multishields in CATV was Re: [BULK] - [TowerTalk] RG-11 Source?

[Jim Lux <jimlux@earthlink.net>]

At 04:50 PM 2/9/2005, Tom Rauch wrote:

> A real test would be to put numbers that mean something in
> the real world on this, rather than talking about systems
> where other problems may be confused with shield thickness
> problems.
>
> I always see people crowing about the imagined virtues of
> quad shield.
>
> http://www.uniprisesolutions.com/docs/Uniprise_Electrical_Characteristics.pdf
>
> shows there is barely any measurable difference between quad
> shield and just a single foil with loose braid overlay. As a
> matter of fact, before anyone gets all awestruck by the 50dB
> change from 95% copper to foil you have to keep in mind that
> 50 dB over meaningless is still just meaningless.
>
> Now there are cases where a system right on top of a very
> very strong field (say a 100 kW on-channel TV station that
> is just miles away and line of sight) might benefit from a
> change from single woven braid to a foil shield, but it's a
> HUGE exaggeration to extrapolate that situation to claiming
> a rural CATV drop cable would have a noticeable change from
> single foil to multi foil.


I will fully agree that connectors will probably be the dominant source of 
problems, leakage and/or mismatch in almost any cabling system.

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?


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.  Radiate a 
bit too much power in the wrong bands, and the FCC is down on you pretty 
darn fast (read all those enforcement letters on the fcc website..)

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.

There's a fairly decent signal level inside the coax in most cable TV 
plants.. (something like -49dBm is needed at the set for VHF broadcast.. 
digital signals are at -61dBm (-106.2 ktB + 15.2 dB CNR + 10dB NF+20dB 
"propagation and implementation margin"))

In the cable TV world, they tend to use dBmV (into 75 ohms, presumably), 
but in any case losses should work the same regardless, a typical path from 
"mainline cable" appears to be something like 26 dB for the tap, 2dB for 
the drop, 6dB for inhouse cable and splitters, so 34dB from "main wire" to 
the -40 odd at the set.(I seem to recall something like -33 dBm corresponds 
to +15dBmV, which is a typical level). Figure at least zero, and then 
they'd add some margin, so they're probably running +10 dBm (per carrier) 
or several watts down the wire with all 100+ channels...

(Tom or others can correct me if I'm horribly wrong here).

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?)

The FCC spec is 20 microvolts/meter 3 meters from the point of leakage.  I 
think that's about -90dBm into a dipole.  There's some space loss over the 
3 meters (but not a huge amount maybe 30-40 dB), so if your signal leakage 
starts getting up into the -50dBm or -40dBm range, you'd have a problem.

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).


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