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A newbie in need of aerial wiring advice.

Discussion in 'Audio Visual' started by opps, 26 Jul 2020.

  1. opps

    opps

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    Hi all.

    I live in a 2 bedroom terraced house in west london.

    The aerial on the chimney is the cheapo Yagi Uda type- the ones where the horizontal bars seem to be equidistant and then a plate with slots at 90 degrees at the end. From there I have CT100 cable that runs in to the attic- at a guess about 3m.

    In the attic there used to be a powered amplifier with two outputs. There are a total of 4 devices running off the aerial, 3 tellys and I radio.

    During the lock down I decided to buy a proper F Type tool and plugs and a basic 6 way splitter. I naively thought that I could do away with the amplified unit and use the passive splitter. I soon discovered that without the amp I could only power one telly and that as soon as I did so the signal on the first telly I tried dropped from 75% signal strength when using a straight female F type in to the feed to 48% when using the passive F-type splitter. In both cases the signal quality remained 100%. The next TV I tried (master bedroom), with the first TV removed from the equation only had 32% signal and 74% quality when using the female direct in to the feed, as soon as I plugged it into the splitter it went to 40% signal and zero quality. That telly however is a bit of a Frankenstein, the feed runs down to the B&0 telly down stairs and then runs up to the tv in the bedroom which is also a B&O. The coax from the telly down stairs sends whatever is on the telly down stairs up to the tv in the master bedroom via a digi box and the other functions are sent via a B&O masterlink cable.

    The third telly is in the kitchen. All have CT100 cable running to them. The B&0 radio/cd player has an old coax.

    All worked until recently until the 20 year old powered amplified unit died. I picked up a new one from toolstation today and additionally picked up a masthead amplifier. I have only installed the in loft unit thus far. Everthing is working but I am guessing that some channels will still pixelate (as they did previously). I will fit the masthead amplifier but I need to wait until I pick up the roof/apex conversion kit for my ladder.

    The one I picked up has 4 outputs. My (lay) understanding is that I need to fit it with in 1m of the aerial. I am happy to do that, but before opening the box I would like some advice. Should I get a better aerial and return the masthead amplier or if I stick with the MA should I extend the cables to meet it and ditch the cheap powered amplifier.

    Will the MA and powered amp potentially cause problems? Should I use the MA with just one output and the powered amp?

    Sorry if I am being awfully vague, it is nothing more than evidence of my ignorance.
     
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  3. HERTS P&D

    HERTS P&D

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    Lucid will be along shortly.

    Andy
     
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  4. HERTS P&D

    HERTS P&D

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  5. Techn0

    Techn0

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    You can place the Masthead amp inside the loft near the entry point if you don’t have access to the aerial. Take the output from the Masthead power supply to a single TV to test signal,if OK, then route through a powered splitter which would feed as many TVs as you need.
     
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  6. ericmark

    ericmark

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    Mast head amplifiers need power, there are two methods, some TV's can be configured to give an output, otherwise you need a power supply which looks like a second amplifier, there are units to combine the output of two aerials, but I will be frank never used one, but clearly to run radio and TV you need two aerials, and a unit to combine and split the signal. The problem is some of the band pass devices stop DC so would stop the power to the mast head amplifier, @Lucid will likely know, but would be good if you give details of the mast head amplifier and any splitters or combiners you are using.

    I had problems as I was using the old digi eye which also needs DC through the coax, and I had the wrong plate which would not allow DC to pass.
     
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  7. Lucid

    Lucid

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    Hi @opps and thanks for a useful amount of info. I'm going to try to pick out the main points and address them, but if there's anything I miss then please feel free to flag it up.

    'Your old aerial is giving you 100% quality and 74% strength direct in to one TV'
    - this suggests that you're in a place where the signal from the transmitter is fairly strong. My guess would be that you're picking up from Crystal Palace (CP).

    From your description, it sounds a bit like you have contract aerial. This could be a digital era aerial, or something that was installed when we just had analogue but it still works okay today; lots do. These may not be exactly what your aerial looks like, but something along these lines.

    Horizontal-Vertical-aerial-polarisations-500W-L5.jpg

    Contract aerials are mostly about keeping the cost down, so they sacrifice some performance in order to achieve a lower price. They work, but don't generate as much signal power as you get with a good alternative. Since quality is a measure of the gap between signal level and background noise, then the more signal that the aerial can pull in compared to the baseline of background noise then the better the quality measure.

    At this point you're probably thinking "but my quality is already 100%?"

    It is, according to one TV. The thing is TVs often overstate their measures of quality and strength. The TV metering isn't always that accurate (read as: rarely accurate).


