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TV Aerial for Sandy Heath

Discussion in 'Audio Visual' started by Gary_Funwick, 6 May 2020.

  1. Gary_Funwick

    Gary_Funwick

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    I had a loft aerial install around 5 years ago. I have lost the paperwork but I believe that it has a masthead amplifier (there's power in the loft and the original spec probably involved feeding a multiswitch).

    I don't recall it ever provided a flawless signal but because I use a PC based video server and I do have a well performing and accessible satellite dish I've never sorted it out (I have 15 foot ceilings so loft access is not trivial).

    I'm trying it again because I think that my initial problems were cause by too strong a signal over driving the TV capture card - that's been dealt with using some passive attenuators and picture quality now seems good and stable (not seen any green breakup in the last few days despite forcing the TV server to use the DVB-T tuners in preference to DVB-S.

    However the range of TV channels I receive is very small. The aerial is pointed at Sandy Heath which according to https://ukfree.tv/transmitters/tv/Sandy_Heath is broadcasting C21+, C24, C27, C36, C48, C55, C56
    I get (this is with a 10dB attenuator)
    C21+(474.2MHz) 82 signal strength/100% quality; all channels listed in mux received
    C24 (498MHz) 82 signal strength/100% quality; all channels listed in mux received
    C27 (522MHz) 82 signal strength/100% quality; all channels listed in mux received
    C36 (594MHz) 72 signal strength/0%; no lock
    C48 (690MHz) 72 signal strength/0%; no lock
    C51 (714MHz) 100 signal strength/0%; no lock
    C55 (746MHz) 100 signal strength/0%; no lock
    C56 (754Mhz) 100 signal strength/0%; no lock

    My conclusion is that there is a narrow band aerial fitted and/or the amplifier doesn't cover a wide range and if I wanted to fix it the best way would be a log-periodic aerial and to try disconnecting the amp (sorry, I don't have software that will return a meaningful signal strength number, but it took about 30dB of attenuation to cause the lock on C21+/C24/C27 to be lost).

    Any advice please - I'm happy to replace the aerial but climbing into the loft just for a look see to confirm my suspicions isn't appealing.
     
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  3. Sureitsoff?

    Sureitsoff?

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  4. Lucid

    Lucid

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    Your aerial is still over-amplified. I say that because you've got 100% Strength and little or no Quality.

    If possible, try bypassing the masthead amp at the aerial end. Check your readings again
     
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  5. winston1

    winston1

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    You most likely have an A group aerial which is no longer appropriate. Nor is a K group so don’t get one of those. You actually need a T group 21 to 60 to cover com 7 and 8.

    A log periodic type is generally a good choice. A local good aerial installer would (hopefully) know of any local problems
     
  6. Rodders53

    Rodders53

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    Location? Most important for reception of any transmitter. (Postcode of a nearby shop/pub/church/school is usually close enough. I don't need your exact address).

    But I'd agree with Lucid in that over-amplification is the root cause of your problems. Attenuating after an amplifier is not going to cure issues within the amplifier electronics from high signal levels.

    You may need to get the ladders out and up into the loft. But do be ultra-careful!

    I'm on Sandy about 9 miles or so and my bedroom TVs work off a bit of wet string (one is a "EID dipole" design a colleague used when we first moved into the White City building, that involves just a bit off coax cable and a TV plug).
     
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  7. Gary_Funwick

    Gary_Funwick

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    @Sureitsoff, @Rodders53
    Wolfbane reports the following for Sandy Heath to my location (it actually prefers Crystal Palace)
    Field strength: 47dBuV/m
    Distance: 26 miles
    Bearing: 298 degrees (this is straight into the setting sun and I do reliably get breakup just before sun sets - is this inevitable unless I use a different transmitter?)

    @Lucid
    Is the 100% strength/0% quality an artifact of the amplifier or typical of digital signals when actual reception falls to nothing? I understand its quality that matters with strength much less important.
     
