History
The route between Granite Hill (Aberdeen) and Rosemarkie (Inverness) was developed originally for the extension of BBC television to north east Scotland. Early network maps suggest the Post Office may have considered further links beyond Rosemarkie however it was the BBC and IBA who would eventually plan and construct a link from Rosemarkie to the Eitshal transmitter (Isle of Lewis) in the 1970s.
As with Core Hill (BBC Meldrum transmiter) the Post Office built their own station at Rosemarkie, perhaps in the expectation of future development. A BBC Research paper in 1963 describes the linking arrangement as "The television and v. h. f. sound transmssions from Meldrum are received at Fochabers and fed by G.P.O. microwave link to Rosemarkie" - this intermediate site was possibly Tor Sliasg, approximately five miles east of Fochabers.
Although Rosemarkie entered service in 1957 a "fully engineered" link was not provided until some years later: contract 22697 was awarded to GEC in February 1961 for a L6 GHz system at an initial price of £72,794. The "latest price" at April 1966 was given as £81,997 but there is no information as to the original "Ready for Service" date or the eventual date of completion. The nearby ITV Mounteagle transmitter for Grampian Television entered service in September 1961. The ITA was permitted to set up its own SHF link due to delay in Post Office circuits being available.
In theory the link to Rosemarkie involved the addition of just one site at Tor Sliasg, plus the terminal station. However it seems the use of L6 GHz may have led to replacement equipment and a separate tower at Core Hill and to a new tower at Granite Hill... At the new sites an unusual form of tower was used, apparently designed to support two of the "large" horns but at relatively low level. At Tor Sliasg two separate structures were provided, allowing a pair of horns to face in each direction. The option for twin horns was normally used on routes which were expected to use all channels in the 4 GHz and/or L6 GHz bands. This may indicate long term plans for growth of telephony. Little is known of the impact on the existing site at Core Hill which had been set up originally to carry a 4 GHz link for BBC television. At Granite Hill a new "standard tower" was built in 1967 - by which stage the original tower carried a single horn antenna facing Core Hill.
Estimates of expenditure in the late 1960s included £xxxx for the "Aberdeen - Inverness TV Radio Link (BBC1 and ITA)" which was to be the provision of two one-way chanels plus a shared "protection channel" - this would appear to be delivery of the 1961 contract - plus a further £yyyy for an "Aberdeen - Inverness Radio Link" for telephony. The latter was to be a single channel 960-circuit system, sharing the protection channel with the TV link in that direction. There does not appear to have been a plan to add a link for BBC2 at this stage (the channel entered service from Rosemarkie in July 1970 but BBC1 and ITV did not start UHF service until 1973)
By the mid 1980s TV links in the direction Granite Hill to Rosemarkie were provied for all four channels - though Channel 4 was delayed until June 1983, seven months after launch. The BBC used an additional "vision circuit" for VHF radio distribution, serving Meldrum en-route. Just one channel was provided for television in the direction from Inverness to Aberdeen - a short SHF link was added from Inverness to Rosemarkie to link the local Grampian studio to their main site in Aberdeen.
The 960-channel telephony link is understood to have continued via the Inverness - Rosemarkie No. 1 coaxial cable (a distance of around 20 miles - this would have required intermediate repeater stations).
In the 1970s a new site was added at Daviot - close to Inverness and we believe telephony links were re-routed as part of the wider development of services to the offshore oil and gas industry. The television links, however, continued to serve Rosemarkie directly.
In 1968 Specification W6888 was issued for "A Radio Relay System Between Aberdeen and Inverness". This appears to have been for the link required for BBC2: "The initial use of the channels will be for 625-line colour television, but each channel is also required to be [capable of use for 960-circuit telephony]". This was to be a new system operating either on 2 GHz or U6 GHz - perhaps suggesting the original L6 GHz system had reached capacity or was unsuitable for further expansion. The work included provision of new antennas and waveguide and was to be completed by April 1970.