History

The overall Northern Radio Scheme comprised two microwave links together with VHF radio systems and cable sections connecting to Aultmore and Wick. The Post Office Engineer in Chief's Report for 1956-57 noted: "The final stages in the provision of the Northern radio scheme have been reached. All equipment has now been installed on the Braewynner - Thrumster - Kirkwall 4000 Mc/s 240-channel telephony link, and on the V.H.F. radio-relay system which provides a total of 48 channels between Kirkwall and Shurton (Lerwick)". Overall testing was expected to be completed in May 1957.

The scheme was significant as it included the first SHF link integrated into the UK trunk network - other SHF links in the period had been to meet television requirements. Although effectively a prototype system, the section between Braewynner and Thrumster remained in operation until 1978 when replaced by a new route running along the coast, see Offshore Oil & Gas.

The original installation was documented in some detail (see references below) and used equipment of Post Office design (RS 10/2) between Braewynner and Thrumster. STC RL4B equipment (RS 10/3) was used for the two-hop section between Thrumster and Wideford Hill. The southern section, over a 55 mile sea path was noted as "...unusually long for a single-hop link in a 4,000 Mc/s system." [Radio Report 2623, page 3]. The strategy to overcome potential problems with tidal fading was to use a "twin path" system - the link transmitted on two frequencies simultaneously - and "height diversity" with antennas at different levels. At the receiving end of the link the best signal was selected automatically, with fast switching between frequencies and antennas. The twin path option also provided some protection against equipment failure. Propagation tests began in November 1952 with a number of antenna configurations used for periods of up to six weeks and during different seasons. Photographs included in Radio Report 2623 show four dishes fitted to each mast.

The transmitters used for the southern section were designated as systems A/B (Braewynner) and X/Y (Thrumster). The frequencies 4110/4158 MHz (A/B) and 4134/4182 MHz (X/Y) do not fit the standard channel plan for the 4 GHz band. It is not known whether the 1957 link was upgraded prior to the new route entering service. The sites at Thrumster, Olrig Hill and Wideford Hill (Kirkwall) were retained. The mast at Braewynner was removed but the buildings remained.

References

Radio Report 2623 "The Braewynner - Thrumster Radio Link for 240 Circuits" - Radio Branch, June 1958. BT Archives TCB 226/2623 - includes references to other Radio Reports related to the equipment, etc.

Post Office Engineering Department: Engineer-in-Chief's Report on The Work of The Engineering Department for the year 1 April 1956 to 31 March 1957