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  9. winston1

    winston1

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    Never heard of the sun affecting UHF signals, though it does affect Ku band satellite signals.

    100% signal/0% quality could well be too much signal or it could be interference.
     
  10. Lucid

    Lucid

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    It's the TV tuner over-saturating. Give the TV too much signal - even if the quality is 90%+ - and you'll just swamp the tuner and kill reception.

    With a 0-20dB variable attenuator used as a diagnostic tool at the aerial input of the telly, reducing the signal strength will produce an increasing signal quality until it gets to the system's maximum. Allowing the signal to increase again will reduce quality.

    This isn't the only cause, but it's one of the first things to try when troubleshooting. Do the easy checks first before looking for more complex issues. Bypass the amplification and see how you go on from there. If it doesn't help, then you know you have a more complex issue to deal with.
     
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  11. Gary_Funwick

    Gary_Funwick

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    @Lucid
    Thank you for your reply. I'm trying to work out how to get my extra-big A-frame ladder up stairs to mount an assault on the loft (its been up their once, so it must be possible but I cant remember how Mrs Funwick and I managed it).

    BTW, with your experience will I always have problems at sunset as I flagged to Rodders53?
    If so I might start again from scratch and aim for Crystal Palace.
     
  12. Lucid

    Lucid

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    I haven't yet come across a situation where the sun directly knocks out a TV signal. If this were a main phenomenon, then I think far more people here and in sunnier climes would be affected, so we'd know about it as a big problem.
    There have been incidences where the sun causes something to change in a system such as heating up water in a cable so it expands and creates a short, or the heat makes some bit of silicon trip out that's already intermittent.
    I'd still stick with the "do the basics first" method of troubleshooting.
     
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  13. Rodders53

    Rodders53

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    LOCATION????
    Without that, approximately, we can't use the Freeview prediction tool in <detailed view> https://www.freeview.co.uk/help (which includes allowance for interference from the continent/our own transmitters by the Spectrum Planners).

    Sunrise/sunset are times where enhanced propagation interference often peaks? Equally at that time after a long day of sunshine the temperature may be frying the electronics up there?

    270 degrees is West (setting sun) 298 is more West Northwest. I make you to be somewhere south of Saffron Walden. Their Rugby Club Ground postcode CM22 6BQ is close to the distance and bearing you give but not spot on.
    That Club postcode location gives Sudbury as best (all 6 muxes green) 50dBuV/m @10m outside but 49 DB diffracted at 6m agl. Crystal Palace is next the 3 PSBs and COM7/8 are green the other COMs amber and 50dB/49dB but clear l.o.s. Sandy has the 3 PSBs green and the others amber, but is diffracted at both heights 50/49dB.

    The precise predictions for your address postcode with house name/number will likely be important to gauge the most likely to be best. Also consider other factors like nearby houses, or farm buildings, that may shield your aerial from the wanted transmitter. Pointing an aerial through roof tiles is fine... Pointing one through a series of party walls is a very different thing.

    47dBuV/m (if correct - I have never 100% trusted Wolfbane following making analogue measurements outside my home many years ago with a calibrated survey vehicle) in a loft space will be lower - as much as 10dB loss is not atypical - add aerial gain say 10dB and you may be back at the same level.

    Ideal input to a TV is 45 to 65dBuV... so an amplifier is needed if feeding more than one TV. If only one, maybe no amplifier would be needed.

    I'm doubtful that a masthead amplifier - even one with 26dB gain - should cause overload and that a 10db attenuator has little effect. I suspect a second high-gain distribution amplifier (aka amplified splitter) may be cascaded after the masthead?

    But you do need to start with the basics and try the aerial direct, then with the masthead OR distribution amp (and maybe vice versa) etc.,.
    Some mastheads have a variable gain control. If so that can be altered as well.

    Best of luck. If you can, please use a proper extending access ladder once the hatch has been opened using the A-frame one! Take pictures of the aerial and all amplifiers (showing labels/make/model) as that could help enormously.
     
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