Wanted to move the BT Hub so bought a 10M ADSL cable but noticed a significant loss in speed when connected via the cable.

DL speed went from around 20mbps down to around 14mbps ..

Is this normal with a longer cable or possibly due to new cable being of poor quality

10M is a lot of cable. You've just made it longer from the exchange/cabinet.

In the grand scheme of things, 10 metres extra shouldn't make that much of a difference. Probably poor quality cable.

The signal has to go through hundreds or thousands of metres to get to your house, another 10 metres shouldn't result in that much of a speed drop, unless it's really bad cable

true but you're not extending the phone line, you are extending the DSL lead, you can argue whether that should make any difference to extending the phone socket. I would argue it does. However I do think its down to the poor quality of the lead so have such a significant drop in speed.

The DSL signal still has to travel through tons of ancient, rotting BT copper to get to your house. 10m of quality cable shouldn't really do anything to the speeds.

Poor cable would have less of an effect on the phone side.

yes but the DSL lead is more likely to cause an issue than the phone line itself. To be clear I am saying adding a 20m phone extension would be far less detrimental than a 20m DSL(RJ11) cable.

however yes if it is of quality yes ok it will be fine.

It won't make any difference at all provided you're using good twisted-pair cable in both cases. The DSL line is the phone line, the fact that it;s after the filter changes nothing.

This does not make any sense. And in fact a phone line extension is likely to be less good than something sold specifically for DSL!

DSL is not a cable. DSL uses cables.
DSL is a technology. But the quality of the cable used is important, poor quality will limit DSL.
---------
10m extension is fine. It might bring the download speed down 5% - 10%

Of course if adding the extension has brought down the speed significantly then there is a problem.

Maybe the cable, maybe something else.

Yes my terminology is wrong, what I am referring to is more easily referred and understood than saying RJ11 to RJ11, I say DSL instead of ADSL cable since there is VDSL now. In any case I do this as part of my job and way more often its long DSL cables that seem to cause errors and drastic speed loss rather than telephone extensions. While not proven just putting it out there.

Yes of course in all circumstances using good quality cable it won't make a difference but we're in the real world.

This really depends on the cable.

Most DSL cables aren't actually twisted. They're just two bits of what MIGHT be copper, but a lot of the time are simply not.

There can also be problems with the shielding of the cable which causes noise problems.

If you use genuine CW1308 cable with properly terminated jacks, there should be no problem in extending the distance from your phone socket to 10m.

My guess is that the cable is the cause of your loss.

I used a 4 metre length of aluminium rj11 cable for years. I replaced it with a 30 cm copper cable and my broadband speed instantly shot up.