History
The BT Tower, originally the Post Office Tower, was constructed above Museum Exchange starting around 1962 and fully operational around 1967. Strangely although a prominent landmark, and for a time open to the public, the building was not marked on Ordnance Survey maps and its location was classified "Secret" for many years.
Happily a vast amount of archive material is now available detailing the construction of the tower. It was intended to allow SHF links on main routes from London towards and beyond Birmingham and Leeds, East Anglia, Southampton, Bristol and the West and across to France. The height was determined in part by the clearance to give line-of-sight to the next station on each route and partly by a "gamble" that taller buildings would not be built which would block the signals.
As early as 1949, however, Museum Exchange had started to develop as a centre for television network switching with cables to Birmingam and Cardiff carrying the BBC programme feed as the transmitter network expanded. The Engineer-in-Chief's Annual Report to 31 March 1957 reveals "It is proposed to have a central London terminal station for trunk radio-relay systems and approval for a 450-ft. tower on a new building adjacent to Museum Exchange has been sought." The 1960 report optmistically stated "A 487 ft. radio tower is to be erected at Museum Exchange, London and planning, in co-operation with the Ministry of Works, is proceeding. It will serve as the main focal point of radio-relay terminals for links to the midlands, East Anglia, the north, the west and to the continent. Completion is planned for 1962-63." The report for the year ended March 1963 indicates continued optimism depite apparent delays: "The planning of the Museum Tower reached an advanced stage, and separate contracts were let for the foundations and the superstructure. The contract for the foundations has been almost completed and an early start will be made on the superstructure. Completion is scheduled for June 1963. The tower will consist of a hollow, circular, reinforced-concrete shaft, rising to a height of 564 ft. above ground level, with provision for a 40 ft. lattice steel extension at the top."
The next Annual Report advises: "Work on the superstructure started early in the year and by 31 March 1963 the core had reached a height of about 280 ft. [...] Progress fell considerably behind schedule partly due to the severe weather conditions and the Ministry of Public Building and Works re-estimated the final completion date, including the public floors, as October 1964. The main equipment floors, however, should be ready by the end of 1963 and it will be possible for the Post Office to start equipping the lower floors whilst construction work on the upper floors continues." The 1964 Report refers for the first time to "The Post Office Tower" and progress seems to have been steady rather than spectacular: "At the end of the year the main building was practically completed, ready for the installation of equipment; only minor work such as the finishing of staircases was outstanding. The radio tower had reached a height of about 548 ft. and cladding of the public levels was in hand. The main ventilation plant in the lower three floors was completed and the remaining apparatus floors and aerial galleries were very nearly ready for the installation of equipment."
The 1965 Annual Report (to 31 March 1965) confirms the effective completion of the building although there was still much technical work in progress: "During the year the tower reached its full height of about 620 ft., including the lattice-steel extension mast at the summit, and most of the exterior and interior building construction works were completed. Equipment floors became available to the Post Office progressively from May 1964 and the work of installing ventilation plant, lifts, power plant, cabling, radio equipment and aerials, proceeded throughout the second half of the year. [...] It is expected that the first radio system will become available for service in June 1965."
The SHF routes from the new tower continued the long-established London to Birmingham route, now at 4 and 6 GHz using horn antennas, and that from London to Norwich. The Isle of Wight television link was re-routed via Bagshot where a new link towards Bristol diverged. A fourth main route towards France via Fairseat was added. Early 1960s practice had required the four routes to be at approximately 90 degree separation, to avoid interference between the links, and it seems the "master plan" envisaged two horns on each route, ultimately to be used on 11 GHz if required. With improvements in the performance of dish antennas and a realisation that horns might not be suitable for use at 11 GHz the tower eventually carried a vast number of dishes, primarily on the four main routes. It appears from early photos that one of the originally planned 8 horns was never installed (for the Norwich route via Kelvedon Hatch). Two others were removed at some stage, probably in the 1980s, but five remained - disused - until late in 2011 when all the redundant antennas were removed.