Dorchester Beam Station

DORCHESTER RADIO STATION

1927 - 1978

When the Marconi Company built the radio stations known as the Imperial Wireless Chain for the Post Office during 1925-26, they also established their own transmitting station at Dorchester with a receiving station 30 miles away at Somerton.

History before 1950

The original service at Dorchester opened on 16 December 1927 to New York and was followed shortly after by a route to South America. By the end of 1928 services to Japan and Egypt had also been inaugurated.

Aerials, designed by C.S. Franklin, for use on these routes, concentrated the energy radiated into a narrow beam 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 was suspended from triatics attached to 90ft wide crossarms which surmounted 287ft high lattice steel masts. 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 referred to Dorchester as the 'beam station'. The site covered some 460 acres and was one mile west of Dorchester town on the Bridport road.

The early transmitters were Marconi Short-Wave Beam (SWB) 1 type using paraffin cooled CAT2 and CAT3 power valves, which were the first design to incorporate copper to glass seals. They achieved an output of 11kW. A second transmitter hall with 8 SWB1's quickly supplemented the original 4 to enhance the coverage on each route and to open 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 SWB10 transmitters were introduced, They had a 25kW output with water cooled final stage valves and 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.

During 1943 'B' Building was added with 5 SWB10'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 which 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 the formation of a new company, Imperial and International Communications Ltd, this title being changed in 1934 to Cable and Wireless Ltd.

After 1950

However, by the Commonwealth Telegraphs Act of 1949, the UK assets of Cable & Wireless Ltd. and the services and staff operating them passed to Post Office control, first, in 1950, to the Engineer in Chief's Department, and later to the External Telecommunications Executive at its formation on 1st October 1952.

Under Post Office control, standard 3.1 MHz drive systems were introduced, and the original Franklin oscillators replaced by crystal controlled carrier frequency generators which 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 4 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 30kW peak power could be wave-changed to any of 6 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 began at Dorchester using Log Periodic aerials erected on the south east corner of the site for Eastern, Middle East and South African routes. 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 then being: 11 SWB10, 2 SWB8 and 4 DS13D transmitters.

Station power was originally derived from 5 engine sets: 3 X 150HP/90kW 3 cylinder; 2 X 200HP/120kW 4 cylinder. It was not until 1938 that a 3.3kV 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.

After 1971, apart from the continuing Press services, all the transmitters were 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 formed part of a world-wide radiotelegraph service established through Portishead Radio which was 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. Here, 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.