On 5/7/2013 7:33 AM, Paul Christensen wrote:
The standard drop cable is a bonded foil with single braid, it has
been that way since the late 1970's. The current best grade is
"Brightwire" by CommScope. Any good cable will far exceed FCC specs
without a quad shield.
It remained that way with Comcast & AT&T Broadband at least until 2002
when I left AT&T corporate engineering, shortly after the merger. I
doubt much has changed since then.
Concerns are a bit different in the broadband world where downstream
leakage must be minimized between 50 MHz - 1 GHz. The upstream path
is in HF region, but no sane cable operator uses spectrum below about
10 MHz. The only services anywhere near that area of spectrum are
used for data transponders and IPPV from the set-top box where
modulation is almost always QPSK. Telephony and DOCSIS cable modem
service is all relegated to an area above 20 MHz. In the Jacksonville
Comcast system, it's now all above 30 MHz which was made possible
through the use of tighter diplexer specs in the system amps and
fiber-optic nodes. if you could see a spectrum analyzer display of
the return path back at the headend, you would be amazed that the
return path works at all. Any point of ingress results high levels of
interference, most notably SWBC.
I recall writing up a paper in the mid-'90s that predicted a need for
better return path certification and an upward move of the lowest
usable frequency to well above 20 MHz. I pointed to the 11-year
sunspot cycle as an important driver. The non-ham engineers in our
group didn't get it. But the CTO of MediaOne was a ham, and he did
get it. The public did not know it, but there was a real fear between
1995 and 2000 that return path broadband technology would never work.
When you consider all the points of potential failure, especially on a
power-passing system, it truly is a miracle that it works at all.
Consider this: The typical fiber-optic node services between 200-500
home passings. From the comfort of your living room, and with an RF
signal generator, one can wipe out an entire service area when the
return path frequencies of the system are known. This isn't
theoretical, I demonstrated the impact to a sober group of engineers
with an Eico generator. That potential still exists today.
Concerning SANS connectors and wiring, I would take the lead from the
cable operators. They cannot afford to have unreliable cables
anywhere between the customer equipment and the headend or hub
facilities. When you've got thousands of miles of cable plant and
interfacing hardware, that becomes the most important piece in the
network. If a router, modulator or fiber amp fails in the system, the
fix is easy with money. But if you deploy bad cable and hardware into
a system, you'll feel the pain a long time as it affects long-term
service call volume, unhappy customers, and angry government leaders
who generally hate the cable operators.
Paul, W9AC
About six months ago, our garbage truck hit the sagging telephone/CATV
drops that serve my house and my neighbor's. It took several days to get
a response from Time Warner despite the fact that the cable was hanging
at neck level at times (we pulled it up the best we could only to see it
get knocked down again by the various delivery trucks that come through
the neighborhood). In any case, when Time Warner finally responded, I
saved the discarded drop cable and took it up to my station in the
desert north of here. I didn't pay close attention to it at the time,
but this discussion got me to wondering what sort of shielding was used
in that drop cable, so I retrieved some last time I was up there. This
evening I took a close look at it.
The drop cable is Times Fiber Communications (TFC/Amphenol) T10
"Teledrop". It look like an RG6 (I didn't measure the diameter) paired
with a messenger on one side and a telephone twisted pair on the other.
The coax is tri-shield (bonded inner tape layer, braid, and outer tape
layer). There also appears to be some sort of sticky flooding compound
which may be TFC's "Lifetime" non-drip/non-leak flooding compound (see
page 85 of the following):
http://www.timesfiber.com/TFC_Cable_Book_III.pdf
The other interesting information in the TFC Cable Book III is Technical
Note 1025 "Drop-Cable Transfer Impedance" starting on page 70. What this
note says is that you can't ignore cable aging due to flexure and
corrosion when looking at shielding effectiveness. Comparing the
shielding effectiveness at beginning of life (BOL) can yield vastly
different conclusion compared with shielding effectiveness done on cable
that has experienced significant environmental exposure. Quad shield
seems to have been developed to address the problems that come from foil
seam separation that occurs as the result of drop cable flexing. It's
actually a pretty complicated trade space which may explain some of the
lack of consensus in this discussion.
73, Mike W4EF................
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
|