This microwave link was established in the early sixties to provide a higher quality 405 line feed of BBC1 from Rowridge to North Hessary Tor. It was bi-directional and so enabled the sending of material from BBC Plymouth and Outside Broadcasts from Devon and Cornwall to London and other centres.
North Hessary Tor on air
The North Hessary Tor BBC television 405 line transmitter opened on 17th December 1954 on Channel 2. Its official programme source was RBL Wenvoe Channel 5. Reception of Wenvoe would often be subject to CCI and an alternative feed was therefore provided by reception of Rowridge Channel 3. As further transmitters opened throughout Europe, Channel 3 suffered increasingly from interference particularly under Sporadic E conditions. In the mid-fifties the BBC installed a Rowridge receiver at Start Point and fed it via an SHF link to North Hessary Tor, presumably calculating that a second receiver at approximately sea level would provide improved reception. One possibility of a source of Channel 3 interference may have been Tacolneston whose ERP was increased at that time to 45kW.
The severity of interference was variable, and although mainly being at its worst in summer, I recall one Christmas Day in the early sixties pictures from NHT during Billy Smart’s Circus covered in interference. As the afternoon progressed picture quality deteriorated leading to a NHT generated fault caption being displayed occasionally.
As interference on NHT’s programme feeds of Wenvoe and Rowridge became a larger issue, plans were drawn up for a solution. At the time the GPO retained a monopoly on the licensing and provision of all broadcast circuits, and would usually only allow self-provision by the broadcaster on the grounds of cost or lack of availability of GPO facilities. They were unable to provide vision facilities at NHT.
On 20th April 1961 the first television news opt-out from BBC Plymouth, News from the South West, went on air – nine days ahead of the commencement of its competitor Westward Television. This required the provision of a BBC provided SHF link from BBC Plymouth to NHT. A dish was installed on a mast, still present today at Plymouth Broadcasting House, with the receive dish near the bottom of the NHT mast. Plymouth controlled the opt-out switch between studio output and the feed of network from WV or ROW. At first this was a hot-switch since Plymouth did not have access to a network vision feed, and once opted out they could not see what they were switching back to. This was helped by providing a low-grade vision cue circuit down an audio pair from NHT to Plymouth.
Now there was a television newsroom and studio, a contribution circuit to the rest of the country, especially London, for news feeds and live interviews was required.
There was additional pressure from Outside Broadcasts. Mobile videotape machines were few and far between leading to most outside broadcasts requiring vision links to the nearest network production centre where VT was available. This made OBs in Devon and Cornwall expensive and difficult to provide. It was at this time that Goonhilly was establishing itself as an international satellite station with the spine network being extended between Cornwall and London. Increasing international demand for circuits meant that the hire of inter-regional vision circuits in that part of the country was expensive, unreliable and frequently not available.
Eggardon Link approval and provision
These pressures resulted in approval for the BBC to engineer an SHF link, installed by the mid-sixties, to feed BBC television network pictures from Rowridge. The 2-hop reversible link was engineered with the midpoint adjacent to an old Iron Age fort at Eggardon Hill, near Bridport, Dorset. Both hops passed over a stretch of water; from the Isle of Wight across Poole Bay, and from Eggardon across Lyme Bay. The path profile showed the link was liable to two forms of signal problems; fading due to temperature inversion over land, and additional fading due to the water paths. Both of these conditions were commonly observed on this link. There was also a tee-off the link which fired towards Cranmore Tower on the Mendips.
Some protection for fading was provided by vertical space diversity using two dishes spaced at pre-determined heights above each other. Sadly, however, this was not completely effective. The very weather conditions which brought severe CCI could also produce fading on the SHF link. It was fairly common to see NHT’s dilemma: choosing between a fading link and an RBR covered in interference.
The link also provided other benefits. Plymouth could usually see a feed of network before opting back, rather than another regional programme. They were also connected to the BBC network so could contribute items to London and elsewhere. For this reason Plymouth was given control of the link, enabling them to reverse the circuit in case of bookings arising.
Outside Broadcasts benefitted, since any programme which could be routed to NHT was then able to connect to the BBC’s contribution network via the link. A masthead rotator dish with an OB receiver was provided 180m up the mast at NHT increasing OB range.
On 17th March 1971 BBC1 Caradon Hill entered service. Soon afterwards a standards converter and off-air receiver were installed at NHT to provide stable 405-line pictures for the first time. Television sound, previously fed by line, was also sourced off-air.It was finally time to bid farewell to the SHF link from Plymouth, since opt-outs were then being routed down a new 625-line circuit to Caradon Hill, and to decommission the Eggardon Link, which was not 625 line capable.
The mast at Plymouth BH was retained and converted for OBs. A panning head was mounted behind the radome dish connected by flexible rubber waveguide to the rigid waveguide which ran down the mast and into the new Radio Links OB room. In later years the BBC used the mast for self-provision of some circuits, and BBC South West installed a 7GHz/2GHz rotator. The latter was discontinued when 2GHz frequencies were removed from Outside Broadcast allocation, by which point Plymouth had installed a downlink dish for its new satellite newsgathering vehicle.
--Oldcommsboy 13:09, 25 October 2012 (PDT)