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Lost COM7 & COM8 Channels, Replaced Aerial, Still the same?

Discussion in 'Audio Visual' started by twixx, 4 Jan 2019.

  1. twixx

    twixx

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    In October we moved home, recently the wife has noticed that some channels have disappeared. Upon investigation, it seems that the channels that have gone were "recently" moved to COM7 and COM8 multiplexes.

    Not knowing anything about the TV aerial setup in the home (though it appears to be a Blake UK 32 ELEMENT HIGH GAIN AERIAL DMX05T/F) I decided to replace it with a Labgear LAB450T Very High Gain TV Aerial, which never let me down in our old house.

    When getting up to the aerial I noticed it was facing West, whereas all our neighbours are pointing North. For reference, North would be aiming towards the Sheffield/Emley Moor transmitter. In our case, West, is aiming more towards the Chesterfield transmitter.

    Presuming this was the issue I've turned the aerial to match that of the neighbours, but still the channels are missing.

    I have no tools to check signal strength (other than that provided by my YouView receiver, which suggests signal is perfect).

    Is there anything glaringly obvious I have missed?
     
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  3. Lucid

    Lucid

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    A couple of thoughts: COM 7 & 8 require the TV to have a Freeview HD tuner. They aren't receivable on TV with an ordinary standard def (DVB-T) tuner. It has to be a hi-def DVB-T2 tuner, even if the channel itself isn't broadcast in HD.

    If all your neighbour's aerials are pointing towards Chesterfield, then that might suggest that some or all of the Emley Moor stations can't be received.

    EM is powerful, but if there are hills or some other physical obstruction then that might explain why your neighbours are using Chesterfield. I haven't decent access to the Web to check if Chesterfield is a relay. If it is, then it may not even carry COM 7 & 8.




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  4. Sureitsoff?

    Sureitsoff?

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    to hijack this thread, I didnt know that. Thanks Lucid
     
  5. twixx

    twixx

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    It is my understanding that Chesterfield is a relay and it does not carry COM7 & COM8.

    Your understanding of my description is slightly skewed though. Currently I appear to point to Chesterfield, and my neighbours Emley Moor.

    I'm saying they're pointing to Emley Moor, purely because thats known to be the biggest transmitter nearby, but by pointing "in the direction of Emley Moor", they will also take in the Sheffield transmitter (by the by, Sheffield & EM transmit COM7 & COM8).

    My receivers are both Freeview HD.

    I plan to re-align my aerial this weekend, and reset my devices so that I get the choice of Freeview Regions again. Will see how that goes.
     
  6. Lucid

    Lucid

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    My apologies at mixing up the Chesterfield/EM directions. However, if you are aligned on Chesterfield then that explains why there's no COM 7 & 8 signals.

    I wouldn't worry about picking up Sheffield. It's a relay transmitter, so it is polarised vertically. That means the three legs of your boom will fan out parallel to the ground if the aerial is correctly aligned for the Sheffield transmitter. Emley Moor is a main transmitter. It is horizontally polarised. The orientation of the aerial is the same as you'll see in most adverts for it.

    Horizontal and vertical polarisation is a way for two transmitters to operate in the same area with significant interference with each other. With TV transmitters, the relays are there to fill in any coverage gaps from the main transmitter. In the case of Sheffield, the hills on which the city is built form a natural barrier to the signals from EM. The relay fills in where the EM signals don't reach. The relay is polarised differently so that in the fringe areas where the signals do overlap, the two don't interfere with each other. The receiving aerial is polarised to match one or the other transmitter, and so the aerial becomes far less sensitive to the signals from the opposite polarisation.

    You'll also notice that the Sheffield relay operates on far lower power. It doesn't need to reach that far. That means that the signals from it will be far weaker by the time they reach your home.

    I've only used tri-boom aerials such as your LAB450T on a couple of jobs. They were both over a decade ago when I was just getting in to aerials in a small way and really didn't know too much. I was persuaded by someone I later found out had only marginally-better knowledge than me at that point that these were the best I could get. Certainly reading the blurb and some info online, the idea of being wideband and very high gain sounded ideal. Who wouldn't want that combination? I came to realise later that there was far more to it than that.

    Most TV aerials have a lumpy gain curve. That means they're only 'high gain' for a small portion of the reception range. Your tri-boom follows the same pattern. If you want to see what the gain curve looks like then there a very good website from a company very close to where you live: aerialsandTV

    When we look at the coverage checker you linked (and thanks for that), the main Emley Moor muxes are all high power (174kW) and coincide at frequencies where your tri-boom aerial is relatively efficient. COM7 & 8 are roughly 1/3rd of the power, and at frequencies where the aerial isn't so good. Once your aerial is aligned on EM though, the coverage checker still predicts decent reception for those two muxes.

    If anything, the combination of a strong signal from EM where you live (the Field Strength) and the peaky high gain of the tri-boom would make me concerned about over-saturating the tuner. If you also have an amplified splitter, then that could be adding to the problem. Too much signal is just as bad as too little, but people rarely recognise this when it occurs. They usually think in terms of adding amplification, and not about reducing the overall level with some attenuation.

