References

Cable Attenuation Chart

CATV Channel Allocation Chart

Hex-Crimp Connectors

DOCSIS

dBm Conversion Chart

From the May, 2000 Issue of Communications Technology International Magazine:

Q: I am taking a CNT course in college, and a question has arisen about an item in our textbook. We came across RG-58/U, A/U and C/U thinnet cable types. Our question: How do you get the extension names of A, U, C? We read of stranded and solid core wire, but how did the industry decide how to name these?

Jim's Answer: The slash designations, such as /U, /A/U and so on, were assigned to a cable by its original manufacturer to designate it however he wanted. Thus, there is no consistent meaning to the slash numbers. The "RG," however, came from the military--the old U.S. Army Signal Corp, I believe--which developed coaxial cables during World War II. RG stood for "Radio Grade," perhaps named as such to differentiate from power cables.

The numbers designate the size and the characteristic impedance, but you have to know what they mean. RG58 is a small 50 ohm coax used for receiving and for some lower-power transmitter applications. RG8 is a large version with lower loss and higher-power capability. RG59 and RG9 are the corresponding 75 ohm cables, but RG6 and RG11 are larger sizes of 75 ohm cable that are much more popular than RG9 (I had some RG9 around my ham shack when I was a teenager).

Anybody can sell a cable and call it an RG coax--which has driven the cable industry crazy in recent years. Some stores have sold inferior cables, calling them RG59. It is intended to let homeowners install their own cable TV extensions. Good-quality cable is available, but you can save money by skimping on the braid covering, for example. The problem is that the cable tends to leak signal, and actually can drive a cable system out of compliance with the FCC rules on leakage. Since the cable is in a private home, the operator cannot do anything about it except refuse to serve the customer, which of course cable operators aren't happy to do.

By the way, I hope your textbook talked about why most coax is 50 ohms. Apparently, many of the antennas the Army wanted to use during the war happened to have about that amount of impedance, so it became a standard. It is still used, but many antennas must be matched intentionally to it. The cable TV industry uses 75 ohm cable because you can prove that this impedance results in the lowest loss in cable for the amount of metal in the cable. Heavy route Ethernet uses a higher impedance.

As for stranded or solid wire, you usually use stranded where you need flexibility in the cable. Solid wire is stiffer and has a larger minimum bend radius for a given gage wire, but you don't have to worry about controlling the individual strands. There is also a fused stranded wire where the wire is stranded, but the strands are held together with solder, so the wire is really a cross between stranded and solid. The choice often is reduced to how much flexing the wire has to handle.

Ron's Answer: The subject of the origin of the RG-x designation for coaxial cables came up in an SCTE-List discussion in June 1998. The consensus seemed to indicate that "RG" stands for radio grade or radio guide, but the question wasn't really resolved. Rex Porter, editor-in-chief of IC's sister publication Communications Technology, commented that RG originally stood for radio (government) grade. As the commercial sector began to manufacture RG-type cables, the "government" designation was dropped in favor of radio grade. This was in part because of the concern that RG-designated cables that had not been tested for compliance with MIL-C-17 might be interpreted as MIL-SPEC cables when in fact they were not, so they were simply called RG-type, with RG standing for radio grade.

When this discussion appeared on the SCTE-List, I did a little research and came up with the following. I tracked down the "Army-Navy List of Preferred Cables," which was derived from "RF Transmission Lines and Fittings," MIL-HDBK-216 (4 January 1962, revised 18 May 1965). As a side note, requirements for listed cables can be found separately in MIL-C-17.

The "Army-Navy List of Preferred Cables" didn't specifically define the acronym "RG" but included 75-ohm cables (the common impedance for coaxial cables used in cable TV networks) among the many listed. These were classified as "JAN Type RG-" cables. RG was shown as an indicator, applicable to the family name "cables, RF bulk."

The following list of 75-ohm "JAN Type RG-" cables are nominally 75 ohms, because some actually are 70 ohms, 72 ohms, 75 ohms, 76 ohms and so forth. Here are the listed 75 ohm cables: Single braid: 11A/U, 12A/U, 34B/U, 35B/U, 59B/U, 84A/U, 85A/U, 164A/U, and 307A/U; Double braid: 6A/U, 216A/U; High temperature, single braid: 144/U, 187A/U, and 302/U

Even after digging up this information, there was still some question as to just what "RG" means. I then contacted Edwards Publishing, the company that published "The Encyclopedia of Connectors," a multi-volume reference series based on industry standards, MIL-SPEC documents and similar material. RG" was an arbitrary designation for the family name "cables, RF bulk," much like "UG" was an arbitrary designation for the family name "connectors," according to an individual with whom I spoke at Edwards Publishing.

A contributor to the SCTE-List discussion on this subject referenced a late 1960s version of the "RCA Field Engineers Technical Manual" and found this description: Component indicator: RG Family name: cables and transmission lines, bulk, RF Definition: RF cable, waveguides, etc. without terminals

Yet another SCTE-List subscriber said a US Navy spokesperson told him that the designations were indeed arbitrary, because the military supposedly had huge lists of multi-letter designations for all kinds of materials, supplies, hardware and parts. That cables wound up with the RG designation, was a coincidence, according to that particular source.

Whether or not the designations really are arbitrary is open to debate. Anecdotal evidence suggests RG originally was an acronym for radio government (and later radio grade); the UG designation for connectors is "universal grade;" and the /U in many of the RG cable part numbers simply stands for "universal." Even so, I'm going to have to say the jury is still out on this one.