Ref: 11640059
Title: Cabling Basics:  Transmission Ground Parameters
Date: 2/21/90

Copyright 3Com Corporation, 1991.  All rights reserved.

All transmission lines have certain ground parameters which define
their characteristics.  They are resistance, capacitance,
characteristic impedance, attenuation, velocity of propagation, and
inductance.


RESISTANCE - "R"    measured in Ohms/Unit length

Resistance is a measure of a material's resistance to the flow of electrons
when voltage is applied.  If the resistance is too high, the electrical
signal being transmitted will decay to a lesser strength and may not
reach the receiving end.  The resistance of the conductor is important
in determining the signal-carrying capacity of the transmission line.  The
resistance value is the ratio of the electrical resistance of the conductor
material to its physical cross-sectional area.


CAPACITANCE - "C"   measured in Farad/Unit length

Capacitance is the ability of a dielectric material between conductors
to store electricity, when a difference of potential exists between the
conductors themselves.  For general transmission applications, it is
desirable to use cables with low levels of mutual capacitance to allow
longer transmission distances.   It is best to use low-dielectric
constant materials like polypropylene or cellular polyethylene.


CHARACTERISTIC IMPEDANCE - "Zo"   measured in Ohms

Characteristic impedance is the total bucking that a circuit offers to the
flow of electrons.  For long distance, high-pulse-rate transmission systems,
it is important to use cables with a characteristic impedance corresponding
to that of the transmitting and receiving systems.  If a difference of
characteristic impedance occurs at this junction, the resultant electrical
reflection will distort both signal strength and quality.  Equally
important, particularly on coaxial lines, cable geometry and dielectric
uniformity must not be constant or random and periodic reflections will
result.


ATTENUATION - "a"    measured in dB/Unit length

Attenuation determines the transmission level of a line.  The attenuation
level is usually expressed as a ratio of the input and output signals.
Attenuation of a digital pulse results in a degradation and distortion of
the signal in terms of loss in peak voltage and slower pulse rise time.
The two main factors that cause attenuation are resistive loss in the
conductor (mainly due to conductor skin effect at high frequencies) and
loss in the dielectric.  The sum of these two factors represents the total
attenuation of a given transmission line.


VELOCITY OF PROPAGATION - "Vp"    measured in Percent "%"

Velocity of propagation (or propagation delay) is the time required for a
signal to travel through a transmission line.  Vp is the ratio of the
signal speed in air to the signal speed in the transmission medium,
measured in percent.  This value is essentially a function of the dielectric
constant of insulating material.


INDUCTANCE - "L"    expressed in Henry/Unit length

When two conductors (coaxial, twinax, twisted pair, parallel pair) are
crossed by equal and opposed currents, a magnetic field between the two
conductors is originated.  The ratio or flow which crosses the space between
the two conductors, and the current which produces it, are called
inductance.  Inductance represents a primary constant of the cable.  It is
influenced by the distance of the two conductors, diameter of conductors,
and relative magnetic permeability of materials which compose the
conductors (for example, Copper "Cu" and Aluminum "Al":  permeability = 1).