    My home is about 10km further away from Winter Hill than yours is from EM. The WH signal here is very strong, even though the transmitter is lower powered (100kW vs 174kW). My aerial is a low gain Log Periodic (see the shallow black curve on the graph from the ATV site), and yet I get 100% Quality and 90% Strength on all the muxes I can receive. In fact, there's enough signal coming off this low gain aerial that I could passively split and run three TVs without an issue. I just wonder if you really needed such a big aerial?

    If you're still having problems receiving COM7 & 8 then try tuning in channels 32 and 34 manually. If that's still a problem, try bypassing any amplified splitters and have a think about insering a 0-20dB variable attenuator in line to see what happens when you reduce the signal strength.

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  7. twixx

    twixx

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    Thanks for your great reply!

    I did rotate the aerial from Chesterfield to Sheffield/EM to no avail. I can’t remember if I left it in H or V. Though from what you’re saying, it shouldn’t really matter. It will auto select Sheffield/EM, based on H/V.

    I used to get the option on the TV to choose the Freeview region, but no longer get that. I suspect that I’ve perhaps positioned the aerial out of line where there was contention?

    With regards to the Labgear and why? I went a bit Jeremy Clarkson thinking bigger is better. What I’ve actually found is that it’s made not an iota of difference. I fitted one at my old house and had zero issues, so that was another reason for going Lab also.

    The tests you list are all on my to do. Specifically bypassing the amplifier, which is a slx 28103FG Four Outlet Satellite Distribution Amplifier
     
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  9. Lucid

    Lucid

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    Like most Freeview TVs, yours will remember the stations it has seen last time it did a 'from scratch' tuning. If the set is just rescanned, then all it will do is refresh the channels received from the existing muxes. That's okay if you're still pointing the aerial at the same transmitter, but no good if there are new muxes to pick up.

    The problem you may face though is that if your aerial alignment is a bit off, then the telly won't find the new muxes from Emley Moor after you do a factory reset / shipping condition reset /first time installation reset (whatever they call it on your models of TV). Incidentally, this is also why you don't get the option to to select a region. It's a good idea then to bone up on manual tuning rather than simply relying on autoscan tuning, just in case. The channel numbers for the muxes are given on the coverage checker you linked to previously.

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  10. twixx

    twixx

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    I’m performing a full ‘channel initialization’ Each scan, which clears out the previous settings, if I get an hour child free I’ll have a proper play later
     
  11. twixx

    twixx

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    After performing the above tests (with the exception of fitting an attenuator) I'm no better off, short of getting a professional in I think I'm going to cut my losses. I may aswell fit the old aerial again as its a lot smaller, especially as COM7 and COM8 are only temporary right? During this process I have noticed that when viewing signal strength/quality on my YouView box that its not exactly great, 60-70% so perhaps we're just being blocked by neighbours or we're all in the same situation.

    We have an old Sky dish fitted which supplies me with Freesat. When I can be bothered I may split the LNB feed, that way we'll have Freeview & Freesat, Freesat can then plug the gaps on Freesports and CBeebies HD.
     
  12. Lucid

    Lucid

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    I think you've got too much signal.

    The signal for digital TV reception is split in to two components as measures of performance. There's Strength - the least important - and Quality which is where your attention should be focused.

    Strength is simply a measure of the signal power. Adding an amplifier to a lousy signal will make it stronger, so it becomes a 'loud' lousy signal.

    Where you want to know if the signal is any good then Quality is the thing to look at. This tells us how big a gap there is between the signal we want and any background noise in the system that's trying to get in the way. A lower Quality reading means a smaller gap wich means less signal and more noise.

    When a signal is over-amplified, then the Quality reading goes down. Add a variable attenuator to the aerial downlead before it gets to any amplified splitter. Turn it up to the maximum attenuation (20dB). Have someone watch one of the TVs with the Quality and Strength display shown. Have them relay the results as you adjust.

    Turn the attenuator down gradually. Quality should go high but Strength low. Strength will gradually increase until it reaches a point where Quality then starts to go down. Back off the setting slightly. You're now set.

    Repeat the process with channels from the other muxes. When you're ready, try a manual tune of Com7 and Com8
     
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  13. Lucid

    Lucid

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    By the way, you can't split satellite dish signals in the same way that you do with aerials. Satellite doesn't work that way.

    Run additional cables from the spare sockets on the LNB.
     
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  14. twixx

    twixx

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    When I say split, I meant re-route one of the current LNB feeds. At present the sky dish has a twin feed from the LNB running to the living room, my plan would be to "split" these, so one routes to the living room and the other into the other room.

    I've completely given up on the DVB-T aerial. We're back pointing to Chesterfield, so no matter what I do, I won't pull in COM7 or COM8 as they're not being relayed.

    One benefit I have is that we use NEEO remotes (Harmony-a-like), so I can hide the input change behind the remote steps. So essentially, she won't need to know what the source is (Freesat/Freeview), she'll just choose to watch FreeSports, or CBeebies HD and the remote will perform the necessary input changes and set the channel.

    Cheers for your help & advice with this!
     
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