"tuning" in a Freeview digital TV set-top box

You know that business of old, with indoor rabbit ear antenna. Ask someone to hold the aerial while you monitor the picture and the best position is with your assistant holding the aerial at an impossible arm's length , in middle space.

The same with one of these digital boxes. Good external Yaggi and downlead and the signal bar is 80 to 100 percent on all stationss except two. Moving the plastic cased box around there is a point , in mid space, where the "no signal" box disappears and the signal bar goes to 20 to 30 percent on those channels. Have to be barefoot to work, also wearing a glove nullifies the effect. Any advice on how to progress, grounded aluminium around the box? local ghosting effect ? if so why only 2 stations affected out of the 10 or more on that one multiplexed UHF channel

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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N Cook
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But is it a proper wideband digital antenna ? Some of the multiplexes that 'belong' together, are at opposite ends of the band, so a wideband antenna is essential. A 'channelised' type, when added to cable slope and other assorted vagaries of a 'standard' analogue setup, can result in such oddities as individual channel suckouts, as you are describing.

Where's Bill Wright ? We need him in on this one. He'd put you straight.

Arfa

Reply to
Arfa Daily

If there's that much pickup on the box or downlead, there's something wrong. Short in the co-ax or connector?

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*I never drink anything stronger than gin before breakfast *

    Dave Plowman        dave@davenoise.co.uk           London SW
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Dave Plowman (News)

Looking a bit deeper the main problem stations are on UHF Channel 33, Mux D the highest frequency of the local (Rowridge) group. Unless anyone knows how to push the problem into the totally junk and so deleted shopping channels, looks like a new yagi and downlead. A near neighbour has no problem with any freeview channel with a relatively short Yaggi in his loft space. The immediate neighbour, so yaggi only a couple of feet away and parallel, uses a line powered headend amp which perhaps could be radiating/interfering/coupling. Perhaps a different mounting position should be considered also.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook

If you can ascertain the age of the aerial in certain areas, as has been said, it may not be suitable for all the FreeView muxes - in the same way as some weren't suitable for Ch5. It all depends on the frequencies used locally. In the London area the muxes are within the existing grouping - you'd need to look it up for Rowridge. There are probably some yagis still in use from the early BBC2 days which were peaked for just the one frequency.

Could be a wideband type.

A few feet can make a difference. But while you're up there you might as well change it. If you're in a reasonable field strength area go for a log beam. Nice smooth response and good DP. And use a decent satellite cable rather than plain UHF. I like these people for such things - and their strap chimney mounting is a delight to use.

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*Gravity is a myth, the earth sucks *

    Dave Plowman        dave@davenoise.co.uk           London SW
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Dave Plowman (News)

I agree on all points. A decent log periodic is the way to go. Also remember that any amp, if required, must also be 'digital specified' which in practice means wideband and masthead. CT100 is a good choice for the coax.

A different position for the aerial can make all the difference. Many years ago, in the early UHF colour days, I was installing a TV in a village that was a very difficult reception area, being in a complete hollow in the countryside. We arranged for the aerial man to be there at the same time as us. I clearly remember him walking backwards and forwards along the house's ridge line, with an aerial 6 foot long on a 10 foot pole in his arms looking for somewhere to get a signal. Success was the difference between a chimney stack on one end of the house, and another one thirty feet away at the other end ...

Arfa

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Arfa Daily

Well, of course Charles, we all know in the UK that digital doesn't suffer from multipath. In fact it's immune from everything ! The government says so ... d;~}

Arfa

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Arfa Daily

That's pretty much the feeling here too, Charles. When they originally started foisting the 'digital TV revolution' on us, they ran TV adverts with a well-known fat comedian and his knitted monkey puppet, to tell everyone how they were going to be able to just buy a set-top box or digital TV set, and jam it on the end of their existing UHF antenna. Well, of course, that worked well, as you can imagine ...

Since then, the digital modulation format has changed in an effort to make the signals more robust. Needless to say, not all existing STBs could cope with the change in the number of carriers. It's now pretty much accepted that in many areas of the country, in order to stand a cat's chance in hell of receiving these transmissions, which are often contained in multiplexes spread from one end of the band at 470 MHz to the other at around 820 MHz, a serious antenna upgrade to a wideband ugly toast rack and new double screened coax at a total cost of around £150 ( $280) is required. If you do have trouble with multipath, which as you say, is not uncommon, then you can be doubling those amounts.

