 |
UK Broadcast Transmission |
         |
|
    |
|
|
|
|
|||||||||
|
|||||||||||
Rebroadcast Links to the Channel Islands
These days the provision of programme feeds to transmitters serving the Channel Islands is relatively easy, using under-sea fibre optic cables or satellites. Before these technologies became available, the only practical and affordable method was via re-broadcast links from transmitters serving the Southern UK. The closest distance from the south coast of England to the Channel Islands is 90 km (56 miles) from Portland Bill to Alderney. However, the main transmitters in the Southern UK are further away, being either inland, or further along the coast in the case of the transmitters on the Isle of Wight. The sites chosen for the main transmitters to serve the Channel Islands are both on Jersey, at Les Platons and Fremont Point Jersey is the southernmost Channel Island and therefore most distant island from the UK mainland. To reduce the path length, both the BBC & ITA/IBA used remote receiving stations on the more northerly islands of Guernsey and Alderney. This outline map shows the locations of the sites involved in this story.
BBC 405-line VHF TV
Practical Television, November 1955The provision of the programme feed was via a receiving station at Torteval on Guernsey and then onwards to Les Platons via an SHF link. At Torteval there was equipment for receiving the off-air transmissions from North Hessary Tor, Rowridge and Wenvoe, although initially both North Hessary Tor and Rowridge were operating at low power from temporary masts. At Les Platons there was also equipment to receive North Hessary Tor directly.
In January 1968 a new and improved receiving aerial system was brought into use at Torteval which is documented in BBC Research Department report 1969/21 This probably remained in use until the BBC 1 405-line transmitter closed in January 1984, as it would have saved the cost of installing a line store converter at Les Platons.
ITV 405-line VHF TV.
Channel Television launched on 1-Sep-1962 from the ITA transmitter at Fremont Point on Jersey. The provision of the programme feed was via a receiving station built by the GPO on Alderney, the most northerly of the Channel Islands. The onward feed to Fremont Point was via an SHF link. At Alderney there was equipment for receiving the off-air transmission from Caradon Hill, Chillerton Down and Stockland Hill This IEE report has details of the Alderney receiving system.
BBC FM Radio
Practical Wireless, December 1961The original receive arrangements for BBC FM Radio are not documented, although a 4-element band II Yagi pointing towards North Hessary Tor was installed on the Les Platons mast. I suspect that the primary receive point was at Torteval, as was the case for BBC 405-line TV.
Stereo The Les Platons site was re-engineered for both mixed polarisation and stereo in the 1980's with the mast replaced by the current 52m tower. Stereo transmissions commenced in January 1984.
As FM stereo is more demanding of the performance of a re-broadcast link, the reception site was relocated to Alderney, being the closest island to the south coast of England and significantly closer to Rowridge. At Alderney the re-broadcast link was achieved by diversity reception of either North Hessary Tor or Rowridge. Under normal reception conditions the system selected North Hessary Tor, receiving Radios 2 and 3 on three 6-element yagi aerials at a height of 4.5m on the tower and Radio 4 on two 6-element yagi aerials at a separate location away from the tower, selected to give natural screening from a co-channel French station. If the signal strength of North Hessary Tor fell below a predetermined level, whilst that of Rowridge was above, the system automatically selected Rowridge, using three 6-element yagi aerials at a height of 4.5m to receive Radios 2, 3 and 4. The programmes were fed to Les Platons on an SHF link. Les Platons also retained the ability to receive North Hessary Tor directly, although the decreased signal levels are only sufficient for monophonic reception (RBS).
In 1984, a new international band II frequency plan was drawn up at an ITU conference in Geneva. The GE84 plan for FM Sound Broadcasting. When implemented, Rowridge would be required to radiate around 8 dB less in the direction of France. This would reduce the Rowridge signal level received at Alderney, rendering it unsuitable for re-broadcast purposes. The solution devised by the BBC was to extend their digital radio distribution system to Les Platons by means of two digitally modulated links from Stockland Hill, using a spare UHF TV channel. This feed was brought into use in July 1989.
BBC Research Department also published three reports on this digital UHF link. Reports 1987/07 1987/08 1987/09 An undersea fibre optic circuit eventually replaced the UHF link in the 1990's.
625-line UHF TV
During the period from 1962 to 1971, the BBC carried out some long-term propagation measurements across the English Channel at UHF to provide statistical data necessary for determining the requirements for programme feeds to the Channel Islands in this band. Report 1972/06 The use of an SHF link between the Isle of Purbeck and Alderney for both television and telephony was also studied by the Home Office.
By 1974 both the IBA and BBC had decided that the re-broadcast link should be from Stockland Hill to Alderney. Stockland Hill had commenced UHF TV transmissions during the autumn of 1971 and BBC Research Department published a report on behalf of both organisations to further investigate UHF scatter propagation across the English Channel. Report 1977/10 The use of adaptive receiving systems was also investigated in report 1971/32 and report 1974/23 The 1974 report seemed to conclude that the BBC had given up on trying to get such a system to work. The IBA persevered and eventually built a working adaptive system known by the acronym SABRE (Steerable adaptive broadcast reception equipment). 625-line colour UHF TV transmissions to the Channel Islands commenced in July 1976 from the Fremont Point transmitter, which was fed from Alderney via SHF links.
The BBC used two aerial systems working in diversity. An array of 24 UHF log periodic aerials, and a 9.14m parabolic reflector (dish). Report 1977/22 and report 1978/08 provides us with the details. Later, it is believed the BBC used the IBA SABRE system. From July 2003 until digital switchover in November 2010, Fremont Point relayed the Astra 2D satellite feeds of BBC One Channel Islands and BBC Two England.
The IBA used the 9.14m parabolic reflector operating in diversity with their SABRE system
The prototype SABRE aerial undergoing tests at the IBA's Engineering Centre in Hampshire.
Alderney. The SABRE aerials are the four panels. Each panel contains an array of 8 by 2 dipoles. Above is the BBC array of
24 UHF log periodics and on the other side of the tower is the SHF antenna facing Fremont Point. (Picture: Peter Lonsdale)
The 9.14m parabolic reflectors at Alderney (Picture: Peter Lonsdale)From the beginning of 1986 the IBA used a new link which received the ITV and Channel 4 transmissions from Rowridge at the French transmitter site at Digosville, near Cherbourg. This was brought about because Channel TV wanted to change their contract for the supply of ITV network programmes from TSW to TVS. The Digosville aerial system comprised a half-size SABRE array which used two panels each with 16 dipole elements and 16 x-y correction units. The adaptive-array operated in diversity with two trough aerials. The onwards feed to Fremont Point was via a single-hop 8.5 GHz SHF link.
Digosville (Picture: Martin Watkins)At some later time ITV and Channel 4 were fed via an undersea fibre optic circuit until digital switchover in November 2010.
ENG INF cuttings by kind permission of Martin Ellen at BBCeng.info
Features index | Steerable Adaptive Broadcast Reception Equipment article
mb21 by Mike Brown Hosted by 
