Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Transmission Lines - What Are They??
Message-ID: <1989Nov12.043426.11834@utzoo.uucp>
Organization: U of Toronto Zoology
References: <868@encad.Wichita.NCR.COM> <1989Nov12.013850.7756@utzoo.uucp>
Date: Sun, 12 Nov 89 04:34:26 GMT

In article <1989Nov12.013850.7756@utzoo.uucp> I wrote:
>... current is determined by the "characteristic impedance"
>of the wire pair, which in turn is determined by things like wire size,
>spacing, insulation, etc., plus some effect from nearby objects...

I should have been more detailed here:  characteristic impedance arises
from the resistance of the wires (usually small) plus their capacitance
and inductance.  Hence the importance of physical details and nearby
objects.  If these assorted variables don't change along the wire, the
characteristic impedance is independent of the length of the wire.

Also, another complication arises:  characteristic impedance is somewhat
frequency-dependent, which is lots of fun for sharp-edged digital signals
that are a mishmash of harmonics.
-- 
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry henry@zoo.toronto.edu