Question Coax cable only carries some channels. Why?

[delta mike]

I am in Northern Ireland and for years I have been getting Freeview (channels 52-58) and Saorview (Irish Freeview channels 42-43). They come via separate aerials, a diplexer and distribution amp to three TVs. Recently the signal became unreliable and a contractor installed a new broadband diplexer to combine the two separate aerial feeds. One of the TVs worked fine but the other two stopped receiving Freeview and only received Saorview. The contractor checked the outputs from the distribution amp and they were fine. He then changed the Coax cable to one of the two TVs and it started receiving both Freeview and Saorview. Does anyone know why cable would carry one set of channels and not others?

 

[TJT1]

If water had got into the cable that it could cause this. The FV frequencies are quite a bit higher than the SV frequencies that you receive, and it's the higher frequencies that normally 'go down' first. It's a sort of frequency dependent 'shorting out' of the signals.
Shame he didn't check the cables before changing the diplexer (unless it was badly corroded). What about the other TV that had lost FV?

 

[delta mike]

That makes sense as the cable from the distribution amp to the tv ran outdoors and then back in to another room. The cable to the other tv that has lost FV is internal and not exposed to sun or weather but I'll try changing it and see what happens.

 

[TJT1]

Have you tried putting a TV that can 'get' FV on the end of it? But it does sound like there is something wrong somewhere. Check the plugs for stray whiskers first. Presumably the dist amp is in the loft and nice and dry?
Cable outside should last many years unless it is damaged, or water has got into it. Air spaced coax is a bugger , as if water gets into the top end, it runs down the air spacing and can destroy the TV's tuner if it's directly plugged in.

 

[delta mike]

All the TVs have been "getting" FV for years. Then the reception got worse. FV only disappeared after the aerial contractor visited. Maybe there's a lesson?

 

[TJT1]

Yeah. Get the 'aerial contractor' back in to fix it. He broke it.

 

[grahamlthompson]

I reckon the aerial guy fitted a splitter backwards as a combiner that didn't have the required frequency split to avoid interference from one aerial picking up the other transmitter. The proper way is to fit a diplexer with a frequency split between 43 and 52. eg Channel 51 diplexer as here.

Television Aerial Boosters / Amplifiers, Splitters, Diplexers & Triplexers

 

[TJT1]

Interesting theory GLT.

 

[grahamlthompson]

Interesting theory GLT.

Easily tested, disconnect one aerial at a time and check for full channel reception from connected aerial.

 

[Rodders53]

Did this guy arrive on a horse, or have a bucket and wash leathers on his ladder?

Filtered combiners should be used when two aerials are in use. The spectrum planners allow for this in selecting the transmit frequencies. Wideband combiners can produce unwanted signal cancellations and reinforcements due to the signals from the 'wrong aerial' interfering with the 'correct aerial'. Although digital is more forgiving of such things c.f. analogue it is still not best practice. (They also attenuate - lose - more signal than a filtered combiner would!)

However, precise combiner needed will depend on the two transmitters and all the frequencies on each. But the one suggested above may well be correct.

Any good installer would use a meter to check levels at each outlet and at aerial, combiner out, etc.,. to determine where issues lie before changing parts. Then recheck that levels at the TVs are within the advised limits (45 to 70 dB ref 1μV) on all the wanted frequencies.

Location also comes into it with signal levels from the two transmitters: if the Freeview one is distant and weaker, c.f. the Saorview one closer and more powerful, it makes the weak signal more likely to suffer from problems. Especially as a wideband splitter used as combiner loses 4dB on both aerials.

Weather (rain and wind) and reception through trees / distracted off a tree-lined ridge rather than by direct line of sight can give considerable variation of signal levels as well. Better aerials and/or masthead amplification might be necessary.

Either call back or get a better installer in to rectify? Mind, as 90% of problems are down to poor connections and/or damaged cables it's worth going over all of them and remaking any suspect ones that can be safely accessed. {and by-passing any amplifiers to one room at a time to check and measure signal levels/quality: most, if not all, TVs have such meters built in.}