TV Booster, picture fuzzy.

[Dark Angel]

I've just installed some new external TV ariels, which I intended to link to a TV distribution booster/amplifier in the loft.

I checked the picture on them after installation via a TV in the upstairs bedroom, to make sure they were correctly aligned and the picture was crystal clear on both of them (one is for picking up Yorkshire TV, the other for Central).

However, now I've connected them to the booster the picture is somewhat fuzzy, particularly the alternate channels being received from the Central TV aerial. It's not so bad if I connect it to the booster's "full" output, but the individual split outputs leave much to be desired.

Whilst the cable run to the TV's is a bit longer now that I'm running them through the booster, I thought the increased signal gain it provided meant this wouldn't be a problem.

The booster itself is a Videolab 6-way amplifier, like the one at

ideas for the poor performance? Should I get rid of this one and plump for a Maxview or Labgear one instead?

[BigWallop]

Make sure that none of the connections has a short across the screen and inner core of any of the coaxial lines. Even just one strand of the screen can cause major problems on all outputs from and to the amplifier.

Any coils or tight bends in the coaxial can also cause deterioration in the signal levels on the lines, so check that the coaxial are loose and not crimped to tightly or bent to sharply around corners etc.

The length of coaxial cable to the antenna's can also cause problems, and they need to be a multiple of three feet in their length to each input. So you need to check these lengths to make sure that one antenna isn't fighting with the other, and that both have the same length coaxial cable to them to even out the signals received from them to the booster. The length should be a multiple of three feet, so the length needs to be either 3, 6, 9, 12 etc etc.. feet for each antenna, to make the signals equal.

[fred]

In article , BigWallop writes

What's the basis for this? The frequency span from channel 21-86 or even just a few channels is enough to make a nonsense of any attempt at tuned lengths. Proper termination should make such a practice unnecessary.

[Bob Eager]

Yes, I make mine a multiple of a millimetre. Works well too.

[Dark Angel]

So you reckon the problem could be where the cables are joined/terminated?

[Frank Erskine]

What a load of twaddle! If the aerial, the cable and the amplifier have the same characteristic impedance (75 ohms), the cable lengths make absolutely no difference (apart from cable loss) - that's precisely the idea of impedance matching.

The OP should try to have some idea of the signal strengths from each aerial on each channel. It could be that one or more signal is overloading the distribution amplifier.

The ideal signal strength presented to a TV set, or a distribution amplifier (as opposed to a booster amplifier) is between 60 and 70 dBuV (1 - 3 mV). Overloading the amplifier is likely to cause severe distortion of all channels.

[fred]

In article , Dark Angel writes

Sorry my post wasn't very helpful to you in the big picture, I just didn't want you going down any time consuming potential dead ends. It's confusing that you have a good picture at the input to the amp (true?) and ok at the full o/p but poor at distributed o/ps. I notice your full o/p is +21dB which is a huge gain (a bit OTT) but that the distributed o/ps are only +5dB and are poor. This would suggest that the i/p to your amp was low (check with the telly again with amp disconnected) or that you hadn't plugged the amp in. The final suggestion is that the gain on your amp may be too great and that the amp is overloaded.

My step by step trace would be:

Check for good signal at amp input. Re-check results at amp output and if bad use a lossless splitter to check if aerial will split ok w/o amp consider trying an attenuator at the amp input

In any case, it would be good practice to google groups for posts by Bill Wright who is an aerial installer who is regularly published in the trade press and is somewhat of an authority, he's often found at uk.tech.digital-tv and his site at

a mine of information.

[Dave Plowman (News)]

How have you combined them?

[Dark Angel]

I used one of those Y-shaped combiners/splitters and joined them in the loft.

[charlieB]

Both aerials are receiving signals from both transmitters. When you tune in a set to Central you are seeing the picture from the Central Antenna combined with the picture received by the Yorkshire antenna. Since the Yorkshire antenna isn't going to pick up Central "as well" its going to give you some fuzzyness.

Also, since your antennas are is slightly different positions, the phase of the signal received at each antenna is different. You would be able to tune this out for one channel by varying the lengths of coax to change the transmission times down the cables - but you're unlikely to have the required kit in your toolbox.

Charlie

Dark Angel wrote:

[tony sayer]

In article , charlieB writes

Correct..

Sorry but thats not the way to go about that. Yes you can vary the time with the length of the cable, velocity factor etc, but you'll be seeing all manner of reflections off buildings and other terrain obstacles and obstructions etc which will give rise to reflections and thus smear the signal as it were. If the channels are in different groups then a frequency dividing filter is the way to do that.

I suggest that you have a post over on uk.tech.digital-tv as theres more to this than meets the eye, if you want to do it properly;)

I suspect that the amp may well be faulty or perhaps not a very good design...

[Dave Liquorice]

Thats a lot politer than I could come up with...

On the OPs problem it has been touched on in a few recent posts. It's probably down to signal levels of *all* stations from both aerials.

Both aerials will be picking up the signals from all transmitters and due to them not occupying the same physical bit of space will be in different phase relationship if you simply join these two aerial together you'll get differing amounts of cancellation on differing frequencies (channels).

The OP needs to look at the RF channels in use from wanted transmitters and pick a suitable combiner. This will filter the "unwanted" signals much reducing any cancellation problems.

Why it works on the high output might simply because there is so much signal present the TV can sort it all out via it's AGC.

[Andy Luckman (AJL Electronics)]

There's your answer. You can't just combine aerial feeds willy nilly. In particular, your 75p resistive combiner needs chucking out and a proper filtered combiner suitable for the groups in use fitted. If the groups overlap, then you can't combine at all.

[fred]

In article , fred writes

Sorry again, in a Sunday evening haze I missed the aerials (plural) bit, my answer is crap but valid for a single aerial, those who have advised that you cannot do this (without expensive filters or switching) are correct but the bit below is valid, Bill regularly advises on combining feeds but most say thanks and walk away due to cost, have a google groups.

[Dave Plowman (News)]

Ah. What you need is a frequency selective combiner. So that it rejects unwanted signals that will come in on the 'back' of each aerial. You'll need a specialist for these, though. Someone like Aerial Technique of Bournemouth.

[Andy Luckman (AJL Electronics)]

Ahem!

[BigWallop]

LOL Maybe you should make the links BOLD type Andy. LOL :-)

[Mark Carver]

Which as several others have pointed out is not the right component.

You need a frequency selective combiner. They come with various crossover channels, and it's important to know which transmitters you are using.

Yorkshire TV Emley or Belmont (or a relay) ? Central TV Waltham or Sutton Coldfield (or a relay) ?

See:-

's the sort of thing you need

[Dave Plowman (News)]

Ah - sorry Andy. ;-)

[Dark Angel]

Yorkshire - Belmont Central - Waltham