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 subsequent 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.

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 but two new sites were required at Stokenchurch and Wotton-under-Edge. Both were planned as concrete towers and planning delays impacted the development of Stokenchurch in particular. Bagshot was also a new site. It is unclear whether the original intention was to use the existing tower at Museum exchange, but the new Post Office tower was originally planned to be ready in the early 1960s.

Photos show the tower at Stokenchurch was structurally complete by 1963 however antenna installation was still in progress during 1966. The route was unable to enter service until the London tower was sufficiently complete and the site at Bagshot was operational.

The section between Stokenchurch and Wotton-under-Edge was shared with the "Backbone" network - it is possible this became operational agead of the main route. This traffic is understood to have been a single channel in the L6 GHz band.

It seems the use of L6 GHz links to Southampton reduced the number available towards Bristol sufficiently for 4 GHz systems to be ordered before the route was actually operational. Contract 23551 was awarded to STC in September 1963. 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 was 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 ?? 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.

Contract 23579 was awarded to GEC in ?? for "additional channels" at L6 GHz. There is no specific comment about the non-competitive award however a pattern was established where additions to an existing system were awarded to the original supplier.

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.