I don't see the relevance to modem cable

In reply to a post by eckiedoo:

Just noticed that the Thick impedance is given as "50 ohm", so the same for the Thin Ethernet, as it was possible to connect them end to end , using a special but simple adaptor.

I did this on a few occasions for demonstrations to customers.

10 BASE T was 50 Ohms and most used a BNC connector connected to a BNC T-Piece and each end having a 50 Ohms BNC Terminator.

But yeah I recall seeing each segment being a set fixed length, we had to coil up the cables so that they didn't get tangled up behind the desk.

UHF TV and Sat Cable are 75 Ohms.

Paul

The OP had mentioned the length of cable.

When dealing with RF signals, such as the BB ADSL and VDSL signals, these are in the RF spectrum 2 MHz to 17 MHz, so reflections, standing waves etc, can be coming in to play.

I have particularly encountered this with Ethernet cabling, 10 MHz, so used that to illustrate what can happen, especially with the spread of RF frequencies used by xDSL, and the general reduction of wavelengths in cables compared to "free space".

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You may have noticed the posting by threeleg, (learned) Wed 16-Aug-17 18:21:15, regarding incidental cleaning of "dirt/corrosion" may be having an effect.

That repeated insertion/withdrawal may also be clearing dendrites that form in gold, plated version particularly, as encountered in plugs and sockets.

But no obvious difference to the casual user - "The fault just disappeared and all I did was unplug and plug back in".

What is the wavelength of vdsl rf carried by a modem cable?

Taking 10 MHz in "free space", it is 30 Metres.

In a cable of any sort, it will be distinctly shorter than that, depending on the cable characteristic, let's say 20 Metres.

(It may be even less - in my training, it was mentioned that the transmission speed in cables was generally around 1/3rd of the free space figure, which would give a result of 10 Metres rather than 20 M)

You then have to particularly consider the "Quarter-Wavelength" and multiples, so now down to 5 Metres @ 10 MHz.

Given that Ethernet of the "10 Base x" is 10 MHz; xDSL, and all the channels involved, imply a "Broadband" mixture of wavelengths, ranging from in free space, about 60 Metres @ 5 MHz to 20 Metres @ 15 MHz.

Hence the relatively higher Attenuation figures seen for the generally much shorter FTTC via PCP to Home, seen when changing from ADSL to VDS.

ADSL - 24 db for 1,286 Metres
VDSL - 13 db for 300 Metres on a 40/10 contract. (Expect higher on 80/20 Contract)

Leading on to the recommendation for some that VDSL may not lead to a BB Speed increase; and may actually cause a decrease.

So the length of a modem cable is irrelevant?

In reply to a post by obroad:

Given the distance the signal has to travel I'm quite surprised that the modem cable makes any difference at all,

indeed, however that last 2 metres is in the noisiest (electrically) enviornments the line goes through. typically having computers screens printers and various psu devices nearby.

It clearly is Relevant, as it adds in Length; and by implication, Attenuation, Noise etc.

Only experimentation as the OP has been doing, might establish "The Perfect Modem Cable" for the specific Installation.

BT cable with two RJ11 plugs, sync 65m which is way down.

Your first "guess" was about right for coax. Speed of propagation in coax is about 2/3 that of free space and does vary. Using 0.6, 0.65, 0.66, or 0.7 is normally reasonable enough.