The London to Birmingham Television Link operated at 900 MHz – not technically a microwave band, and UHF rather than SHF. It is, however, often cited as the start of the Post Office microwave network and most of the sites were re-used for SHF links. The 900 MHz equipment was removed by 1961 but the steel towers remained in use into the mid-1960s for a 2 GHz link. The original buildings survive at some sites.
Starting from London the sites were:
In the mid-1960s London and Birmingham were developed with concrete towers and became main switching centres for television and major nodes in the general-purpose SHF network. A concrete tower was provided at Charwelton and new steel towers at Harrow Weald and Dunstable. Turners Hill, however, was not used in the core Post Office network although the site continued to be owned by PO and BT.
Initial field trials for the London to Birmingham television link were carried out in December 1946 with further tests from the chosen sites in 1947. The Post Office Engineer in Chief’s Report for the year ended 31 March 1948 reported “A contract has been placed with the General Electric Company Ltd. for a two-way radio relay system for transmitting 405-line television signals between London and Birmingham. This system will employ amplitude modulation on carrier frequencies of the order of 900 Mc/s., and will transmit video frequencies up to rather more than 3 Mc/s.”
It seems the original requirement was to link “not quite central London” to “not quite central Birmingham” – assumed to be Harrow Weald to Turners Hill – but the Engineer in Chief’s Report to 31 March 1949 confirmed the final scope: “The results of reception tests carried out during the tendering procedure indicated that it was feasible to plan the radio system from Museum Exchange (London) to Telephone House (Birmingham). As the costs of the cable tails involved in the earlier scheme would have been no less than the additional costs on the radio system, the contract was extended to include radio transmission to the two Post Office switching centres at Museum Exchange and Telephone House. During 1948 considerable progress was made in obtaining sites, providing buildings, and erecting permanent towers at the terminal stations and temporary masts at the intermediate stations. The contractor made good progress in testing prototypes and manufacturing the final equipments.”
The link opened initially on a one-way (reversible) basis and was available for use by the start of BBC transmissions from Sutton Coldfield in December 1949. A cable between London and Birmingham had also been provided – some accounts say it was the preferred option. The reversal procedure was meant to be rapid but initially proved troublesome and was in any case a temporary provision.
As mentioned in the 1949 Report, masts were used at the intermediate sites, these appear to have been of a standard Post Office type with square section – in some cases not of great height. Two masts were used at Turners Hill, probably due to the relatively small angle between the directions of Charwelton and central Birmingham. Initially a single dish was used in each direction – these were in the form of a 14 ft paraoblic reflector – but reduced to 10 ft horizontally to allow a more compact mounting. A dipole and reflector were mounted at the focus and electrically heated to avoid ice forming. Conventional coaxial cable was used and the equipment housed in brick buildings to a standard pattern used for other Post Office facilities such as (cable) repeater stations. The scheme eventually used two frequencies in each direction, alternating for each section of the route. From London 870 MHz was used, then 890 MHz at Harrow Weald, 870 MHz at Dunstable, and so on. The return path started with a 917 MHz transmission from Birmingham, changing to 937 MHz at Turners Hill and back to 917 MHz at Charwelton.
The 1950 Chief Engineer’s Report notes that work was in hand to provide the planned two-way link and the 1951 Report confirms “The system operated satisfactorily until 1 October 1950 when service was opened on the alternative cable system. After two month operation of the cable system, the radio system was withdrawn from service to facilitate completion of the outstanding work to provide
full two-way operation.” For the two-way link the intermediate sites were equipped with short self-supporting towers of relatively large cross-section. The equipment was housed in a hut at the top of the tower to reduce feeder losses.
The 900 MHz link presumably returned to service at some point during 1951 but is not specifically mentioned in the annual reports until 1955, when: “The original 900 Mc/s radio link London ? Birmingham has been overhauled, preparatory to its forthcoming use by the I.T.A. and is now being used by the B.B.C. to carry normal program service whilst the London ? Birmingham cable link is being overhauled and re-lined.” The same report also mentions a planned link at 2 GHz between Birmingham and the ITA Lichfield transmitter.
By 1956 there were further developments planned: “A contract has been placed for the provision of a one-way 2000 Mc/s radio-relay link for the I.T.A. between London (Museum Exchange) and Birmingham (Telephone House) which will comprise two parallel channels; main and standby. […] It will follow the same route as the 900 Mc/s television link and will be accommodated in prefabricated huts where permanent buildings are not available.” It’s not clear whether Turners Hill remained in use at this point.
The 1957 Report advises that the 2 GHz link entered service on 18 July 1956. The Lichfield Transmitter was on-air from Feburary 1956 using either one of the available cables or the 900 MHz system, indeed the report also confirms “With the provision of a second London to Birmingham vision channel the Manchester program contractors have a choice of two independent London programs at the Birmingham network switching centre”.
In the 1958 Report a further link between Birmingham and London for the ITA is mentioned as “planned” – this is assumed to be also at 2 GHz – and in 1959 “Plans were made for the recovery of the existing 900 Mc/s radio-relay link between London and Birmingham since the frequencies used are required for other services. This link […] will be replaced by an additional broadband channel in each direction on the 2000 Mc/s system.”
Further progress was reported in 1960: “Five television channels using 2000 Mc/s equipment of new design have been provided on the London – Birmingham route and will be brought into service later in 1960. The 900 Mc/s […] and two television channels using 2000 Mc/s equipment of early design are being recovered.” The 1961 Report however states that the new link carries three channels (two working, one standby) in each direction. There is also mention of a plan to add telephony to the existing television link, and “The aerials […] at Dunstable on the London to Birmingham link have been raised to improve the propagation path”. A 1965 archive photo suggests the 2 GHz dishes had probably been moved to the position previously occupied by those for the original 900 MHz system.
Subsequent development of the London to Birmingham route involved a migration to 4 and 6 GHz bands and replacement structures at all sites (except Turners Hill which was no longer required). This is covered in [article to be added]