The Post Office, together with the Irish Department of Posts & Telegraphs, operated a trunk route between Belfast and Dublin, the original link dating from 1893. By lated 1960s a 600-circuit coaxial cable was in operation and a microwave link was under construction.
From north to south the sites were
A short article in the July 1971 Post Office Electrical Engineers’ Journal has helped to confirm and clarify the details of this link, although not the in-service date.
The link was reported as providing additional telephony channels between the Republic of Ireland and Northern Ireland and also between the ROI and England (which suggests additional capacity may have been added between Belfast and Carlisle at the same time).
“This is the first radio-link to be installed in the British Isles by Philips of Holland. The installation and testing of the radio equipment was carried out by seven Dutch technicians. […] The route consists of two stations in Northern Ireland, Standing Stones (Terminal) and Deadman’s Hill (Intermediate) and three stations in the Republic of Ireland, Mount Oriel and Cappagh (Intermediate) and Dame Court (Terminal).
The radio-link has a maximum capacity of six main channels and two protection channels, each channel being capable of carrying one colour television channel or one direction of transmission of a 960-circuit telephony system. Initially one main and one protection channel have been installed in each direction. The first extension to the system is due for completion in March 1972 and will provide one additional main telephony channel in each direction. The protection channels are used for television occasional broadcasts.” One such use of the protection channels was for the April 1971 Eurovision Song Contest broadcast from Dublin (another POEEJ article confirms there were also RTE/BBC links available).
The equipment operated in the Upper 6 GHz band and was solid state except for the travelling-wave tube output stage which produced 10 W. This is believed to have been Philips model 8SR 600, with Post Office designation Radio System 10/26. The output power was “…necessary owing to long spacing between intermediate stations. Automatic baseband switching is provided between main and protection channels in case of failures. The supervisory system is transmitted via the radio-link using the vacant sub-base-band portion of the first main channel. On failure of the first main channel, the telephony traffic plus the supervisory system are automatically switched to the protection channel.”
The article confirms the use of standard buildings and the “daffodil” modular tower design for the two sites in Northern Ireland – the only sites other than Sibleys where this type of tower was used. BT Archive photos dated July/August 1969 suggest the stations were largely complete and a list of radio equipment shows RS 10/26 as first used in 1970. The manufacturer of the antennas is not known however they may have been the first use of the Andrew Antennas “drum” style for the article notes: “The 10 ft diameter Cassegrain aerials used are connected to the radio equipment by Flexwell waveguide. An unusual feature of these aerials is the use of radomes to protect the aerials against adverse weather. The radomes degrade the signal level by approximately 0.5 dB”
The northern terminal at Standing Stones is five miles west of central Belfast and “The radio system is extended to Belfast repeater station by means of a C.E.L 1006 small-bore co-axial system [a 960-circuit cable] All circuits between Dublin and Great Britain are transferred to Belfast-Cross Channel systems without being demodulated beyond group configuration”. This description clarifies the relationship between Standing Stones and Ballygomartin – traffic from the ROI to the UK mainland would have been routed via Belfast and then either via cable or the SHF link via Enoch Hill to Carlisle.
The two intermediate sites in the ROI were subject of planning applications (from Philips Electrical) in early 1967 and featured “pylon” type towers together with buildings constructed from concrete blocks. Subsequent applications in 1974 extended the buildings, which may indicate an expansion beyond the original “6+2 channel” capacity. Later planning documents record the addition of “welfare” facilities for military guards – during The Troubles this link would have been a potential target. (It is very likely the two sites in Northern Ireland also required enhanced security.)
The route appears to have been migrated to a digital link during the 1980s – RS 10/67 was a 140 Mbit/s U6 GHz system, model DRS-48-14Q from AT&T. This is noted as “Solid state permanent for Dublin – Belfast link 16QAM” and in-service 1984.