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Topic Title: Jointing Cat5e
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Created On: 11 November 2017 10:14 PM
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 12 November 2017 06:47 PM
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MHRestorations

Posts: 40
Joined: 22 October 2017

BNC are available in 75 ohm impedence, but to be honest, these days I'd use the F connectors for satellite, MUCH easier to fit, rated at 75 ohm, and mechanically secure. They're fine on broadcast tv, cable tv, satellite, and even baseband (cctv).
 12 November 2017 07:08 PM
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Fm

Posts: 1733
Joined: 24 August 2011

My earlier post was a bit quick
Our IT dept recently used one of these, just not from screwfix
I have a gigabitconnection to my office pc and it works fine, it's short term until I knock the building down in a few years so was the most cost effective solution as below my office is a public facing environment

http://www.screwfix.com/p/cat-5e-coupler-rj45/10436
 13 November 2017 07:59 AM
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gkenyon

Posts: 4954
Joined: 06 May 2002

Box with cordgrip for coax and Cat5 ... Yes ... and NO - the problem is, that a screw-down cord grip, if you squash the cable, its performance is ruined.

Cat5 junction boxes will have serrated-edge type cord retainer at the cutout which lightly nicks the outer sheath without squashing the cable.

Solder will change the characteristic impedance of the cable, and cause signal reflections. Unless you are lucky, it will likely lower the data rate to well below the rated 100 Mb/s (if you think that's not a problem, consider the fact that broadband speeds to homes are now getting above 50 Mb/s). The characteristics of a soldered joint change over time, so the problem might not manifest itself immediately ... this is far more pronounced with traditional lead-solder, less of a problem (we are told) with modern silver solder provided the silver-soldered joint is properly made.

Jelly crimps and a suitable box, or a dedicated Cat5 junction box, probably the best.



Similarly, with coax it's important to use a coax coupler with the correct characteristic impedance, or the signal attenuation might be significant.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 13 November 2017 08:02 AM
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gkenyon

Posts: 4954
Joined: 06 May 2002

Originally posted by: Nospark

This is the thing, what impact in practice would well soldered joins and a small section of a say a centimetre were they aren't twisted and shielded have..

What is a proper connector or back to back barrel?
Coax ... could well cause significant attenuation of the signal. If it's on the output of an amplifier, could damage the amplifier if there were significant reflections (although unlikely).

With Cat5 or Cat6, may well completely slug the data-rates to well below the performance (i.e. well below 100Mb/s or 1Gb/s respectively).

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 13 November 2017 11:45 AM
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davezawadi

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Graham

What you have said is grossly misleading I'm afraid. At 100Mb/s on CAT5 the effect of a cord grip or discontinuity of impedance of 1 or 2 cm is negligible, ditto the effect on coax, although the signal amplitude may be reduced by a tiny amount. The propagation velocity is about 5 ns/m and the wavelength of 100MHz about 3 metres, so a discontinuity of a cm or two is nothing and very little is disturbed. Impedance discontinuities in transmission line are unable to make much reflection unless they are a significant proportion of a wavelength, as usual frequency, wavelength, and size of error are directly related. This is why these systems can use very poor types of connectors and still work fine! CAT 5 installations are generally very robust whatever the installer has manages to do provided the electrical continuity is ok.

Coax at microwave frequencies (above 10GHz) is a different matter, as is impedance matching inside the electronics, but this low frequency stuff is trivial. You may remember that the original multi-drop coax Ethernat was problematic, but this was due to loadings and lengths of the "stubs" connecting to various equipment. We have come a long way since the original specifications were drawn up!

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 13 November 2017 01:55 PM
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mapj1

Posts: 9568
Joined: 22 July 2004

And Ethernet bit rates and frequencies do not go up in step - the wave forms on gigabit ethernet are not 10 times faster than those on 100megabit.
So the disturbance you can have in a cable in terms of reflection, ringing and delay skew is not perhaps as demanding as the headline speed figures suggest.
Rather the system has more states - instead of the voltage being either up or down to represent a single one or zero, like the Grand Old Duke of York, there are intermediate states, so the slew rate of the waveform is slow, but each change of level represents several bit states per sample
so rather than a simple 'eye' as per things like 10 base T

(This is not actually from 10BaseT but its a nice clear picture of a 2 level system)
we have a multi level one, and with 10 gig ethernet it gets worse. The price for this trick is that the amount of extra noise voltage to cause a state to be misread is less, so the signal to noise requirement is tighter, and so the limit on cross talk between adjacent cables is more stringent.
(10base T will work over short lengths of phone line. 100 base T won't.)
Example of a multi level scheme.


