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From: ISW@cup.portal.com (Isaac S Wingfield)
Newsgroups: sci.electronics
Subject: Re: Ridiculous Impedance
Message-ID: <33145@cup.portal.com>
Date: 24 Aug 90 00:51:05 GMT
References: <1770009@otter.hpl.hp.com> <1770010@otter.hpl.hp.com>
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Organization: The Portal System (TM)
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Larry Lippman writes:

>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.
               ^^^^^^^^^^^^^^

According to some things I read a few years ago, the two values 50 and
75 ohms are, in fact  "magic".

75 Ohms answers the question "For a given outer diameter, what impedance
provides the lowest attenuation per unit length"? This is good to know if
you are going to distribute video or cable TV, for example. The fact
that dipole antennas are also 75 ohms is coincidental.

50 (or 51, or 51.5) Ohms answers the question "For a given outer
diameter, what impedance cable can handle the maximum amount of power?"
The answer is really around 35 ohms, but the minimum is quite broad;
50 ohms is only fractionally poorer, and also matches vertical
quarter-wave antennas, so that's why it was chosen.

Reference: Schaum's Outline Series volume on transmission lines.
There's also some fascinating information in there on frequency dispersion
in ordinary wires at audio frequencies which should be of interest
to people who believe that such effects are non-existent in speaker
cable, for example.

BTW, I'll bet that the Trans-Atlantic cable impedance was carefully
chosen to provide an optimum set of parameters such as minimizing
loss while providing sufficient breakdown to allow the voltages
necessary to series feed the repeaters, or some such. Ma Bell doesn't work
by guess or by golly. At least one of the TA cables used about
15KV at each end...(And you think YOU have ground bounce problems.)

Isaac
isw@cup.portal.com