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BRESSAY - Shetland
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e.r.p. |
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| All Photos By Dave Stephen and
additional information provided by Ian Anderson of SIBC 96.2 http://www.sibc.co.uk The Bressay transmitting station in
Shetland was opened by the BBC on 15th April 1964 to bring VHF
monochrome television to the islands. There are two towers on the
site, one essentially for TV and one essentially for radio. VHF
television has now ceased but in 2004 Bressay transmits UHF analogue
colour television from BBC1, BBC2, ITV and Channel Four (but not Channel
Five) and UHF Digital Terrestrial Television (DTT).
The second tower is used for VHF/FM radio broadcasting and this transmits the high power BBC national and local opt-out services together with the lower power Independent Local Radio station SIBC (Shetland Isles Broadcasting Company). The programmes for SIBC were originally fed to the Bressay site from the studio in Lerwick via a low power radio link on 102.2 MHz (20 watts - vertical), which also acts as a small filler for the town. Since 1994, however, the feed from the studio to the transmitter has been via U.H.F. (Ultra High Frequency) radio link. The 102.2 MHz v.h.f. transmitter is now essentially a standby service that listeners can retune to should Bressay fail, which has happened. These details have been provided by Ian Anderson of SIBC kindly e-mailed us to confirm the technical details of the station and also points out that SIBC uses the two vertically polarised Marconi R1000 aerials seen in several of the photographs below. Thanks Ian! The transmitting aerials used by the BBC radio services are the six tiers of large "Coniex" crossed dipole aerial panels at the top of the 'radio mast'. In SIBC's Ian Anderson's e-mail he also notes that: "Your e.r.p. for SIBC is undercalculated, although it is difficult for us to give precise figures. If the BBC is "high power" we cannot be "low power" since were are on parity with the weakest of the BBC services (the variance in the Q of the aerial system means that the services erps are not all the same) in some lobes, and no more than 4-5dB down in general. It has been like that for 10 years, with 1 dB increased reflection from the Orange metal work (just above) added a few years ago. It just shows that a well placed dipole pair with reflectors can be relatively efficient, even against eighteen Coniex crossed dipole panels". * In reply to the effective radiated power observation raised by Ian, I must point out that the power figures that we generally quote come from either the BBC of Ofcom. The 4kW figure quoted in the technical panel at the top of our page is therefore that published by Ofcom and previously by the Radio Authority. I did, however, read in "Communication" (the monthly publication of the BDXC - British DX Club) in around 1994/5 that SIBC had been given authorisation to increase transmission power from 4kW vertical to 50kW mixed (i.e. 25kW (V) + 25kW (H)), making it most powerful ILR station in the UK. Although the current technical parameters published by Ofcom show the permitted maximum e.r.p to be 50kW, they continue to list the actual output as 4kW vertical! I am guessing from what Ian has written that the output from SIBC is probably nearer to 8kW vertical (i.e. about 4dB down from the BBC's 43kW mixed i.e. 43kW ÷ 2 = 21.5kW less 4dB = 8.6 kW). Thanks to Ian Anderson of SIBC for the useful contribution. More below.... The first photographs show the tower that carries the radio transmitting aerials:  |
 The tower carrying VHF radio aerials. The six stacks of Coniex mixed polarised aerial panels that carry high power BBC VHF/FM radio are positioned at the top of the tower. Just above the Coniex panels there are UHF aerials which carry two of the DTT (digital terrestrial TV) multiplexes. The other digital terrestrial tv multiplexes (BBC, Freeview etc) are carried via the main UHF aerials on the other tower on the site. |
 A close up of the main mixed polarised aerial panels for high power BBC VHF/FM |
 This photograph details two sets of aerials: 1/ Two horizontal log periodics (top right) that appear to be VHF receive (RBR) aerials for BBC Radio 2/ Below the pair of Orange mobile phone aerial panels are two vertical Marconi R1000 dipoles, (bottom left with white dipole centres) with rear reflector, that are used by the ILR station SIBC that produce near 8 kW. |
 A view from the hill with the radio tower on the left and the television tower on the right |
The following photographs, below,
show the tower that carries the main television transmitting aerials: |
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 A close up of the main UHF GRP aerial cylinder and top mast light |
 Note the six VHF yagi antennas on the left hand side about one third of the way up the tower, just below the mast light |
 A close up of two of the six VHF yagi antennas Thanks to Dave Stephen for very kindly sending in these photographs. |
| Continuing the subject of radio
coverage from the Bressay transmission tower, Ian Anderson of SIBC adds: The e.r.p. of SIBC is no worse than 4-5 dB on the BBC in some directions and parity in others. The BBC's horizontal pattern is like the petals of a flower and the parity is in the directions where we have a gain and they have a slight null. In one direction (roughly towards Lerwick) we have massive reflection of the tower (we are low down on the tower so there is a lot of metal behind us, and to the side) and Orange's metal work above added 1 dB when it was installed by reflecting our signal downwards. The Radio Authority did a computer model and gave our maximum lobe as 9dB because of the metal work, so that would be 16kW less feeder loss (c 1dB). We are probably about 12.5kW max in that direction, which is only 3dB down for the maximum of 25kW vertical that we are allowed. We usually quote about 12.5 kW vertical towards Lerwick, Brae etc. The horizontal radiation is interesting. With no horizontal element, we are actually less than 10dB down on horizontal, all because of reflections of the tower, the hillsides and twisting in free space, giving about 1kW all for free!!!!. We are also less prone to some kinds of multipath than the BBC curiously. When the hills are wet from rain, the soaked peat also helps to conduct our signal over to the back of hills. Normally there is increasing attenuation when the declination behind a hill is greater than 7 degrees, but wet hills help. Attenuation due to mist and drizzle the short distance between Lerwick and Bressay, combined with scatter reflection of the small droplets, can be as much as 1dB. And when very large ship enters the harbour we can see a change in signal strength from -3dB to 2dB. Recently a crane worked 100 yards away and the same changes were witnessed. The moral of the story is that you can used scorched earth power like the BBC and lose it all because local circumstances. Ian Anderson SIBC |
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