Path: utzoo!utgpu!water!watmath!clyde!bellcore!tness7!killer!pollux!dalsqnt!uunet!mcvax!unido!tub!lobo
From: lobo@tub.UUCP (Alexander Lobodzinski)
Newsgroups: comp.dcom.lans
Subject: Re: Using 48.? ohm RG 62A/U cable for Thin Ethernet?
Keywords: Thin Ethernet, Cheapernet, Thinnet...
Message-ID: <533@tub.UUCP>
Date: 1 Jun 88 16:29:01 GMT
References: <2471@ritcsh.UUCP> <22226@tis.llnl.gov> <875@ucsd.EDU>
Reply-To: lobo@tub.UUCP (Alexander Lobodzinski)
Organization: Technical University of Berlin, Germany
Lines: 29
Posted: Wed Jun  1 17:29:01 1988

In article <875@ucsd.EDU> brian@ucsd.edu (Brian Kantor) writes:
>RG59/U is a nominal 75 ohm cable; the standard for Ethernet (both thick
>and thin) is a nominal 50 ohms.  Thus you have an impedance mismatch
>which can cause standing waves (i.e., reflections) which may cause or
>mask collisions on the cable.

Beware that RG58/U cable has 53 ohms impedance; RG58C/U gives you
the 50 ohms you want.

>Foam-dialectric cable [...] has a velocity of propagation that is
>around .79C instead of .66C. [...]
>Minimum spacing requirements for taps into the cable have to be
>recalculated; they would have to be .79/.66 = 1.2 times further
>apart in order to assure the same minimum time delay between taps.

I doubt that; as far as I know, the Cheapernet specs (max 185m,
max 30 taps, min 2m between taps) are calculated based on .79C;
not accidentally, this velocity matches that of TEthernet.

The difference between RG58C/U and Belden 9907 (*the* Cheapernet
cable) is just that relative velocity (and the pretty yellow
insulation, not to forget :-)

    Ciao, l.
-- 
Alexander Lobodzinski, , 
UUCP: ...!pyramid!tub!lobo (Overseas)
      ...!unido!tub!lobo (Europe only)
					Drink positive!		--RKL