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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: Coax cable specifications (General info wanted)
Message-ID: <8593@utzoo.UUCP>
Date: Wed, 16-Sep-87 17:24:08 EDT
Article-I.D.: utzoo.8593
Posted: Wed Sep 16 17:24:08 1987
Date-Received: Wed, 16-Sep-87 17:24:08 EDT
References: <1284@bgsuvax.UUCP>
Organization: U of Toronto Zoology
Lines: 34

> 	Can anyone explain how coax cable specifications are derived?         
> Does 75ohm coax get its designation by ohms per thousand foot or is this 
> a rating given from a measurement of impedance at frequency?

Sort of the latter.  Consider a longish cable with *something* at the
other end.  Suddenly we apply a voltage to the near end.  How much current
flows?  Remember that speed-of-light lag means that the impedance at the
other end is unknown and cannot influence the answer!  The answer is that
the properties of the cable (and its immediate environment) determine a
"characteristic impedance" which is what you plug into Ohm's Law to find
the instantaneous current.

Of course, after speed-of-light lag has had time to occur, then there will
be trouble if the impedance at the other end doesn't equal the cable's
characteristic impedance.  What happens then is "reflections", which
propagate back (and forth) until everything eventually settles down to
the steady-state situation that you would expect if you ignored the
finite speed of light.  Unfortunately, reflections are usually undesirable,
because they can be mistaken for signal changes when fast circuits are
involved.

To avoid reflections, the basic approach is to keep the same impedance all
the way along, from the transmitter output to the other end.  In particular,
this means cable with fairly constant characteristic impedance and a
terminator of that same impedance at the other end.  The c.i. of a single
wire varies a lot depending on what is near it, although various tactics
can limit the variations.  Coax's c.i. is largely insensitive to its
surroundings, making a constant-impedance system much easier to build.

Some of the serious electron-pushers may want to correct or elaborate on
the above; I'm just a software type with one foot in hardware...
-- 
"There's a lot more to do in space   |  Henry Spencer @ U of Toronto Zoology
than sending people to Mars." --Bova | {allegra,ihnp4,decvax,utai}!utzoo!henry