Path: utzoo!attcan!uunet!husc6!mailrus!ames!lll-tis!helios.ee.lbl.gov!nosc!ucsd!brian From: brian@ucsd.EDU (Brian Kantor) Newsgroups: comp.dcom.lans Subject: Re: Using 48.? ohm RG 62A/U cable for Thin Ethernet? Keywords: IBM COAX, Thin Ethernet, Cheapernet, Thinnet... Message-ID: <875@ucsd.EDU> Date: 28 May 88 16:16:30 GMT References: <2471@ritcsh.UUCP> <2631@ritcsh.UUCP> <22226@tis.llnl.gov> Reply-To: brian@ucsd.edu (Brian Kantor) Organization: The Avant-Garde of the Now, Ltd. Lines: 47 In article <22226@tis.llnl.gov> bae@ati.tis.llnl.gov (Hwa Jin Bae) writes: >How about RG 59? (as opposed to RG 58 or RG 11). >Can anyone think of any reason not to >use RG 59? It seems to be a better cable than 58 and a nice compromise >between 58 and 11. Maybe I don't know what I'm talking about... 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. Note also that much of the RG59/U cable sold nowadays is foam dialectric rather than solid, since that is what is popular in the cable-TV industry. Foam-dialectric cable (the center insulator is made of polyethylene foam instead of solid polyethylene) has a velocity of propagation that is around .79C instead of .66C; thus the speed at which signals travel is different. 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. Further, foamed cable is much less resistant to damage. RG6/U cable is much the same as RG59; it's the slightly-thicker stuff that most cable-TV companies are using nowadays for house drops (and often for house wiring too); it has lower loss and is physically stronger, since it has a CopperWeld (copper over steel) center conductor, but it's still 75 ohm and it's also usually foam. RG11/U is very thick 75 ohm cable; its primary use used to be long video cable runs or special antenna feeds. RG55/U is double-shielded RG58; it should work well for thin Ethernet if you really need the extra shielding (perhaps if your Ethernet runs past a transmitting antenna) but the outside diameter is a bit larger and you'll have to find special connectors for it. Finally, a note to you ham radio operators: thick Ethernet cable (the yellow stuff) works pretty well at up to 450MHz, so that's a pretty nifty way to use up those little scraps that were left over. At current prices, it might be cheaper than RG8 at the local hamstore, although Belden 9913 seems to be the cable of choice above 30 MHz these days. Brian Kantor UCSD Office of Academic Computing Academic Network Operations Group UCSD B-028, La Jolla, CA 92093 USA brian@ucsd.edu ucsd!brian BRIAN@UCSD