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)
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?
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
I think it is a capacitve measurement rather than a true TDR tester...
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
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! ;-)
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..
A little knowledge is a dangerous thing eh Steve? Next time keep yer mouth closed..
10-Gig-E on copper? Jesus I hate to think. It's bad enough fiddling with it on Fiber.
cheers dan.
Well cat5e / cat6 is just under that at 49.
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...)
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
Quite funny though, didn't you think?
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.
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
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.
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.
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:
It's English, I'll give it that.
Apart from that, I don't think I've read such a load of bollocks in my life.
The first few zeroes at the end, eh? It means nothing to me... oh Vienna.
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