Dorchester




Introduction When, during 1925-26, the Marconi Company built the radio stations known as the Imperial Wireless Chain for the Post Office, they also established their own transmitting station at Dorchester together with a receiving station 30 miles away at Somerton.
History before 1950 The original service from Dorchester was to New York which opened on 16 December 1927, followed shortly by a service to South America. By the end of 1928, services to Japan and Egypt had also been opened.
Aerials designed by C.S.Franklin for use on these routes concentrated the radiated energy into a narrow beam, thus increasing the effective radiated power (erp) to give a much-improved signal to noise ratio at the receiver.

The 'uniform' array of stacked dipoles producing the beam were suspended from triatics attached to 90ft wide crossarms which surmounted 287ft high lattice steel masts. These masts were spaced at 640ft intervals and the width of the beam aerials was about 500ft. The physical length of each dipole had to be cut to match the appropriate wavelength, the range covered being in the band 15 to 75 metres (frequency range 4-20 MHz).
The beam aerials were prominent on the Dorchester skyline and although the last one came down in 1966, local people still refer to Dorchester as the 'beam station'. The site covers some 460 acres and lies one mile west of Dorchester town.
The early transmitters were Marconi Short-Wave Beam (SWB-1) type using paraffin-cooled CAT2 and CAT3 power valves, which were the first valves to incorporate copper to glass seals: they achieved an output of 11kW. A second transmitter hall with eight SWB-1's quickly supplemented the original four to enhance the coverage on each route and to provide new services to Bangkok, Nairobi and Teheran. Manual changeover links in the aerial feeder system enabled a change of aerial to be made.

In 1938, SWB-10 transmitters were introduced. These had an output of 25kW, using water-cooled final stage valves, with components fully enclosed and interlocked. Later, further beam aerials were erected to take the South African services from Bodmin and, in 1941, the Australian service from Grimsby.
Station power was originally derived from five engine sets: 3 X 150HP/90kW 3-cylinder and 2 X 200HP/120kW 4-cylinder. It was not until 1938 that a 3.3kW Public Mains AC supply was brought into the station. After the war, the 440V DC generators were replaced by two standby English Electric Diesels each of 500kVA at 3.3kV.
During 1943, 'B' Building was added with five SWB-10's, which were capable of being operated from the main station. The beam services were highly profitable and traffic speeds of up to 160 words per minute were possible. So successful were they that in 1928, an Imperial Wireless and cable Conference was convened "to examine the situation that had arisen as a result of the competition of Beam Wireless with the Cable Services". It recommended and received Government approval for the overseas cable and wireless resources of the Empire to be merged into one system controlled by a newly-formed company, Imperial and International Communications Ltd, this title being changed in 1934 to Cable and Wireless Ltd.
After 1950
Under the Commonwealth Telegraphs Act of 1949, control of the UK assets of Cable & Wireless Ltd. and the services and staff operating them, passed initially to the Post Office in 1950, then to the Engineer in Chief's Department, and later to the External Telecommunications Executive when it formed on 1st October 1952. Under Post Office control, standard 3.1 MHz drive systems were introduced, and the original Franklin oscillators were replaced by crystal controlled carrier frequency generators that gave greater stability and versatility of service. The 3.1 MHz drives allowed frequency division multiplex (e.g. WH11, TA5, HL13) multi-channel telegraphy transmission. In 1959, four STC type DS13D transmitters were installed in an extension of 'B' Station, for use on highly-loaded routes such Barbados, Lagos and Karachi where up to 18 telegraphy channels could be required simultaneously. The transmitters, rated at 30kW peak power could be wave-changed to any of six pre-assigned frequencies in 30 seconds from remote control panels in the main station. The early 1960's saw the demise of the beam aerials; their high maintenance costs, susceptibility to storm damage, and fixed frequency working led to their replacement by Rhombic broadband type aerials. In 1966, three Associated Press Services for Eastern, Middle Eastern and South African routes began at Dorchester using Log Periodic aerials erected on the south east corner of the site ; the latter route closed in 1975. Because of the success of satellite communication and the provision of trans-oceanic cables, all point-to-point services had ceased or were transferred from Dorchester by 1970. The remaining SWB1 transmitters were scrapped and the 'B' Station SWB10's transferred to the main station. The complement was then eleven SWB10, two SWB8 and four DS13D transmitters. Since 1971, apart from the continuing Press services, all the transmitters have been used for long range maritime telegraphy services using Stacked Quad Dipole aerials for the 8, 12, 16 and 22 MHz bands, and vertical bicone radiators for the 4 and 6 MHz bands. They form part of a worldwide radiotelegraph service established through Portishead Radio which is controlled by the receiver and terminal station at Burnham-on-Sea, Somerset. The HF rationalisation scheme which was then being implemented, eventually concentrated maritime telegraph services at Rugby and Ongar stations. There, more modern versatile transmitters were available, following the transfer of many point-to-point services to satellite working via Goonhilly Satellite Earth Station. Portishead Services were finally transferred, and the station closed on 1st February 1978, with Dorchester following in April 1978.