Impedance in CAT5 wiring

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From - Wed Jul 28 18:05:48 1999
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From: jeffl@comix.santa-cruz.ca.us (Jeff Liebermann)
Newsgroups: comp.unix.sco.misc
Subject: Re: Hubs
Date: Wed, 28 Jul 1999 18:24:58 GMT
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On Mon, 26 Jul 1999 23:36:16 GMT, sarnold@redwood.willamette.edu (Seth R
Arnold) wrote:

>I have also heard that the electrons travel along the outside of the wire;
>stranded wire has more surface area than solid wire, therefore (the thinking
>goes :) less resistance.

Think impedance instead of resistance.  When you're working with
100-250MHz carrier rates and 50Mhz symbol rates, you're playing with RF
not DC resistance.  Any difference in DC resistance of stranded vs solid
over the maximum 100meter segment length is negligible when compared to
the 100ohms impedance of the twisted pair.

Geometry will tell you that stranded wire has a greater surface area and
therefore less loss at RF frequencies.  However, life is not so simple
above 50Mhz or so.  Bends in the wire, irregularities, proximity to
metallic objects, and variations in twist rate, will all contribute to
losses and reflections far in exess of any differences caused by a
surface (skin effect) losses.  Stranded wire tends to be worse in most of
the aformentioned.  It bends easier, therefore edge reflections are worse.
It's more irregular, therefore dispersion is worse.  Because it's more
flexible, the twist rate tends to move around.

However, all the above are very minor compared to losses from crappy
cable assembly techniques and the disgusting RF characteristics of the
common RJ-45 connector.  If I had to put a number on the contribution of
the differences between solid and stranded CAT5, I would say that it the
surface area contributed to perhaps 1/1,000 of the total loss picture.

Belden has a nice article with mangled formatting on the topic.  Note how
the impedance varies radically along the cable length, and is only close
to 100ohms in the graphs.  This is the real difference between CAT5 and
the newer cables that are rated to much higher frequencies.  The newer
cables have much less manufacturing and therefore impedance changes.
        http://www.belden.com/products/ftvimptp.htm

The impedance becomes critical only in 1000baseT.  1000baseT uses hybrids
to seperate the direction of data flow on each pair of wires.  Each end
has 4ea hybrids for each of the pairs running at 125Mhz symbol rate.  The
directivity (how well it seperates data going in opposite directions) of
these hybrids is heavily dependent upon the termination impedance which
is primarily determined by the cable impedance.  If the cable impedance
is all over the graph, or if variations in impedance along its length
cause waveform distortion, 1000baseT is going to have problems.

-- 
Jeff Liebermann  150 Felker St #D  Santa Cruz CA 95060
(831)421-6491 pgr (831)426-1240 fax (831)336-2558 home
http://www.cruzio.com/~jeffl   WB6SSY
jeffl@comix.santa-cruz.ca.us   jeffl@cruzio.com

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