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Broadcasting to ships had been taking place since the early days of radio; the GPO's long-wave stations at Poldhu and Caernarvon had been conducting two-way traffic with ships within a few hundred miles of the United Kingdom prior to the First World War.
However, no long-range system existed until 1919 when the GPO and the Marconi Wireless Telegraph Company agreed to convert a redundant Imperial Wireless chain receiving station at Devizes in Wiltshire for long-range maritime use. Comprising of a receiver and a 6-Kilowatt valve transmitter, station ''GKT' was opened for service early in 1920, with a guaranteed range of 1,500 miles.
The radio officers at 'GKT' were housed in old army huts, with radiotelegrams being sent to and received from ships up to 5 days from any British port at the rate of 11d (just less than 5p) per word. Radio traffic was keyed to and from the London Central Telegraph office from then operating station for onward delivery.
This two-way "long-range" service proved to be immensely popular, and by 1924 it became necessary to expand the station at Devizes to cope with the increased demand. The GPO constructed a second long-wave transmitter and built a new receiving station at Highbridge (near Burnham-on-Sea) in Somerset, to which most of the radio officers transferred.
By 1926, experiments on short wavelengths had established that world-wide communication could take place. The GPO installed the first maritime short-wave transmitter at Devizes, keyed by operators with receiving equipment at Highbridge that same year. Initial tests proved outstandingly successful, and it became necessary to construct a brand new transmitting station. This station was to be located at Portishead, near Bristol, and thus in 1927 Portishead Radio was born. Three long-wave transmitters were installed, followed in 1929 by a new short-wave transmitter, ultimately resulting in the closure of the Devizes station.