Where I live, we have a clear line of site to the original analogue transmitter about 20 miles away. We could receive perfect pictures on a piece of wet string. Now, it needs a great big metal thing on the roof. Many are either double stacked yagis, or the 'X' element "antighost" types. Now the latest thing is the HD debate. Originally, the government and their mouthpiece Ofcom (the supposedly politically independent broadcast regulating body in the UK) said that when the analogue transmissions were closed out, some of the additional bandspace won, would be used to allow DTTV to carry HD broadcasts. Now, recently, they seem to have reneged on this, and are now saying that no more space will be allocated, and a different compression system will have to be employed, to win the necessary space. This has gone down like a lead balloon with the manufacturers, who have been selling "HD Ready" TV sets, based on the existing compression scheme, for some time.

This makes it all quite clear that the whole thing is fundamentally a government cash-raising exercise when they sell off the spectrum space for billions to the mobile phone operators, glued on the back of a bunch of flim-flam designed to make the public believe that they are somehow getting a better deal than they had before.

Now that we have the same free digital transmissions available by satellite - the new Freesat service - I can't see any point in the terrestrial service, with all its bandwith and performance constraints, continuing. But then, perhaps that's the intention ? Maybe I'm just being cynical, but that would leave a 400 MHz swathe of bandspace, completely available for sale ...

Arfa

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Arfa Daily

That really doesn't make sense to me.

However, the ERP of the digital services will be upped when the analogue ones are switched off.

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*Just remember...if the world didn\'t suck, we\'d all fall off*

    Dave Plowman        dave@davenoise.co.uk           London SW
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Dave Plowman (News)

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Could a multipath problem "knotch" out one specific frequency for all times of day and weather conditions. I think of it more with wavering effects that come and go with moving metalwork or weather/time variation.

Reply to
N Cook

Nor me really. I haven't bothered with equipping the house with DTTV, as I can get all the services I need via Sky, so I haven't looked into why these monster antennas seem to be needed. Of course, there's no reason why an antighost antenna shouldn't work just as well without any ghosting problem to overcome, so perhaps riggers are just carrying one type of 'catch-all' antenna on their vans ? I know that in the nearby town, there have been many issues with receiving reliable DTTV signals, so that might go along with my suspicion.

I suppose another possibility could be to do with the reduced ERPs of the digital transmitters, and also that not all of the DTTV transmitters are co-located with their analogue counterparts. Maybe where I am, that is the case, and in the direction that the DTTV transmitter lies, there is in fact some obstruction or adverse lie of the land. I'll have to have a bit closer look to see if the toast racks are pointing in the same direction as the discrete little 10 ele yagis on the houses that are still thusways equipped.

Arfa

Reply to
Arfa Daily

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Hah, reminds me of the neighbor behind my summer cabin on the fringes of Flagstaff AZ. This area is served by 8 or 9 repeater stations carrying signals from the Phoenix channels. Repeater stations in the colonies, incidentally, are not required at this time to switch over to digital - most of the repeaters that we can receive at our cabin are staying analog for the near future.

Anyway, the neighbor is the chief engineer for one of the Phoenix stations, stands to reason he has the best TV reception in the area. When I asked him what he did to get such good reception, he mentioned the type of antenna, the type of coax, and the fact that he borrowed a portable spectrum analyzer from work and walked his roof until he found the best possible signal.

Jerry

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Jerry

Hah, reminds me of the neighbor behind my summer cabin on the fringes of Flagstaff AZ. This area is served by 8 or 9 repeater stations carrying signals from the Phoenix channels. Repeater stations in the colonies, incidentally, are not required at this time to switch over to digital - most of the repeaters that we can receive at our cabin are staying analog for the near future.

Anyway, the neighbor is the chief engineer for one of the Phoenix stations, stands to reason he has the best TV reception in the area. When I asked him what he did to get such good reception, he mentioned the type of antenna, the type of coax, and the fact that he borrowed a portable spectrum analyzer from work and walked his roof until he found the best possible signal.

Jerry

There ya go then ! Identical story from 4000 miles away !

Arfa

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Arfa Daily

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Freeview crap. cuhulin

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cuhulin

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