From: mcovingt@athena.cs.uga.edu (Michael A. Covington)
Newsgroups: sci.electronics
Subject: Re: Coax Question
Summary: 50 and 75 ohms explained
Date: 22 Jul 92 05:25:19 GMT
Organization: University of Georgia, Athens

In article <1992Jul20.140143.19341@yang.earlham.edu> jeremiah@yang.earlham.edu (Jeremiah Wilton) writes:
>I am wondering what kind of coax is used in Ethernet thin and thick wire 
>connections.  I am unconvinced that some really expensive special stuff 
>is necessary.  Might not RG-58 or 59 work for thinwire?  How about 
>composite video cable (the thin cheap stuff)?

Thinwire Ethernet is RG-58 (52-ohm).

>Why is RG-58 used in low-power FM applications (like receiving) while 
>RG-59 is used for TV?  I know about 50 ohms vs. 75 ohms, but what does that
>mean, and why are they different sizes? 

Well, let's see.

Cable impedance depends on the diameter of the central wire, diameter of
the shield, and properties of the dielectric.  So you can have the same
impedance in various diameters (larger ones have lower loss, generally).

75 ohms is the characteristic impedance of a dipole antenna.  TV and FM
receivers generally use horizontally polarized dipole antennas, and hence
75-ohm cable (or 300-ohm cable for folded dipoles; actually the most
common setup nowadays, due to haphazard evolution, is a 300-ohm antenna
and a transformer matching it to 75-ohm cable).

In commercial 2-way radio setups, the antenna is not a dipole, but rather
a "ground plane" antenna (1 vertical segment on one side, and 3 or 4
drooping almost-horizontal segments on the other side).  The characteristic
impedance is somewhere around 50 ohms, hence the popularity of 52-ohm
cable.

What _are_ these ohms?  Well, imagine a cable that is millions of miles
long.  Connect a battery at one end.  Obviously, a current flows into
the cable so that the battery voltage can get to the other end.  This
current is proportional to the voltage (of course) and is determined by
the inductance and capacitance along the cable.  

That is, from the battery's viewpoint, the cable looks like a resistor
with a particular resistance.

This is only true _until_ the voltage gets all the way to the other end.
(That's why I asked you to imagine a tremendously long cable.)
When the voltage finally gets to the other end, either of 3 things will
happen:

- if the end is open-circuited, current flow will stop, and the effect
  will be as if a voltage source had suddenly appeared there
  (a "reflection" of the original voltage source);

- if the end is shorted, current flow will increase, as if a voltage source
  of _opposite_ polarity had suddenly appeared there (again, a "reflection");

- if the end is terminated with a resistor of exactly the same resistance
  as the characteristic impedance of the cable, then the current will just
  keep flowing same as before.

That's why we terminate Ethernet cables with resistors.  It prevents
reflections.




-- 
==========================================================================
Michael A. Covington, Ph.D. |  mcovingt@uga.cc.uga.edu  |  ham radio N4TMI
Artificial Intelligence Programs | U of Georgia | Athens, GA 30602  U.S.A.
==========================================================================
Newsgroups: sci.electronics
From: tchannon@black.demon.co.uk (Tim Channon)