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Unusual signal strength variatons on Mendip BBC HD

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    JohnH77

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    I am experiencing strange signal strength variations on Mendip with the BBC 1 and 2 HD mux on C35, and the Drama/5USA mux on C48.

    Previously everything has been fine for years.

    It started a day or two ago. When the picture is present the signal strength is 74%. But the picture randomly disappears for seconds or many minutes during which time the signal strength is 0%. When I watch the signal strength I never see any intermediate strength - it is either 74% or 0%.

    When the strength is 74% the quality is 100% When it is 0% quality is 0%.

    I would never expect the signal strength to drop to 0%. My neighbour sees the same problem.

    Has anyone seen anything similar?

    My line-of-sight to the transmitter passed through solid earth and I wonder if groundwater during wet weather affects it. I have contacted the BBC to see what they suggest.

    Mon 30 Dec 2019 19:48:38 #1 |
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    A1944

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    There are interference problems over much of the country:

    https://www.bbc.co.uk/news/technology-50945421

    Mon 30 Dec 2019 19:55:02 #2 |
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    JohnH77

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    Thanks for that as I hadn't spotted it. I am very marginal so it just takes something small to give me problems.

    The Downdetector website looks useful apart from the ignorance and arrogance of 90% of the posters. It is rather dispiriting and I fear a sign of our times.

    @FreeviewAdvice please explain why you’ve told my elderly parents in rural North East Scotland that they have had absolutely no Freeview signal at all for 3 DAYS due to ‘atmospheric pressure’ and still refuse to send an engineer to help! Disgraceful!

    No you idiot. There is no signal so no point in sending anyone out. Where are engineers going to magic a working signal from?

    The BBC page says it is a temperature inversuion caused by the high pressure which is the problem.

    However, it isn't the high pressure itself which causes interference with TV signals. It's the presence of what is known as an atmospheric temperature inversion. A temperature inversion is when a layer of warm air overlays cooler air at the surface. Temperature usually decreases with height above the earth's surface, but when there is an area of high pressure, the air aloft sinks down towards the earth's surface and it warms up as it does so.

    This creates a sharp thermal contrast in the atmosphere which TV and radio waves see as a physical boundary. There are many TV and radio waves travelling through the lower atmosphere and in such atmospheric conditions, these waves can be refracted or bounce off the inversion overhead and this enables them to travel much further than they would otherwise be able to.

    In this way, the usually strong Freeview signals can experience significant interference with other, normally distant, signals from other sources.

    Will these conditions persist? Inversions tend to break up. The current high pressure over the UK is likely to slowly weaken over the next couple of days.

    Inversions may still form overnight for the next couple of nights, particularly in the south of the UK, but they are not expected to be as sharp or close to the ground and so should have less of an effect on Freeview signals.

    Mon 30 Dec 2019 20:07:48 #3 |
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    JohnH77

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    The Beeb has responded:

    Hello John

    Thanks for getting in touch with us.

    Some viewers and listeners in particular areas might experience short term interference to BBC television and Radio services. This is being caused by a zone of high pressure, which causes problems to the links between transmitters and increases interfering signals in the area, which results in poor reception. The majority of the BBC's transmissions are not affected by this period of interference, as the BBC's networks are engineered to be generally very robust. Television viewers may experience slight occasional pixilation or break-up or loss of service. Cable and satellite services are not affected. At these times, there is nothing you can do but wait until the weather changes. You should not re-tune your television when this happens.

    Interestingly they include "problems to the links between transmitters".

    Mon 30 Dec 2019 20:15:16 #4 |
  5. Trev

    Trev

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    General suggestion

    DO NOT RETUNE.

    You will just make things worse.

    Tue 31 Dec 2019 0:28:44 #5 |
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    JohnH77

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    Trev

    Absolutely. There is nothing to tune so I would lose everything.

    Interestingly all seems better tonight and I have all channels back.

    Tue 31 Dec 2019 1:16:23 #6 |
  7. Trev

    Trev

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    Yeah. Mine were coming and going last night, but I have two aerials in different locations that I can swap between with a switch and if one 'doesn't work' the other one often does. It's usually (but not always) the BBCB (HD) multiplex that I loose first.
    I live in the south east and get quite a bit of co-channel interference from the continent at the slightest suspicion of a lift.

    Tue 31 Dec 2019 12:22:13 #7 |
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    JohnH77

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    I too have two aerials where "one works when the other doesn't".

    I set them up on the basis that they are a different path length from the antenna so when one is in a null, the other shouldn't be; and vice versa. As the height of the inversion layer reflection changes the path length of the signal reflected from the layer will vary and the location of the nulls will change.

    It works for reflections from hills and for reflection off the surface of the sea where some people receiving across water lose signal in sync with tide height!

    Tue 31 Dec 2019 15:08:09 #8 |

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