> I will fully agree that connectors will probably be the
dominant source of
> problems, leakage and/or mismatch in almost any cabling
system.
Of course they are. Anyone who has worked on or understands
those systems knows that.
> That chart is a big odd because it has dB AND a log scale
(log of a log?).
> And, it's really hard to read because the graphics didn't
scan well in the
> pdf process. It kind of looks like it's perhaps 30dB from
95% copper to
> foil/braid, and then another 20-30 to quad?
Yeah, it is hard to read. But my point is this.....
if we express shielding effectiveness as a ratio (and dB is
a ratio!!) we have to know at what distance and what
standard we measure it. Measurement of a ratio not knowing
and fully understanding how that measurement was made is
useless for anything.
It's like dBi, dBd, and dB in an antenna advertisement.
> I suspect that a bigger reason, and one where the 50dB
might make a
> difference, in a CATV system is leakage OUT of the cable
plant.
Yep. The level is very high at some points in the system and
you can't tolerate ANY leakage out.
On the other hand radiation IN to the cable is almost never
an issue, and like going out the connectors and connections
are a much larger problem than the number of shields.
> The quadshield stuff is most useful because the shielding
remains effective
> if the installer bends the cable too tightly, clamps it a
bit to
> vigorously, staples it to the studs or pulls it through
the holes real
> tight to make it reach, or the cable flexes a lot in the
wind.
Absolutely.
> In the cable TV world, they tend to use dBmV (into 75
ohms, presumably),
We shot for about 0 dBmv at the set terminals maximum. On
local strong channels we needed more, but to overcome
problems in the TV to cable interface that allowed signals
in. It was never accumulated signal over distance getting
into the cable.
> Say you've got +10, and you've attenuated it 100 with the
shield, you're
> down to -90, which is still easily above the noise floor
with a narrow band
> receiver (like an aircraft or public safety radio) (I
think that there's a
> hole in the cable channel band plan to avoid the aviation
bands, just for
> this reason?)
Yes. On the other hand look at the level you need getting
INTO the cable to bother the +20dBmV running down the drop!
> Now consider all those thousands and thousands of feet of
coax in the
> typical cable plant, and quad shielding is a good bit of
insurance against
> flunking the annual proof of performance test. The
incremental cost of the
> extra shield is probably negligible (onve you've built the
tooling, most of
> the cost in coax is in the handling, not the materials).
>From there and other very special situations it carries over
to become folklore that we need it between the set and a
dish antenna.
73 Tom
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