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