Richard Clark wrote in
:

That article does seem to have a few vague contradictions,
but I think the point about a thin foil that is adequate for UHF screening
being inadequate for HF is interesting, and I've seen that point claimed
before.

In coaxial application, the performance of the foil is limited to its
thickness, which in turn can be penetrated by low frequencies. We
know this as an example of penetration depth. The surrounding wire is
probably 10 to 100 to 1000 times thicker in that regard. The wire
will always satisfy most typical applications (VLF and up) and where
it would not is found in "coverage." Such issues are very rare and
are not elevated to important simply because you are straining to
catch a weak signal.


Ok, if I take that with the point about lower HF loss in RG6, it seems that
my easiest option of a copper-braid RG6 will be good, but this still begs one
question: If BT are using BT2002 double-braided copper and no foil, at
greater cost, what does it do for them that RG6 will not do? Would it be a
matter of transmission power, or something else?

In article ,
Lostgallifreyan wrote:

Ok, if I take that with the point about lower HF loss in RG6, it seems that
my easiest option of a copper-braid RG6 will be good, but this still begs one
question: If BT are using BT2002 double-braided copper and no foil, at
greater cost, what does it do for them that RG6 will not do? Would it be a
matter of transmission power, or something else?

One issue which may be relevant in some applications (transmitters and
repeaters) is internally-generated cable noise.

Foil-and-braid cable has developed a somewhat evil reputation among
repeater operators. The story, as I have been told it, is that the
braid, and the conductive layer on the foil, don't make particularly
good (or continuous) contact. As RF power flows through the cable,
some of the current can jump back and forth between braid and foil,
through imperfect connections each time. This leads to some amount of
discontinuity in the current flow (diodic junction effects or
"micro-arcing") and rectifies a small amount of the RF power into
broadband noise. The same effect might be capable of generating
intermodulation noise, if the cable is carrying two or more strong
signals at the same time.

In many applications this effect is irrelevant. In a duplex
application (e.g. an FM repeater) it can be nasty... the broadband
noise from the transmit channel goes right through the duplexer stage
into the receiver, and can swamp out the desired incoming signal. You
can lose several dB of receiver sensitivity due to this effect.

For this reason, repeater builders prefer to use a non-foil-shielded
cable. Cables with double silver-plated copper braid shields are
popular, as are heliax-type semi-hardline cables.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Sun, 31 Jan 2010 13:41:26 -0800, (Dave Platt)
wrote:

The story, as I have been told it, is that the
braid, and the conductive layer on the foil, don't make particularly
good (or continuous) contact.

Hi Dave,

I've seen this exact same statement expressed in regard to problems
introduced by the weave of wires in the shield of coax. And yet your
story teller relates that doubling the amount of shielding with woven
wires is the preferred solution.

Given the elaborate logic one must invest their faith in, for the one
explanation to make sense in regard to foil and then to be wholly
unremarkable in woven wire seems to make this rather apocryphal.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Sun, 31 Jan 2010 13:41:26 -0800, (Dave Platt)
wrote:

The story, as I have been told it, is that the
braid, and the conductive layer on the foil, don't make particularly
good (or continuous) contact.

Hi Dave,

I've seen this exact same statement expressed in regard to problems
introduced by the weave of wires in the shield of coax. And yet your
story teller relates that doubling the amount of shielding with woven
wires is the preferred solution.

Given the elaborate logic one must invest their faith in, for the one
explanation to make sense in regard to foil and then to be wholly
unremarkable in woven wire seems to make this rather apocryphal.


There seems to be two different meanings of "foil" in this discussion.

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.

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.

Another kind of solid metal "foil" is bonded onto the outside of the
centre insulation. I've only ever seen this in aluminium; the foil is
extremely thin and solidly bonded to the polyethylene, making it very
vulnerable to damage by bending. A braided cover is provided, but once
again there can be problems with connector assembly.

These points are confirmed by Owen, VK1OD at:
http://www.vk1od.net/transmissionline/RG6/index.htm

In this wet climate I wouldn't ever use a cable containing aluminium;
but Australia's different, of course.

Finally, beware of ALL "RG" designations. The military RG cable
specifications have been obsolete for many years and the carpetbaggers
have moved in. "RG8" was the first to fall, and "RG6" can mean both
anything and nothing.

Even in the days of MIL specs, "RG58" covered several different types of
cable - the copper could be either bare or tinned, the centre conductor
either solid or stranded - so it has always been necessary to check what
kind of construction you were buying. In modern times you also need to
check the quality.

