Jochen writes:
I suspect CI cables are just ordinary 50 Ohm coax, but
heavily
overspeced in best DEC tradition. ;-)
Dave writes:
I agree (now) that they're most likely 50-ohm, but
that particular
coax is anything but ordinary. ;) I'm sure they over-specced it like crazy.
OK I found some hanks (chopped up from computer room removal) of old blue
"thick" CI cable. I was actually gonna take it home and use it for some
recabling of my radio station anyway when this subject came up on cctalk.
Externally it is 0.4" like many RG-8 and RG-213 types.
Internally the center conductor is 11AWG, silver plated solid copper. Yep, silver plated.
The dielectric is 0.238" diameter polyethelyne foam. Again, a little smaller diameter
than usual for RG-8 types, but I think in ratio with the inner conductor being smaller and
the properties of foamed poly, gives 50 ohm impedance.
Then, the amazing thing is what's outside the dielectric. There is a layer of foil, a
THICK layer of braid, then another layer of foil, then another THICK layer of braid. I
don't think I've ever seen a RG-8 type coax with 4 layer shielding. Most RG-213
types have a single thinner layer (and some have foil). I don't think the braid is
silver plated, I think it's just tinned, may test that out later.
If you wanted to put crimp-on N or UHF connectors onto this stuff, the funny dielectric
diameter might cause some heartache, as could the extra thickness of the double braids.
I have some hanks of "reddish-orange" DEC Thicknet Ethernet cables too, will
check out what's inside them. I seem to recall them being solid polyethelyne
dielectric but it's been more than a decade since I cut into any.
Tim N3QE