Only I was just thinking it may be a easy way to measure an installed
(but as yet unterminated) length of cable using a LAN tester with cable
length measurment capability...

(the default assumed capacitance on the tester is set for cat5, but you
can vary it over quite a range)



--
Cheers,

John.

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I did once calculate an approximate figure for T&E capacitance -it was
absolutely tiny - picofarads for normal cable lengths, if I remember
correctly.

However I thought data cable length measurement used TDR -time domain
reflectometry - only suited to cables that can be considered to behave
like transmission lines?

"John Rumm" wrote in message
...

Only I was just thinking it may be a easy way to measure an installed (but
as yet unterminated) length of cable using a LAN tester with cable length
measurment capability...

(the default assumed capacitance on the tester is set for cat5, but you
can vary it over quite a range)

It would depend on the manufacturer as they can have different dimensions
AFAICS.

Does the tester have TDR?
It should be better.
You can calibrate it by trying it on a known length.

Makes you wonder why they don't put length markings on the cable like they
do for cat5+.

Almost nothing. The line capacitance will never get charged. Not the same
in data cable though. If loads of 1's and 0's are going down the wire at
ten megabits per second (to say nothing of 10GE) then a stream of loads of
1's will charge the line capacitance and thus teh first few zero's at the
end will get missed - hence stuff like bit interleaving etc.
Steve


"John Rumm" wrote in message
...

Only I was just thinking it may be a easy way to measure an installed (but
as yet unterminated) length of cable using a LAN tester with cable length
measurment capability...

(the default assumed capacitance on the tester is set for cat5, but you
can vary it over quite a range)



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

dennis@home wrote:

Does the tester have TDR?

I think it is a capacitve measurement rather than a true TDR tester...

It should be better.
You can calibrate it by trying it on a known length.

That was my next plan, but I thought I would ask first in case the
answer was way out of its adjustment range[1]. It can cope with 23.5
pF/m to 131.5 pF/m

Makes you wonder why they don't put length markings on the cable like they
do for cat5+.

That would make it simpler!

[1] I was about to say, it would also save having to make up a RJ45 to
crock clip adaptor for attaching to the cable, but then I looked in the
pouch and noticed that is actually one of the standard test lead
harnesses that is supplied with it! ;-)

--
Cheers,

John.

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\================================================= ================/

wrote:
I did once calculate an approximate figure for T&E capacitance -it was
absolutely tiny - picofarads for normal cable lengths, if I remember
correctly.

However I thought data cable length measurement used TDR -time domain
reflectometry - only suited to cables that can be considered to behave
like transmission lines?

I'd guess at 50pF per meter meself..that sort of order..more than ten,
less than a hundred..

If you are really bothered I have an LCR bridge in the workshop..

Keyser Sose wrote:
Almost nothing. The line capacitance will never get charged. Not the same
in data cable though. If loads of 1's and 0's are going down the wire at
ten megabits per second (to say nothing of 10GE) then a stream of loads of
1's will charge the line capacitance and thus teh first few zero's at the
end will get missed - hence stuff like bit interleaving etc.
Steve


A little knowledge is a dangerous thing eh Steve?
Next time keep yer mouth closed..

"Keyser Sose" wrote in message
...
Almost nothing. The line capacitance will never get charged. Not the
same in data cable though. If loads of 1's and 0's are going down the
wire at ten megabits per second (to say nothing of 10GE) then a stream of
loads of 1's will charge the line capacitance and thus teh first few
zero's at the end will get missed - hence stuff like bit interleaving etc.
Steve

10-Gig-E on copper? Jesus I hate to think. It's bad enough fiddling with
it on Fiber.

cheers
dan.

The Natural Philosopher wrote:

I'd guess at 50pF per meter meself..that sort of order..more than ten,
less than a hundred..

Well cat5e / cat6 is just under that at 49.

If you are really bothered I have an LCR bridge in the workshop..

I can probably do a couple of emprical experiments with some cut lengths
and match the value to the length.

(in fact the handbook suggests using a 50ft length and ramping the pF/m
up or down until you reach the "right" answer...)


--
Cheers,

John.

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| Internode Ltd - http://www.internode.co.uk |
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| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In message , Keyser Sose
writes
Almost nothing. The line capacitance will never get charged. Not the same
in data cable though. If loads of 1's and 0's are going down the wire at
ten megabits per second (to say nothing of 10GE) then a stream of loads of
1's will charge the line capacitance and thus teh first few zero's at the
end will get missed - hence stuff like bit interleaving etc.
Steve

???

You seem to be an idea short of a clue there

Any two wires laid parallel to each other will have a capacitance (in
fact it's intimately bound up with the definition. For a given geometry,
the capacitance will be proportional to the length

and don't top post, and sort your sig sep !


