may have been intended as a link for BBC2. Further justification for the non-competitive tender was given "Techical requirements and urgency...STC are the only firm to have met 600 channel telephony performance and are prepared to demonstrate 960 channel performance IN THE 4000 MC/S BAND".

Despite the optimism, further comments indicate delays: "Work due to [sic] completion Mid May 1967. Delay due to Firm's prodn. problems". The cost at September 1966 had risen to £299,723. A further contact 23570 was issued in April 1964 for "additional channels" at 4 GHz, to be RFS 1 January 1966 - this was awarded to STC in the sum of £165,127. GEC did not bid and an offer from Marconi of £341,000 was rejected. The price had risen to £223,838 by September 1966.

Notwithstanding the issue of limited availability, Contract 23579 was awarded to GEC in November 1964 for "additional channels" at L6 GHz and Contract 300526 in December 1966, again at 6 GHz but with no other details.

The introduction and expansion of satellite communication had a significant impact on the London - Bristol link. All international traffic was routed via London and the Bristol route carried a proportion of the telephony traffic from both Goonhilly and Madley Earth Stations, together with domestic telephone and television circuits. Although several trunk cables were also in use over the route these were unsuitable for television purposes.

A 1984 network diagram for the Bristol Television Network Switching Centre shows the IBA had three vision circuits between London and Bristol (one for Channel 4, the others to allow HTV and Westward independent feeds from the ITV network). Circuits were also provided for BBC1 and BBC2. In the reverse direction two circuits were available for IBA purposes and one for BBC "contribution" use.

During the 1980s the trunk telephone network was migrated to digital links in the 11 GHz band. Network maps and other information from the period suggest the London - Bristol route was included in the digital trunk network.