COAX History - Thought somone might enjoy reading this
from [Tom Hammond]
Wired for Sound: by Steve Lampen
Coax: RG Means Radio Guide
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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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