History
The London - Birmingham Televison Link operated at 900 MHz - probably no longer considered "microwave" though the term has been used to cover any frequency above 300 MHz. In the 1980s the 900 MHz band was used for mobile phones but in 1949 it was unused spectrum considered suitable for point-to-point linking. The link was effectively experimental but carried BBC and ITV programme feeds at various times between December 1949 and the early 1960s. It has an important place in the "pre-history" of the Post Office/BT microwave network.The problem of extending television coverage outside London was under consideration soon after broadcasts started in 1936. An article in the Post Office Electrical Engineers' Journal, April 1950, comments:
"For the sake of completeness it should be recorded that early in 1939 the Post Office placed a contract with E.M.I., Ltd. for an experiment in the relaying of television signals to Birmingham. In this case the radio signals from Alexandra Palace were to be received at Dunstable and transmitted over a radio link to Sharmans Hill, Charwelton, some 40 miles distant towards Birmingham; thus carrying the signals two-thirds of the way from London to Birmingham. Unfortunately the war intervened and this early experiment in the radio relaying of television signals necessarily had to be abandoned."Little more is known about this EMI experiment. The Post Office Engineering Department Engineer-in-Chief's Report for the year ended 31 March 1947 comments:
"Work was carried out by E.M.I. Ltd. on the experimental television link, but the future of this experiment has not yet been decided. [...] a specification for a complete two-way radio link between London and Birmingham was completed and tenders have been received."This two-way link appears to have been planned using the same general arrangement as the EMI experiment: the exact location of "Dunstable" is not known but Sharmans Hill is approximately two miles north east of the Charwelton"site established for the 1949 link. The two-way link was not originally intended to run directly between the centres of London and Birmingham: cable would be used for the first and last "hop". It was, indeed, feasible to carry a 405-line television signal over coaxial cable and a parallel project was undertaken, also described in the EIC's Report
In June 1946 it was decided to proceed with the manufacture and laying of a new composite coaxial cable to Birmingham to include two 1-in. diameter coaxial tubes, initially for 405-line and later for higher definition television. [...] Repeater stations are being constructed at 3 1/4 mile intervals, this short spacing being required at present for higher definition [...] transmissions, but initially equipment will be installed only at 12-mile intervals for 405-line transmission".A "repeater station" - in basic terms and amplifier - is needed to overcome loss in the cable. Such stations were an established part of trunk telephony routes but at the wider spacing. This form of cable with the 1" "tubes" was not used for other televsion links as other developments allowed 405-line television to be carried via 3/8" coaxial cable used on other trunk routes. The "L - BM No 4" cable was installed by 1949 but reported as "being equipped" during 1950 and became fully operational from 7 October 1950.
Turning now to the detail of the 900 MHz link, the EIC's Report to 31 March 1948 confirmed a contract had been awarded to GEC. The system would use amplitude modulation. Also a contract had been placed for 20 kVA auto-start generator sets to be used at the "four intermediate stations" of the televison link. In the Report to 31 March 1949 we are informed that the "control circuit" (a dedicated telephone line) for the television link was operational and in use by GEC contractors. Post Office staff had been trained and were assisting with installation and testing. More significantly, the scope of the link itself had been altered:
"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 cost 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."This decision established the precedent of a "radio terminal" in central London which ultimately became the Post Office Tower. (The same was not immediately true for Birmingham, see: London - Birmingham - Manchester) The choice of intermediate sites at Harrow Weald, Dunstable and Charwelton - in part inherited from the 1939 EMI experiment - determined the route for subsequent London - Birmingham SHF systems. Turners Hill, however, was found not to be required.
The radio link was initially one-way (normally London > Birmingham) but reversible. It seems the BBC were happier with the cable once it became available. In 1950 the link was withdrawn to complete the conversion to full two-way working. There is not much further discussion of the link until the report on the year ended March 1955 where we learn:
"The original 900 Mc/s radio link London - Birmingham has been overhauled, preparatory for its fortcoming 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 Independent Television Authority was responsible for engineering and regulatory matters and became a major Post Office customer as ITV franchises were awarded and transmitters built. The ITA transmitter serving the Midlands, at Lichfield, entered service on 17 February 1956 and the EIC's Report at the end of March confirms the arrangements made.
