On Mon, 1 Feb 2010 08:04:08 +0000, Ian White GM3SEK
wrote:
There seems to be two different meanings of "foil" in this discussion.
Hi Ian,
You don't offer another meaning, simply different examples.
Most of the criticism seems to have been about "foil" made from
aluminized plastic. I'd agree this is very dubious because the effective
thickness of metal is unknown, especially in low-cost cables. The
presence of a so-called drain wire is also an indication that it's
difficult to make direct contact with the metal in the shield.
Actually, the drain wire is not specifically needed for termination,
but having said that, it is needed for termination - in a practical
sense. The drain wire does not run the length of the cable simply to
provide a handy length of wire available at any arbitrary point of
cut. The drain wire is "so-called" because it serves as a current
drain. It is a necessary component to the electrical design much as
the "so-called" drain lead on an FET is.
The foil has an atrocious conductivity for any significant length. If
it were to be relied upon alone, you could as easily assign it the
name of distributed resistor instead of shield (and yet even a
distributed resistor would satisfy some purpose of shielding).
The drain wire insures that this significant length of atrocious
conductivity is no greater than half the circumference of the inner
insulated wire. At this length, the foil path resistance is a quite
suitably low resistance.
The sense of drain, is electrostatic drain. If the term appears to be
"so-called" it is by purpose and historical application.
However, "foil" can also mean a thin but solid metal sheet. When applied
as an overlapping wrap of 360deg, this kind of "foil" has close to
perfect shielding properties at HF and above. Its main weakness is that
the metal can tear if the cable is bent too sharply, and the main
purpose of the braided copper cover is to bridge any resulting gaps.
Both copper and aluminium foil-covered cables are available, and copper
will obviously provide a more reliable contact between a connector and
the shield.
Every cable has what is called its minimum turn radius. In use, this
can be violated and the physical and electrical properties can become
compromised. This is not a fault of design.
That a user can put a cable to misfortune is not remarkable insight,
but attributing the tear in this foil to becoming a great misfortune
seems to be hysterical as that tear is drawing down the shield
coverage from 100% to 99.9999999% except at one specific and
distraught bend where it might actually reduce it to 96% (the native
coverage of the woven shield that embraces it) for an eighth inch. It
is very hard to imagine a situation where this local discontinuity
serves to bring down an entire system when it is a design redundancy.
The user having violated the minimum radius rule should be more
concerned with the inner wire migration through insulation and causing
a short - a vastly higher probability of an issue of greater concern.
Most Hams are quite aware of that consequence, and it alone (if
nothing other) motivates them to observe the minimum bend radius
prohibition.
Those Hams who are not aware of this consequence lead a superstitious
existence where failure arrives by the fault of some mysterious and
elaborate agency:
I have heard these stories of torn foil for years. And yet each and
every one of them has been testimonial, not research based in their
having been the cause of misfortune. Evidence would demand that the
entire length of jacket and woven shield be stripped off the cable in
some form of ritual much like an autopsy. That operation alone is
suggestive of general destruction, a self fulfilling prophecy once you
get down to the fragile foil layer.
This level of examination is something only a producer would embark
upon, and once they discovered a systemic failure, they would resolve
it (cynics can chime in here with their chorus of "no they wouldn't").
A Ham would look at a kink in a cable, open it up, discover torn foil,
and it would be immediate proof of the problem. Simply fill in the
blank of what that problem is, and add that to the list of ills that
proceeds from using foil shielded cable.
Now, if some scribbler wants to invest foil with toxicity for their
current situation, it might do to follow the lead of that foil being
(in flexion at a rotor, for instance) a source of triboelectricity.
Ponder the genesis of the following observation:
Another kind of solid metal "foil" is bonded onto the outside of the
centre insulation.
which serves to resolve that (the manufactures DO perform autopsies
and they DO provide resolutions).
If you turn to Wikipedia to consult what the term triboelectric means
(few here are going to have encountered it knowingly), it will only be
loosely descriptive, but sufficiently so. A more suitable
introduction can be found at:
http://www.systemswire.com/low-noise...ric-cable.html
One extract can be informative:
"The size of the triboelectric voltage spikes
in the cable is very much a function of the materials
selected by the cable designers. Copper and foamed
polyethylene, for instance, are two of the lowest
triboelectric generators available today. Adding
conductive low-noise layers can also reduce the
noise levels from tens of milivolts to the microvolt
range. The cable noise reduction noise occurs as
a result of draining the triboelectric induced charge
away from the wire insulation."
.... and we encounter that "so-called drain" once again.
73's
Richard Clark, KB7QHC