boze@cpe.UUCP (08/05/87)
Can someone explain the difference between RG58 and RG58C/U coax, in particular as it applies to suitability for use as thinwire ethernet cable? I'm frequently asked for advice on setting up thinwire ethernets, and while I've heard that it's a bad idea to use anything but RG58 C/U, I'm always pressed for an explanation as to why a cheaper or more readily available substitute can't be used (e.g. "plain" RG58, RG58 A/U, or old RGB cables). Usually I insist on RG58 C/U, but the asker winds up getting some cheaper cable to work and giving me a raspberry. This time the cable is going in my office, though, so I'd like to do a little more homework; if nobody knows the answer, a pointer to the document that describes the cable nomenclature would help. Thanks. Chris The RG58 Cable has an Impedance of 52 to 53 Ohms. The Manufacturers of Ethernet hardware specify the cable to be used as 50 +/- 4%. Typically the RG58 cable will exceed this when used in larger/longer networks. On a small network the RG58 usually will function, but the larger the network grows the more problem occur. The Network used here worked fine using the RG58 cable until the network was about 250 ft long. The Network is now at the maximum length using RG58 A/U or RG58 C/U as specified, with no problem due to the cable. Also there is a difference in the Capacitance per foot of RG58 Cable versus RG58 A/U or RG58 C/U.
hans@umd5.umd.edu (Hans Breitenlohner) (08/20/87)
In article <617@bbking.PRC.Unisys.COM> rmarks@bbking.PRC.Unisys.COM (richard marks) writes: >Question: I've read for years about the 50 vs 53 ohm impedance difference. >Everyone says that the difference may cause transmission errors. Now I've >got a couple of thousand feet (several nets) of RG58A/U (53 ohm) and everything >works well. > >The question is: Is this 50 vs 53 ohm difference a real problem or is everyone >just giving lip service to it? Has anyone has problems that were fixed by >going to true 50 ohm thin cable? > Whenever you have a discontinuity in cable impedance, some of the signal travelling across that discontinuity will be reflected. This effect is cumulative, and if you have several discontinuities in close proximity the problem gets worse because original signal, reflection, and reflections from adjacent splices may interfere with each other. I don't have a formula for the amount of reflection. I used to have a good reference in a TI book (Line drivers and receivers, or Linear and Interface, or something similar), unfortunately I can't find it any more. In any case the effect should be of the order of a few percent at most. It may reduce the 600 ft. maximum for the thin Ethernet segment proportionally. You need to avoid the case of alternating 50 and 53 ohm segments, as would happen if you have a backbone of one type of cable, and used the other type for splicing in new drops. I would expect that six transitions on one cable segment would not cause an appreciable problem. If you are concerned about the well-being of your Ethernet, a time domain reflectometer is the tool to answer all your questions. Lacking such a device, a simple pulse generator and oscilloscope is quite suitable to point out any problems due to cable mismatches, bad splices, bad terminators, kinked cables, or almost anything else which might be affecting your Ethernet. If your system consists entirely of 53 ohm cable, I can see no possible problem being caused by that. You might consider adjusting the terminators, if you can find suitable 53 ohm resistors.
ornitz@kodak.UUCP (barry ornitz) (08/22/87)
In article <-76592479@cpe> boze@cpe.UUCP writes: > >The RG58 Cable has an Impedance of 52 to 53 Ohms. The Manufacturers of >Ethernet hardware specify the cable to be used as 50 +/- 4%. Typically >the RG58 cable will exceed this when used in larger/longer networks. On >a small network the RG58 usually will function, but the larger the network >grows the more problem occur. The Network used here worked fine using the RG58 >cable until the network was about 250 ft long.The Network is now at the maximum >length using RG58A/U or RG58C/U as specified, with no problem due to the cable. >Also there is a difference in the Capacitance per foot of RG58 Cable versus >RG58A/U or RG58C/U. RG-58 and RG-58 TYPE cable have an impedance of generally 53 to 53.5 ohms. They use a solid center conductor and have a contaminating vinyl jacket. RG-58A/U and RG-58C/U are 50 ohm cables with a stranded center conductor. Also available are foamed insulation RG-58A/U cables, also 50 ohms. I do not believe the minor impedance mismatch between 53.5 and 50 ohms is significant unless you have a _lot_ of connectors. Having used a time domain reflectometer for many years, I have found that many coaxial connectors have more impedance mismatch than you will see between the RG-58/U and RG-58C/U cables. Even the best connectors are sensitive to installation - you have to take _great_ pains to install the connector properly for a good impedance match. The stranded wire makes the cable lossier than the solid wire but this is a very minor point at the low frequencies involved (less than 0.1 db/100 ft). The cables with foamed insulation have losses in between the RG-58/U and RG-58C/U cables. I would not recommend them since they are so easily damaged during installation or use by crushing. The capacitance values of the cable are determined by the dielectric used for the cable insulation. The foamed cables have lower dielectric insulations, hence their capacitance per foot is less. But to keep the impedance constant, the inductance of the cable must be changed. In the foamed RG-58A/U cables, the center conductor is slightly larger than in the non-foamed cables. The foamed RG-58A/U cables have less capacitance per foot than the RG-8 cables, but their loss is twice as high. As you can see, the capacitance value is not what is important here; the loss per unit length is. They are related, but not in a simple fashion. You can intermix any number of cable sizes, types, dielectrics, etc. if their impedances are matched at every point along the transmission line [there are some limits to this statement at microwave frequencies, however]. The other main point which I have made in the past is that only RG-58C/U has non-contaminating vinyl as its jacket material. For outdoor runs or where the cable is to be exposed to elevated temperatures or where the cable must last 20+ years before replacement, I suggest the non-contaminating vinyl jacket. Otherwise, the cheaper material will work fine. ----------------- | ___ ________ | | | / / | | Dr. Barry L. Ornitz UUCP:...!rochester!kodak!ornitz | | / / | | Eastman Kodak Company | |< < K O D A K| | Eastman Chemicals Division Research Laboratories | | \ \ | | P. O. Box 1972 | |__\ \________| | Kingsport, TN 37662 615/229-4904 | | -----------------