edit
Actually for 100base Tx which is the common one in twisted pair, it is a 3 level scheme, in which the voltage advances to the next of 4 phase states (+1,0,-1,0) if the current bit is a 1, or stays put if it is a zero.
To avoid sending a long burst of zeros, there is a packing scheme that makes 4 bits into 5. The highest frequency on the wire results when there is a string of 1s to send , and is about 30MHz (125MHz/4), while gigabit Ethernet uses more pairs in the cable and even more levels, to keep the fastest waveform on any one wire pair below about 60MHz.

-------------------------
regards Mike


Edited: 14 November 2017 at 11:08 PM by mapj1
 14 November 2017 08:49 PM
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Nospark

Posts: 193
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Well, as it turned out, the cables came from unexpected directions so I didn't need to extend any to reach the new positions.
But all good stuff to know never the less.
 14 November 2017 11:36 PM
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sparkingchip

Posts: 10104
Joined: 18 January 2003

When I tried to sort a VGA cable that had been butchered, l called in the a car spares shop looking for Scotchlock connectors.

The guy in the shop lecturered me on not trying to do a "proper job" and produced some crimp connectors, telling me I should slip on over the conductor then twist and solder the conductor before slipping the crimp back over the twisted and soldered connection; and crimping it as well.

Whilst he was putting me right I held of these crimps up to the light, it was blind.

Mental note to self, ignore the guy at the car spares shop when he is dishing out advice on wiring connections.

Andy B.
 15 November 2017 01:02 AM
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mapj1

Posts: 9568
Joined: 22 July 2004

cars and electrics - a volatile mix. Actually I'd be more worried about joining a VGA cable than Ethernet , as the speeds are higher and the eye is a very good detector of certain types of termination artefact such as multiple reflections.

-------------------------
regards Mike
 15 November 2017 12:20 PM
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tjs2

Posts: 50
Joined: 05 February 2005

For coax, any connector without too much loss at the frequency you need (up to 900 MHz for satellite and 600 MHz terrestrial I think?), with the correct impedance (75 ohm) should do. Bear in mind that a lot of BNC connectors are 50 ohm.

A couple of f-type crimp connectors and the coupler pictured above might be good, but pricey if you have to buy the crimper specially...

FWIW, Belling-Lee connectors aren't great from an RF performance point of view, so I don't use them where I have a choice, but they're generally more reliable with the centre pin soldered. especially if the cable is subject to a lot of movement.

Edited: 15 November 2017 at 12:28 PM by tjs2
 17 November 2017 02:34 PM
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gkenyon

Posts: 4954
Joined: 06 May 2002

Originally posted by: davezawadi

Graham



What you have said is grossly misleading I'm afraid. At 100Mb/s on CAT5 the effect of a cord grip or discontinuity of impedance of 1 or 2 cm is negligible, ditto the effect on coax, although the signal amplitude may be reduced by a tiny amount. The propagation velocity is about 5 ns/m and the wavelength of 100MHz about 3 metres, so a discontinuity of a cm or two is nothing and very little is disturbed. Impedance discontinuities in transmission line are unable to make much reflection unless they are a significant proportion of a wavelength, as usual frequency, wavelength, and size of error are directly related. This is why these systems can use very poor types of connectors and still work fine! CAT 5 installations are generally very robust whatever the installer has manages to do provided the electrical continuity is ok.


I'm not convinced. The data rate over twisted pair or coax doesn't behave like a simple transmission line at a single baseband frequency with a bit of bandwidth. The bandwidth of the signal is huge, and the signal:noise ratio can be affected.

Whilst the comms link might "work" or appear "unaffected", the maximum achievable data rate might be lowered by increased errors.

It's also worth considering how the cord restraint works, because if the cable is bent rather than distorted, this can also increase errors.



I've seen the effect of how this adds up on real installations ... but granted, multiple distortions of cable geometry (or, on older coax installations numerous poorly crimped connections on the same run - and that on older 10BASE-5).


A single connection or distortion in a cable, unless very poor, is unlikely to cause too much of a problem - unless you want to push CAT 5 over 100 MB in a GB system, say.

But there is a way you can check whether it's a problem ... TDR test. Simple.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
IET » Wiring and the regulations » Jointing Cat5e

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