The two-way vision link between Museum [exchange, London] and Telephone House, Birmingham, has been set up on the existing 900 Mc/s radio system which has was extensively overhauled and modified for the purpose. A further radio link in the 2000 Mc/s spectrum provides two Go and one Return channels between Telephone House and the Lichfield I.T.A. transmitter. It is also planned to add a second vision channel between London and Birmingham during 1956.By this point it would seem the BBC's link had transferred back to coaxial cable. The arrangements required for ITV were more complicated since the London weekend franchise had been awarded to Associated Television (ATV) who also held the weekday franchise for the Midlands. ATV had facilities in London and Birmingham. Independent Television News was established in 1955 as a separate franchise therefore from the earliest days there was a requirement to connect studios in London with the Midlands transmitter and studios in Birmingham to the London transmitter. ATV was ultimately permitted to commission a private SHF link, presumably on the basis that the Post Office could not provide sufficient "vision circuits" for the company's internal purposes.
The EIC's Report to March 1959 reveals the intention to replace the 900 MHz system.
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, which was the first radio-relay link to be used by the Post Office for the regular relaying of televison programs will be replaced by an additional broadband channel in each direction on the 2000 Mc/s system.The March 1961 report confirms completion of the transfer to 2 GHz. This continued to use three of the original sites, bypasing Turners Hill. The sites at Harrow Weald, Dunstable and Charwelton were retained and upgraded as part of the 1960s development of the main London - Birmingham - Manchester route.
Technical Details
The pioneering nature of the link means much detailed information on the design is still available via papers published at the time.Between London and Birmingham frequencies of 917 and 937 MHz were used, alternating for each "hop", with the reverse direction using 870 and 890 MHz. Initially the intermediate sites used a mast carrying a single antenna for each direction (Turners hill used two separate masts due to the acute angle between the two "lines of shoot". The terminal stations were equipped with 100 ft towers on the roof of the Post Office buildings with provision for two antennas. The equipment was installed in a building at roof level. At the intermediate sites, for "Part I", the equipment was installed in small brick buildings (to a standard Post Office design) but in "Part II" to minimise loss in the feeder cables the tower incorporated an equipment hut at the same level as the antennas. For "Part II" pairs of antennas were used on each side of the tower, allowing full two-way operation.
The antennas took the form of a parabolic reflector ("dish") 14 ft in diamater but with straight vertical sides to produce an effective width of 10 ft allowing a more compact mounting on the tower. The reflector consisted of horziontal tubes at approximately 1/4 wavelength (3") spacing. With a simple horizontal dipole at the focus of the dish a gain of 27.5 db was achieved.
The question of path length versus number of intermediate sites was considered: it was thought that a distance of more than 40 miles would make the cost of (higher) towers uneconomic. In practice the sections either side of Charwelton are just over 39 miles and amongst the longest in the national network - which was to lead to some difficulty when higher frequencies were used over the route. Field trials were carried out in December 1946 and early 1947, at approximately 700 MHz and using 4 ft dishes, with further tests at 900 MHz later in 1947 using the "Part I" masts and antennas of the final design.
Initially it was believed inteference would preclude locating the terminal stations in the city centres, therefore the radio section would run between Harrow Weald and Turners Hill. In practice interference was not an issue but the short link between Turners Hill and Birmingham was retained. Subsequent links ran directly from Charwelton to Birmingham, however.
The masts used for "Part I" were of a standard Post Office design. The towers subsequently used at the four intermediate sites were all to the same pattern but this was not re-used for subsequent microwave routes. An item in the Post Office Electrical Engineers' Journal, January 1954, reported that signs of corrosion had been found with the galvanised steel towers at Birmimingham (which was painted as a remedial measure) and to a lesser extent at Turners Hill and Charwelton. The article notes that the towers at the intermediate stations were built around 18 months later (for "Part II").
reflector was made from