About the only "RG" cables I'd trust today without seeing a sample are
RG213 and 214, and only from a trusted supplier.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

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

Richard Clark wrote:
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.


The purpose of the posting was to identify and distinguish those two
very different meanings of "foil".


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.


Very well, let me re-phrase: the presence of a so-called drain wire can
be taken as an indication that the metalized plastic shield has poor
electrical conductivity and is not suitable for RF applications.


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.

Manufacturers are fully entitled to specify a minimum bending radius.
What's important here is the *result* of bending the cable at a
progressively decreasing radius. A braided shield will slip and stretch
to relieve the stresses, and will often survive quite excessive bending
without breakage of strands; it will then recover leaving relatively
little disturbance.

In contrast, a foil shield has a very sharp failure threshold, beyond
which it will be torn apart; see below.

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.

Those are two separate problems. The "issue of greater concern" is the
simply the one that happens first; but without detailed knowledge of
each specific installation it's impossible to predict which one that may
be.


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.

Here is that story. The cable in question was semi-airspaced with a
shield made from solid copper foil in a 360deg wrap, overlaid by
open-weave copper braid. Having experienced problems with fluctuating
VSWR in a rotor loop, I removed that entire section of cable - and yes,
indeed I did 'autopsy' it.


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.

Rubbish.

The cable jacket was carefully removed by slitting along its length and
gently peeling it off. In the two sections close to where the rotor loop
had been anchored, the foil shield had been torn circumferentially into
several isolated segments, each a few inches long. The overlying braid
was not broken, and was only slightly disturbed by the surgeon's knife.

Such was the objective evidence.

My deductions were that most of the repeated bending of the rotor loop
had been concentrated into those two sections. As for the VSWR
fluctuations, it seemed that the outer braid had not made sufficiently
good contact to bridge over the breaks in the foil when the antenna was
being rotated. I considered both the observed VSWR problem and the
implied shielding problem to be important because the system was
carrying 1kW at 432MHz.

I accept that these problems were entirely due to my poor installation
technique. I now try to distribute the bending more evenly along the
entire length of any rotor loop, but it isn't easy. Therefore I prefer
to use cables that have some tolerance of excessive bending if it should
occur.


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").

No, they wouldn't. They would simply state that this type of cable was
not designed for repeated flexing at close to the minimum bend radius. I
fully accept that; what I don't like is the drastic mode of failure in
which the foil tears completely apart.

In particular, I don't like the type of cable in which the foil shield
is solidly bonded to the underlying PE, because there is no possibility
of 'slip' to relieve the bending stresses.

In practice, hams have to use whatever is most cost-effective and there
is no doubt that solid copper foil has excellent EM shielding
properties, so long as that shield remains undisturbed. For a rotor
loop, one has to balance the risk of tearing the shield against the
disadvantages of splicing in a section of more flexible and tolerant
cable such as RG213.

More modern low-loss cables have both the solid metal foil shield and a
heavier cover of braid to act as backup.


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.

None of us was talking about a severe "kink", only about moderately
excessive bending.


Although I only had that one experience of failure (and didn't let it
happen again), I did take the trouble to find out what had caused it.
Several other hams have related similar experiences with those kinds of
foil shielded cable. I still use them where low loss is important, but
treat them much more carefully than braid shielded cables like RG213.


--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote in
:

There seems to be two different meanings of "foil" in this discussion.

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.

Quality of product is an issue, but the type of 'foil' you describe, and
as part of a system of braid and foil isn't necessarily as poor as you
intimate.

The better ones are quite tough, in fact one might desribe them as
tenacious when trying to terminate cables, and if you look carefully,
they are circumfrentially closed.

As I said earlier, and I think you are agreed Ian, cable bears
inspection. It does take experience to develop the skills and knowledge
to be competent. Above all objectivity is important. I gauge a certain
bias in the OP's approach... but I could be wrong.

Owen

PS: Sitting here is unwanted rain from day to day (I am trying to build
a shed at my new place), I am a bit amused at the throwaway line about
Australian weather. I visited my old house a few days ago, it is about
150km away, and dry as. However, Australia ranges from tropical
rainforest to dry desert and I would not install any form of coax
without adequate protection from water.

I know from work experience that the products of aluminium corrosion can
play havoc with IMD... but then if IMD performance was critical (eg a
communal repeater site), you wouldn't use RG6 in any form, or probably
even braided copper for antenna runs, you would look at solid copper
outer conductor and DIN coax connectors.

Which all shows that there isn't a single "best" coax for all
applications.