OK, guys and gals, lets move smartly on into the 21st century. John, get a
current copy of MIL-C-17. Throw away your MIL-C-17E or earlier. Cables
nomenclatured simply RG-nnn/U haven't been supported by the DOD system for
well over 20 years. Only M17/+++-RGNNN nomenclatured cables are truly
MIL-SPEC. See below.

Jerry: Here is a very condensed history of coaxial cable nomenclature.

Now, in the beginning, coax impedances resulted from standard size wires and
tubing. RG-8/U was 52 ohms, which resulted from the dimensions which were
quickly attainable in wire manufacturing in the 1930s. RG-8A/U was the
same, except the jacket was made from a plastic which did not contaminate
the dielectric through migration of the plasticizer. I recently had access
to some old Federal Radio and Telephone Labs records which contained quite a
discussion on dielectrics and jackets. Plastic technology was pretty
primitive back then. So was wire manufacturing, and braid overlaying.

In the 1960s all common coaxes were normalized to round number impedances
and issued new nomenclature, RG-8/U at 52 ohms becoming RG-213/U at 50.0
ohms. RG-9/U became RG-214/U, and so on ad naseum. RG-11/U was already
exactly 75 ohms, so it remained the same (and then moved on to M17/6-RG11).
RG-13/U, on the other hand, the double shielded half inch 74 ohm coax became
RG-216/U at 75 ohms. The new numbered cables were, for all practical
purposes, the same as their predecessors, except for the impedance. 50.000
ohm network analyzers were now safe.

Then in the 70s came cable TV and other wideband systems, which encountered
performance problems due to unpredictable phase characteristics across the
frequency range. This was found to be typically caused by repetitive
mechanical disturbances in the cable construction. To fully qualify the
cables, swept frequency performance measurements were added to MIL-C-17E,
and the nomenclature changed yet again to the present M17/+++. Now, you
don't see much of this 'new' M17/+++ stuff in ham use because it is another
increment more expensive, and hams typically don't have requirements for
wideband, predictable phase characteristics anyway. But, RG-213/U, which we
had just gotten used to then became M17/163-00001, the cheaper non-swept
direct equivalent, or M17/74-RG213 for the more expensive version swept from
50 MHz to 1 GHz. And cable bearing that nomenclature better meet the
specifications of the current MIL-C-17.

Now, just to be clear, there is no rule anywhere against anyone with cable
manufacturing facilities making coax of any kind and quality (or even rope
or water hose for that matter), and labeling it RG-8, or RG-213, or
RG-(anything else). So asking about RG-8 (or RG-213) is like asking what
the impedance of a wet noodle is.

Finally, a quote from the MIL-C-17 page in the Times Fiber RF Transmission
Line Handbook pretty well summarizes it, even better than the specification
itself. Especially the last sentence:

"Cables that are manufactured to MIL-C-17 specification no longer carry the
RG designation. Instead, they are marked with an M17 designation. For
example, RG-214 has been replaced by M17/75--RG214. In the future, any new
cable design will be designated by a M17 number only.
"In addition to the M17 number, all cables are marked with the
manufacturer's name and government identification number, for example,
'M17/75-RG214, MIL-C-17, Times Fiber Communications, 68999, AA-3409.'
Cables that are not marked with all this information are not qualified and
there is no guarantee of their performance."
--
Crazy George