Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uunet!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Ridiculous Impedance
Summary: Not so ridiculous...
Message-ID: <3965@kitty.UUCP>
Date: 23 Aug 90 02:53:54 GMT
References: <1770009@otter.hpl.hp.com>
Organization: Recognition Research Corp., Clarence, NY
Lines: 49

In article <1770009@otter.hpl.hp.com>, tgg@otter.hpl.hp.com (Tom Gardner) writes:
> Most coax cables have a characteristic impedance of 50 ohms or 75 ohms.
> 
> Why does TAT-7 (Trans-Atlantic Telephone cable number 7) have a characteristic
> impedance of 61.8 ohms?

	Because it's just the way the *physical* design of the cable works
out.  The actual characteristic impedance of such a transmission line is
immaterial, provided that suitable matching is achieved in connections to
any active or passive circuitry.

	50, 75 and 93 ohm coaxial cable are merely common values that have
been standardized upon for many commercial and military applications.  They
are not "magic" numbers for any particular reason.

	Almost all "real" coaxial cables employing a polyethylene (or similar)
dielectric result in some characteristic impedance between 25 and 125 ohms.
Unless one creates a bizarre design - like 30 awg inner conductor surrounded
by 1 inch radius of dielectric :-) - any *practicable* coaxial cable will
develop a characteristic impedance in the above range.

	Bear in mind that characteristic impedance is solely determined by
series resistance, dielectric conductance (i.e,. leakage resistance),
capacitance and inductance, with the latter two being measured at a given
reference frequency.  The geometric *proportion* between inner conductor
diameter, dielectric radius and shield diameter remain within a rather
narrow range in *real* coaxial cables, hence the comparatively narrow
range of possible characteristic impedance values.

	The 61.8 ohm value is pretty much middle-of-the-road, and should
not be a surprise to anyone.  I suspect that this impedance was the result
of other design considerations, that the value was "reasonable", and that
no effort was made to make it conform to standard values.  Such "other"
design considerations probably include: DC resistance (because of series
repeater power considerations); dielectric breakdown (DC repeater power
voltage to ground is probably 3 to 5 kV); shunt capacitance at working RF
frequencies; a minimum inner conductor diameter chosen for mechanical
strength considerations; a minimum dieletric cross section chosen to
provide mechanical flexibility; etc.

> I'll give a piece of wedding cake to the first person with the correct 
> answer.

	If you determine that I am the first person with the correct
answer, I will settle for a bag of Tender Vittles sent to my cats. :-)

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
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