    'However, once it has gone through the distribution system you're getting some pixelisation on certain channels'
    - CP used to have all of its transmission frequencies bunched up in the lower 1/3rd of the ch21-68 transmission range. It was common then for installers to use a type of aerial tuned to that narrower band in order to get more signal strength and a better quality measure. These are called Grouped Aerials. The one that matches CP is a Group A aerial. These are identified by coloured markers. Group A aerials have a red marker. It's generally the end cap on the boom.

    The marker will be difficult to spot from the ground unless you have suitable binoculars. Even then, an aerial installed for a number of years might have resulted in the plastic marker becoming sun bleached, so it's still hard to tell. It could also be covered in dirty or soot, or have even fallen out.

    The point about Group A aerials is they're excellent at reception within their band, but they're not so good outside of their range. This could be one reason why some channels are pixelating.

    For much of the past thirty-five years or so, CP has used frequency channels 22 to 34. That was a good match for Group A aerials which are recommended to cover channels 21~37. What changed was the start of the channel shuffling to allow the sell-off some of the TV channel range for 4G mobile telecoms use. In the case of CP, there were some additional channels allocated at c55 and c56. These are transmissions where the TV needs to have an HD tuner, so they won't show up on a TV that has just a Freeview SD tuner. However, if the TV is properly equipped, and the aerial you have is a Group A aerial, then even despite the transmission power of CP, I would expect you to struggle to get good signal for these channels. They're only temporary too. In fact, the one on c56 has already been switched off (June 30th, 2020).


    Other reasons for pixelation on certain channels....... They're not broadcast at the same power as the main channels. CP transmits at 200,000W for the main channels, but the temporary mux and local channels transmit at less than a quarter of that power.
     

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  9. Lucid

    Lucid

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    'Should I get a new aerial?'
    - Short answer - Yes

    Where you have a contract aerial (presuming that is what it is), then the amount of signal strength it gathers won't be as much as a good Group A standard aerial. (see above)



    'What aerial should I get?'
    - That's a slightly trickier question.

    Long term, CP will return to being best served by a Group A aerial for maximum signal power, but unless the local reception conditions at your house are very difficult, then I suspect that being so close to CP would mean that a simple Log Periodic would give more than enough signal strength to achieve a passive 4-way split.

    Short term, a Group A will struggle to pick up c55 and c56, but that only matters until the end of 2020 for c55. As of June 30th 2020, the mux on c 56 has already been switched off. By the end of the year, the mux on c55 will have gone too. That's because the channel shuffling for CP will have come to an end. The channels carried by those temporary muxes will have been folded in to the Group A range. The other point is that the affected channels are only receivable with TVs or PVRs that have an HD tuner. Some of these channels have SD equivalents within the Group A range.

    Where you have a contract aerial (presuming that is what it is), then the amount of signal strength it gathers won't be as much as a good Group A standard aerial (see above), and might not even match a good Log Periodic wideband aerial in some parts of the range.

    CP is one of the UK's most powerful transmitters. The main BBC and ITV SD channels are kicked out at 200,000W. Even the 'local' channels transmit at 30,000W. For someone in say Little Ealing (W5), they'd be around 11 miles from the transmitter and could probably get decent results with a bit of wet string :LOL:

    For comparison, my home is 30 miles from Winter Hill. The main channels broadcast at half the power of CP, so 100,000W. I can also pick up the local area stations for Liverpool @3kW and Manchester @2kW all on a low-gain Log Periodic. My TV reports 100% Strength and 100% Quality for the main channels including HD. Liverpool and Mcr local channels both deliver 90%+ strength and 100% Quality. To be fair, I live on the Cheshire plain, so there's very little in the way of the signal path from the transmitter to my aerial, and that does help. It would be different if I lived in a built-up area or in the lee of a hill.


    'Where am I losing signal?'
    - There are a few places: Aerial alignment; The age and condition of the aerial; The capabilities of the aerial i.e. it just doesn't pull in that much signal because of design limitations; Local reception conditions i.e. there's a high rise building or a bunch of trees directly in line-of-sight to the transmitter.

    You'll also lose signal strength through cable. CT100 is decent coax if it's the genuine stuff with a copper core and copper foil + braid shield, but like any coax, there's always some loss. With WF100 and CT100 in good condition you'll see a loss of 0.15dB per metre, and so every 10m sees the signal drop by -1.5dB.

    Connections will lose something too. Figure on about -0.25~0.5dB each.

    Two-way splitters will reduce the signal by half per output (-3dB) plus an extra -0.5dB for the insertion loss.

    View attachment 200128

    All of the above though is passive reductions. This means that the signal strength is reduced, but there's no increase in the noise. If signal quality is 100% to start with, then all else being equal, it will still be 100% after the losses.

    It sounds dramatic to say that you lose half the signal (and a bit more) simply by splitting two-ways, but RF signals are resilient to these kinds of losses. Most TVs will do just fine with a signal level of 50dB at the aerial input, and so it's possible to add up the combined effect of all the losses in the signal chain to work back to a target level for signal at the aerial. For somewhere with a high field strength from the transmitter, plus using a high-gain aerial fed directly in to a TV, they could well end up with too much signal.

