[TowerTalk] COAX History - Thought somone might enjoy reading this

[Tom Hammond]

  Wired for Sound: by Steve Lampen

  Coax: RG Means Radio Guide

  Visit the Reference Room Archive to read more from this author
  and others.


  We’re taking a side trip from cables such as twisted pairs, to
  another common cable: coax. Last time, we talked about coax
  cables and different ways they are made for different
  applications. But we forgot to talk about one key thing: What
  the heck does RG mean anyway? Also, where did it come from?

  RG means "radio guide" and was the original military
  specification for coax cable, starting in the 1930s. So what do
  all the numbers mean — RG-6, RG-8, RG-58, RG-59, RG-62, RG-122,
  RG-213, RG-405 and on and on?

  The number is just a page in a book. RG-1 was the first page
  (and obviously wasn’t a very successful cable design). RG-6,
  the sixth page, was wildly successful. Most CATV/broadband
  cable these days is RG-6.

  RG #   Impedance   Center (AWG)   O.D. (inches)
    6       75            18            .275
    8       50            10            .405
    8X      50            19            .242
   11       75            14            .405
   58       50            24            .195

  (NOTE: Above table best viewed in a NON-proportional font)

  Since each RG number is just a page in a book, it really
  doesn’t mean anything. That’s why you can have RG-58, a 50-ohm
  design, right before RG-59, a 75-ohm design.

  Actually, I lie. As the specs read, RG-58 is a 53.5-ohm design.

  Here is an example of things that went wrong: While the
  military can pick whatever it wants, and pay whatever it costs
  to have stuff built to match that impedance, the commercial
  world (your world) doesn’t always have those options.

  WHAT DO THESE NUMBERS MEAN?

  Here’s a quick guide to RG numbers and what they mean.
  Remember, these descriptions are "general" descriptions. You
  can probably find exceptions to every one listed if you look
  hard enough.

  Remember that RG/Type can mean just about anything, and don’t
  assume that all RG-58s (or any other number) are the same!

  Other questions to ask: Is the conductor solid or stranded? If
  solid, is it bare copper, tinned copper, silver-plated copper,
  copper-clad steel or silver-coated copper-clad steel? If
  stranded, how many strands of what gage and what are the
  strands made of?

  Shielding is another area to explore. Is the shield braid,
  double braid, or braid and foil? What coverage is the braid and
  what is it made of? How many conductors of what size?

  Also study applications. It is unlikely you would use any of
  the 93-ohm versions unless you’re into old-time data wiring.
  Most likely, you’ll stick to the 50-ohm/75-ohm varieties.

  Do not forget there are dozens, maybe even hundreds, of coaxial
  cable designs that don’t fall under any RG or Mil spec. You
  should not limit yourself to just RG if you’re searching for
  the perfect cable!


  All systems you have are 50-ohm systems (on the transmit side).
  So 53.5 ohms would exhibit a fair amount of mismatch. But if
  you made a cable for 50 ohms, since you couldn’t call it RG-58,
  you’d call it RG-58 Type.

  The word "Type" is the key. It means "sort of" in
  technical-ese. So our 50-ohm version is "sort of" like the
  RG-58 spec. Pretty lame, huh?

  DIFFERENT JACKET

  Oh, but that’s not all: As the military itself changed the
  original spec, it brought out different versions. RG-58/A,
  RG-58/B, RG-58/C. And some of these could be considerably
  different from the original. One might have a stranded center
  conductor instead of a solid one. Or it might have a special
  jacket compound so the chemicals in the jacket would not
  contaminate the dielectric underneath, called a
  "non-contaminating" jacket.

  And then, not long after World War II, the military decided
  that the RG system was getting too unwieldy (duh!) so they
  abandoned the entire system and replaced it with the current
  system.

  In the current Mil Spec, coax falls under a heading of C17.
  Each time the spec is changed, a new letter is added. We’re
  currently under C17G. This spec supersedes everything written
  before — not just C17A through C17F, but all RG numbers as
  well. To the people who invented the RG system, RG means
  nothing.

  Of course, there were a few million customers, like you and me,
  who thought the RG numbers were fine. A large number of
  technicians who entered broadcasting after World War II came
  with extensive RG knowledge, so it was logical for those cables
  to continue. And, like any ancient religion, these numbers
  continue to be used (and modified) long after the High Priests
  (the military) had moved on.

  DIFFICULT RATIO

  One other question that often comes up: Why 50 ohms and 75
  ohms, or any other impedance, for that matter?

  These impedances were not chosen by accident. It was known in
  the 1920s that cables of different impedance worked better for
  one application that the other. For instance, it was
  determined, through experimentation, that the best power rating
  was around 30 ohms.

  Because the impedance of any coax cable is the ratio of the
  sizes of the center conductor and the distance to the braid
  (and the quality, or "dielectric constant" of the plastic in
  between), you might wonder why we don’t have 30-ohm coaxes.

  To be sure, there are customers out there who would buy as much
  30-ohm coax as we could make! The problem is that 30 ohms
  represents a ratio very difficult to make — so much so that it
  is quite likely that most of what you would make, you would
  throw away. Only a small percentage would be usable.

  The people who would die for 30-ohm coax are the really
  high-power people. Those are customers such as nuclear
  physicists (those with some kind of atom smasher) or medical
  scanners, such as X-ray, CAT scanners or NMRI machines. They
  all use high power and would love to have 30-ohm coax. It would
  deliver their voltage with even less loss (and higher
  efficiency).

  Those customers who wanted low-signal attenuation found that
  the ideal impedance was 77 ohms. But this was an odd number in
  terms of wire sizes. If you "fudged" just a bit to 75 ohms,
  then standard wire sizes and dimensions could be used. This was
  why all those low-power, low-voltage signal-carrying cables
  (baseband video, CATV/broadband, antenna lead-in) were all
  75-ohm.

  And then there are customers who want to deliver high voltage;
  the ideal impedance for them is around 60 ohms. This is an
  eminently "makeable" cable, but it never really got started,
  mainly because it was soon realized that most high-voltage
  customers were often high-power customers too. Therefore, there
  really needed to be a compromise between voltage and current;
  that compromise was 50 ohms.

  We’ll have more on the strange story of 50-ohm coax, with a
  trip into the world of wireless microphones, in the next
  installment. Let me know if you have any questions or comments.

  Steve Lampen is a senior audio video specialist for Belden Wire
  & Cable Co. in San Francisco. His book, "Wire, Cable, and Fiber
  Optics for Video and Audio Engineers," is published by
  McGraw-Hill. Contact

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