--
geoff

John Rumm wrote:

Only I was just thinking it may be a easy way to measure an installed
(but as yet unterminated) length of cable using a LAN tester with cable
length measurment capability...

(the default assumed capacitance on the tester is set for cat5, but you
can vary it over quite a range)

Probably quicker to measure it than ask on here. Set tester to 50pf/m
and measure a known length, then you'll know its C/m. I wouldnt be
surprised if its lower than cat5.

Nb dont measure a reel of it, as that will have a load of extra
capacity, plus L in the equation too.


NT

The Natural Philosopher wrote:

A little knowledge is a dangerous thing eh Steve?
Next time keep yer mouth closed..

Quite funny though, didn't you think?

--
Andy

John Rumm wrote:

Only I was just thinking it may be a easy way to measure an installed
(but as yet unterminated) length of cable using a LAN tester with cable
length measurment capability...

Point 1: you need to specify between which two cores the capacitance is
to be measured and whether the third core is floating, guarded or
paralleled with one of the other two.

Point 2: PVC is a pretty rotten polar dielectric, so don't expect to get
a precision dielectric constant (epsilon_r) value from anywhere.
http://www.kayelaby.npl.co.uk/genera...2_6/2_6_5.html
suggests values in the range 2.8 - 3.2 for uPVC depending on frequency,
but presence of plasticisers and fillers will doubtless modify this.

An empirical approach with an LCR meter or bridge is probably the best
way forward.

--
Andy

Andy Wade wrote:
John Rumm wrote:

Only I was just thinking it may be a easy way to measure an installed
(but as yet unterminated) length of cable using a LAN tester with cable
length measurment capability...

Point 1: you need to specify between which two cores the capacitance is
to be measured and whether the third core is floating, guarded or
paralleled with one of the other two.

Point 2: PVC is a pretty rotten polar dielectric, so don't expect to get
a precision dielectric constant (epsilon_r) value from anywhere.
http://www.kayelaby.npl.co.uk/genera...2_6/2_6_5.html
suggests values in the range 2.8 - 3.2 for uPVC depending on frequency,
but presence of plasticisers and fillers will doubtless modify this.

An empirical approach with an LCR meter or bridge is probably the best
way forward.

what seems to have been missed so far is that the OP has the equipment
needed already to measure the capacitance, its the very item he wants
to use to measure length.


NT

wrote:

what seems to have been missed so far is that the OP has the equipment
needed already to measure the capacitance, its the very item he wants
to use to measure length.

Indeed I do... It was only intended to be a quick question in case
someone had a nice table of typical values Vs cable size to hand. Since
the answer appears to be "no", I shall do some measurements and report
back.


--
Cheers,

John.

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| Internode Ltd - http://www.internode.co.uk |
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| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Andy Wade wrote:
The Natural Philosopher wrote:

A little knowledge is a dangerous thing eh Steve?
Next time keep yer mouth closed..

Quite funny though, didn't you think?

It brought tears to me eys..tears of utter desperation and sadness
frankly. That education is so desperately bad thesee days that people
can utter complete nonsense will all the aplomb of the truly
knowledgeable, and get a country into e.g. a war because of it.

On Sun, 17 Dec 2006 17:12:00 +0000, Keyser Sose wrote:

Almost nothing. The line capacitance will never get charged. Not the same
in data cable though. If loads of 1's and 0's are going down the wire at
ten megabits per second (to say nothing of 10GE) then a stream of loads of
1's will charge the line capacitance and thus teh first few zero's at the
end will get missed - hence stuff like bit interleaving etc.

Don't forget to put a bit-bucket at the end or you'll and up with a mess
of data on the floor all writhing around at megabits per second!

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "Keyser Sose"
saying something like:

Almost nothing. The line capacitance will never get charged. Not the same
in data cable though. If loads of 1's and 0's are going down the wire at
ten megabits per second (to say nothing of 10GE) then a stream of loads of
1's will charge the line capacitance and thus teh first few zero's at the
end will get missed - hence stuff like bit interleaving etc.

It's English, I'll give it that.

Apart from that, I don't think I've read such a load of ******** in my
life.

The first few zeroes at the end, eh? It means nothing to me... oh
Vienna.
--

Dave

In message , Grimly
Curmudgeon writes
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "Keyser Sose"
saying something like:

Almost nothing. The line capacitance will never get charged. Not the same
in data cable though. If loads of 1's and 0's are going down the wire at
ten megabits per second (to say nothing of 10GE) then a stream of loads of
1's will charge the line capacitance and thus teh first few zero's at the
end will get missed - hence stuff like bit interleaving etc.

It's English, I'll give it that.

Apart from that, I don't think I've read such a load of ******** in my
life.

The first few zeroes at the end, eh? It means nothing to me... oh
Vienna.

Hmmm ... the UKRM band has a singer

--
geoff