    Active splitting and amplification both add loudness and noise to the signal. Sometimes you have to accept that, but I think in your situation the masthead amp would be OTT.

    All amplifiers and active splitters add their own noise to the signal. This has an effect of reducing the gap between the background noise and the signal level itself. Since this signal-to-noise ratio (SNR) is what determines the Quality measure, then anything that reduces the SNR also reduces the quality. Once that is chipped away, there's no way to get it back. The addition amplification increases both the signal level and the background noise by the same amount, so you end up with a louder but noisier signal.

    The amplification is there mainly to compensate for the losses in the signal cable from the amp to the TVs. For this reason, any active amplification goes as close to the aerial as possible; hence why we have masthead amps and active signal splitters in the loft. This is the opposite of best practise for passive splitting. They work best where the signal is left to the latest possible moment before splitting. For this reason, passive splitter are best used closest to the TVs.

    From what you've said about the B&O TV, it sounds as if there's some sort of modulator inside it so it adds its signal to the RF pass-through. This then would class as an active amplifier. Where you're getting pixelation on the TV after the B&Q then I would suspect that the signal Quality wasn't quite good enough going in to that TV first so it couldn't stand up to the degradation, or it was already a strong signal with good quality, and the additional amplification from the B&O TV was pushing it over the threshold for the TV next in the chain.



    'FM reception via the TV aerial'
    - In theory you shouldn't be picking up much of a signal for FM radio at all from a TV aerial. However, in very strong signal areas there can be some breakthrough. If you want proper stereo FM reception though, then the best thing to do is to fit either a half wave dipole or a folded dipole FM aerial along with a TV + FM combiner before your passive splitter. This will give you excellent FM reception.

    Presuming that your splitter is wideband (i.e. does FM frequencies as well as TV), then all four legs of your 4-way splitter will carry the FM signal. Connecting that to the radio aerial input of the B&O cd/radio set will replicate what you've done already, but with much better signal.



    (TL;DR) To recap then, I would change the TV aerial. I wouldn't fit any amplification at all. Use a passive splitter instead. Add an FM aerial and combine the signal with the TV feed before passive splitting.

    Have a look at the Aerials & TV site. The DM Log with tilting clamp would probably be my first choice. The tilt adds another dB or two to the signal level because it optimises reception. Where you think you need something with higher gain, then their Group A Yagi 10K might do the trick without going OTT.



    If this and other replies were helpful to you, then please do the decent thing and click the T-H-A-N-K-S button on all those the posts. This isn't a big thing to ask of you. It takes a couple of seconds to do, and it costs you nothing.

    You'll find that the Thanks button appears when you hover the mouse pointer near the Quote Multi-quote buttons.

    This is the proper way to show your thanks and respect for the time, experience and help someone gave to you when you were in need. Be a good forum user and say thank you in the proper way. It will help to ensure that you continue to receive advice. (y)
     
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  10. opps

    opps

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    Sir, If I had a cap, I would doff it to you. If I were able to carry babies, I would have yours.

    To date that is the most comprehensive answer I recall ever receiving on this wonderful site. Not only do I appreciate your advice, I am mindful of the time that you have taken to compose the reply.

    I have just finished work and am having a quick pint. When I get home I will try to absorb your sound advice and act accordingly.

    You are correct that it was a cheapo (contract) aerial.

    Many thanks.
     
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  11. opps

    opps

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    BTW I am about 7 miles north west of Little Ealing but 500 metres from the outer perimeter of RAF Northolt. Do RAF airports use coms that will/might potentially interfere with Freeview?
     
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  12. Lucid

    Lucid

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    That's one I don't know the answer to. It's the first time anyone has asked me that question. There are Ham radio users here such as @ericmark who might have a better handle on sources of interference from airports. (y)
     
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  13. ericmark

    ericmark

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    When I worked at the RAF airport at mount pleasant Falkland Islands in the late 80's we had not problem with TV or Ham radio, I am sure they had loads of electronic stuff there, but any transmission can be used as a direction finder so they tend to be rather careful about any signals, with the mountain sites with the large radars we did get regular white noise each time the radar rotated, but you knew what that was as it was so regular, but did not get it at the airport.

    Producing any radio signals today would be the same as lighting fires in the last war, so can't see any military establishment causing problems except for radar, the Russians used a low frequency ground hugging radar which we all called the wood pecker, we thought it was an attempt to jam radio waves at the time, but seems it was just the type of radar used.

    Nothing to do with TV, but you may find it amusing, during the war in early days of radar they found loads of dead seagulls around the radar at Orfordness, they sent them for analysis and found they were landing ready cooked, so they tried to make a death ray, the death ray was a failure, but the research resulted in the microwave oven.
     
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