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From: ornitz@kodak.UUCP (barry ornitz)
Newsgroups: comp.dcom.lans
Subject: Re: RG58 vs. RG58 C/U
Message-ID: <921@kodak.UUCP>
Date: Fri, 21-Aug-87 18:36:01 EDT
Article-I.D.: kodak.921
Posted: Fri Aug 21 18:36:01 1987
Date-Received: Sun, 23-Aug-87 05:26:13 EDT
References: <4857@columbia.edu> <-76592479@cpe>
Reply-To: ornitz@kodak.UUCP (barry ornitz)
Organization: Eastman Kodak Co, Rochester, NY
Lines: 57
Keywords: impedance, capacitance, inductance, vinyl
Summary: Cable Capacitance _and_ Inductance

In article <-76592479@cpe> boze@cpe.UUCP writes:
>
>The RG58 Cable has an Impedance of 52 to 53 Ohms. The Manufacturers of
>Ethernet hardware specify the cable to be used as 50 +/- 4%. Typically
>the RG58 cable will exceed this when used in larger/longer networks. On
>a small network the RG58 usually will function, but the larger the network
>grows the more problem occur. The Network used here worked fine using the RG58
>cable until the network was about 250 ft long.The Network is now at the maximum
>length using RG58A/U or RG58C/U as specified, with no problem due to the cable.
>Also there is a difference in the Capacitance per foot of RG58 Cable versus 
>RG58A/U or RG58C/U.

RG-58 and RG-58 TYPE cable have an impedance of generally 53 to 53.5 ohms. They
use a solid center conductor and have a contaminating vinyl jacket.  RG-58A/U
and RG-58C/U are 50 ohm cables with a stranded center conductor. Also available
are foamed insulation RG-58A/U cables, also 50 ohms.  I do not believe the
minor impedance mismatch between 53.5 and 50 ohms is significant unless you
have a _lot_ of connectors.  Having used a time domain reflectometer for many
years, I have found that many coaxial connectors have more impedance mismatch
than you will see between the RG-58/U and RG-58C/U cables.  Even the best
connectors are sensitive to installation - you have to take _great_ pains to
install the connector properly for a good impedance match.

The stranded wire makes the cable lossier than the solid wire but this is a 
very minor point at the low frequencies involved (less than 0.1 db/100 ft).
The cables with foamed insulation have losses in between the RG-58/U and
RG-58C/U cables.  I would not recommend them since they are so easily damaged
during installation or use by crushing.

The capacitance values of the cable are determined by the dielectric used for
the cable insulation.  The foamed cables have lower dielectric insulations,
hence their capacitance per foot is less.  But to keep the impedance constant,
the inductance of the cable must be changed.  In the foamed RG-58A/U cables,
the center conductor is slightly larger than in the non-foamed cables.  The
foamed RG-58A/U cables have less capacitance per foot than the RG-8 cables,
but their loss is twice as high.  As you can see, the capacitance value is not
what is important here; the loss per unit length is.  They are related, but
not in a simple fashion.  You can intermix any number of cable sizes, types,
dielectrics, etc. if their impedances are matched at every point along the
transmission line [there are some limits to this statement at microwave 
frequencies, however].

The other main point which I have made in the past is that only RG-58C/U has
non-contaminating vinyl as its jacket material.  For outdoor runs or where
the cable is to be exposed to elevated temperatures or where the cable must
last 20+ years before replacement, I suggest the non-contaminating vinyl
jacket.  Otherwise, the cheaper material will work fine.

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|  ___  ________  |
| |  / /        | |  Dr. Barry L. Ornitz   UUCP:...!rochester!kodak!ornitz
| | / /         | |  Eastman Kodak Company
| |< < K O D A K| |  Eastman Chemicals Division Research Laboratories
| | \ \         | |  P. O. Box 1972
| |__\ \________| |  Kingsport, TN  37662       615/229-4904
|                 |
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