A very strange freeview problem

[ellal]

We've had a windy 24 hours here, but nothing drastic...

However, we have different problems on the 3 tv's we have running freeview..

All run off the same aerial via a distribution amplifier.

We now have only BBC channels (perfect signal) on 1 tv, nothing on another, and broken signal on the last one...

Visually the aerial seems to be ok, but the neighbours have no problems and are pointed at the same transmitter.

I've tried retuning each one, but to no avail...

I've also changed channels (feeds) on the amplifier, but the result is the same

Has anyone a clue as to what is happening, or what the next move is?

 

[rjm2k]

I read about an imminent move, maybe the increased digital only signal is too much in addition to your amp, can you reduce its output?

 

[ellal]

I've tried bypassing it, but it makes no dififference.

 

[Sam Gangee]

See http://www.satcure.co.uk/tech/freeview_problems.htm

 

[ellal]

That site is down...

 

[Sam Gangee]

Sorry, yes, it went just as I posted the link!
Probably just a temporary glitch. Try again in an hour.

 

[Sam Gangee]

Ha, it's back on-line again.

 

[ellal]

I've done everything that I can on that list...

I've rescanned, and these are the channels I now get:
BBC1
BBC2
ITV1
Ch4
Ch5
ITV2
BBC3
BBC4
Ch4+1
More4
E4
ITV1+1
CBBC
CBeebies
BBC News
BBCParliament

plus 3 x '200 range' channels
2x '300 range' channels
R1
R1X
R2
R3
R4
R5L
R5SX
R6
R4 ex
BBC Asian Net
BBC World

Since the only thing I can't verify is the aerial on the roof (although it looks to be pointing in the same direction), would a small movement be sufficient to lose all the other channels?

 

[Sam Gangee]

Water in the aerial connection would certainly do it. Can you snip the cable as high as possible and connect a temporary aerial in a bedroom or loft, as a test?

 

[daxi]

Digital communication is not as easy to diagnose problems on as with analogue & without a spectrum analyser it is virtually impossible for many issues. As with digital the signal is either strong enough to be read or there is no picture as a result.
With analogue, weak signals & many other issues could be diagnosed by simply look at the screen, thus without that spectrum analyser. With digital many many problems all result in a loss of picture.
One potential issue could be that the antenna has been moved off the bearing needed for the transmitter to be received well & this has resulted in a weaker signal than normal. Then the reason that the TV's are receiving different channels is down to the strength of that signal at the TV & the length of the Coax & it's quality will dictate that. Coax can loose large amounts of signal, which if the signal is strong, is not always an issue, but if it's a weak signal, then it could be that the extra degrading caused by the coax is producing one TV on the lowest loss run to get the best signal.

 

[ellal]

Is there any point trying to connect an aerial in the loft when we've never been able to get a signal that way?

The coax runs are approximately the same (within a few metres).

Our first aerial did indeed get water corrosion (after a couple of years), but this one was supposed to be as protected as possible and has only been up about 18 months..

I'm loathe to get someone out again, but my spiderman days are long gone - and I don't intend doing a Rod Hull...lol

 

[Sam Gangee]

If the cable is more than 5 years old it needs replacing anyway. I would connect a new length of cable to A Freeview TV and point an aerial out of the window in the direction of the transmitter.

Actually, let's start from the beginning and determine your signal strength. What's your postcode? Which transmitter is your aerial pointing at?

 

[ellal]

Done the signal strength check already...

Field dBµV/m: 49
Suggested Antenna: Amplified extra hi-gain

Are you saying that coax should be changed every 5 years, internal and external?

 

[Sam Gangee]

External coax deteriorates if not painted or run in conduit to protect it from the effects of sun (UV) and rain. Internal coax gets destroyed if water gets inside it. I've seen this many times with air-spaced dielectric cables. It's much less common with foam dielectric but water can still "wick" down the braid very quickly.

Rain water gets inside if:
1. The aerial connection box loses its cap.
2. The coax hasn't been properly secured to tiles/bricks, so it flaps about and wears a hole in its casing.
3. The plastic breaks down with the effect of UV and becomes porous with microfractures.

Cable that is painted well or run inside conduit should last for decades if it's properly installed - but it seldom is. A rigger wants to be in and out as quickly as possible. It's not in his interests to treat aerial connections with silicone grease, or to secure the cap with UV resistant cable ties, or to use one more tile clip than is absolutely necessary. As long as it lasts 12 months, he can blame it on the weather. This ignores that fact that the job should be done well enough to withstand bad weather for years.

49 dBuV/m is a substantial signal. I installed a V10-040 log-p on my neighbour's house where the prediction was only 35dBuV/m and it has been trouble-free. So lack of available signal certainly isn't your problem and the "amplified high gain" suggestion is plain stupid. (I'd start thinking about "high gain" if the signal were 30dBuV/m or lower.) I'd use a masthead amplifier if the signal had to be split to several rooms but not for a single 10m run of cable. And I definitely wouldn't use an aerial with a built-in amplifier. They are usually cheap and noisy.

 

[ellal]

I don't know if this helps, but I found a signal strenth monitor in the humax box that feeds one of the tv's.

It doesn't say if it is displaying in dBµV/m, but by checking the channels that I do get, the maximum strength is 35 with lows of 30, although it says 100% quality.

This is where I find it difficult to get my head around the problem...

Do different groups of channels get broadcast at different strengths, so that although the aerial appears to pointing at the transmitter something that has been mentioned is in effect blocking those missing channels?

Or that if the aerial is slightly off direction only the stronger channels are viewable?

I find the notion that a channel can be lost if the signal is too strong difficult to understand too!