Cable Shielding Misunderstandings
 

On Mon, 01 Feb 2010 10:32:24 -0600, Lostgallifreyan
wrote:

You just baldly
stated that no cable can fix whatever my problem might be, purely because YOU
can't see what it is.

Quite true. I've asked several times, as have others. So to do it
once again, beyond cost: what is YOUR problem and not someone else's'
that you overheard?

We have your dozen or more suppositions filtered through anonymous and
linked-to sources of indifferent quality that each in their own right
have issues with a spectrum of cable types - but none of them have
been identified as YOUR problem except in generalized, anticipated
anxiety. Can you state one simple quantified characteristic you
currently experience that we can offer a comment to?

73's
Richard Clark, KB7QHC

Cable Shielding Misunderstandings
 

Lostgallifreyan wrote:
Richard Clark wrote in
:

I like their discussion on materials (which fits my own
view on that subject), and on types of RG6. I just started in on the one
about 'quad screens'.
This is called confirmatorial bias which means you justify a thought
on the basis of having found a source that repeats it back to you.


No. It's called 'go see for yourself and tell me based on YOUR judgement if
it's worth revisiting'. If all I wanted was a pat on the head I wouldn't even
have provided a link. Either that info has technical merit, or it doesn't in
which case perhaps you should berate THEM and not me! You expect me to fully
understand details beyond need, yet you won't even take a look at something
signposted right in front of you is an adequate source of info to learn from.
If you can't do that much, why should I trust your judgement? I'll make up my
own mind anyway. Between my efforts, and the other posts here, I have got my
answers.

Face up to the disillusion being presented in this painted into the
corner scenario you are in. The cheapest cable will probably work as
best as any sensible solution has to offer, simply because your
perceived situation hasn't any prospect of being solved by that choice
of line, or any other.


Well, that's just nonsense. Grandstanding nonsense at that. You just baldly
stated that no cable can fix whatever my problem might be, purely because YOU
can't see what it is. You're painting me into a corner. I'm trying to get out
of one. There's no technical point in what you just said. At least I try.
With your knowledge, you should know better.

Other people here, (and in the pages I linked to but you didn't apparently
see) have shown that foil can be so bad, either from tearing, or dubious
contact, that it's unwise to use it except in fixed situations where you know
it will be ok, and not for someone who is likely to want to reuse a cable
while trying new ideas, or to grab more off the reel to try something else.
I've seen that RG6 types vary so much that there's no point citing its name.
Considering I never used to, and already knew that 75 ohms is a result of
precisely controlled sizes and manufacturing tolerances, I was probably
better off before I saw people telling me that distinctions between RG6 and
RG59 were important. Their context isn't the same as mine. My needs are more
likely to be satisfied by a BT data coax than a satellite coax.

As a matter of interest why are you looking at 75ohm cable, when most
people and equipment use 50ohm.

Jeff

Cable Shielding Misunderstandings
 

Richard Clark wrote in
:

Quite true. I've asked several times, as have others. So to do it
once again, beyond cost: what is YOUR problem and not someone else's'
that you overheard?



I've posted more than enough. If that wasn't enough to show what I wanted (I
described it repeatedly in posts dating back over a month), then any more is
just noise so I won't go there.

One thing I will say: Most of the practical guides I read as a kid were a lot
more vague than I have been, yet I was expected to learn from those. Yet now
you say you can't deduce from my posts what I was trying to do? With all your
knowledge to fill in gaps in what you see? Strange. I'll stop now because if
what I said isn't beeing seen, I don't want to compound that by writing
another word.

Cable Shielding Misunderstandings
 

Jeff wrote in :

As a matter of interest why are you looking at 75ohm cable, when most
people and equipment use 50ohm.


Good point, though last I read of that, it was the other way round. :)
(Depends on context). At least, most times I had a device that needed RF
coax, it specified 75 ohms if it didn't come with cables made for it. In my
current case, it's not clearly known what other impedances are involved in
an SWL setup, but they're almost certainly higher than 75 ohms, so going for
a 50 ohm coax seems unwise.

Cable Shielding Misunderstandings
 

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

Cable Shielding Misunderstandings
 

Lostgallifreyan wrote:
Jeff wrote in :

As a matter of interest why are you looking at 75ohm cable, when most
people and equipment use 50ohm.


Good point, though last I read of that, it was the other way round. :)
(Depends on context). At least, most times I had a device that needed RF
coax, it specified 75 ohms if it didn't come with cables made for it. In my
current case, it's not clearly known what other impedances are involved in
an SWL setup, but they're almost certainly higher than 75 ohms, so going for
a 50 ohm coax seems unwise.

Virtually all radio equipment is standardized on 50 ohms, with the
exception of CATV etc.

Any receiving equipment you get will almost certainly have a 50 ohm
(nominal!) input, so any higher antenna impedances will need to be
matched back to 50ohms anyway. How much loss you will encounter by using
75ohm cable will depend on the actual set up.

Jeff

Cable Shielding Misunderstandings
 

Jeff wrote in :

Any receiving equipment you get will almost certainly have a 50 ohm
(nominal!) input, so any higher antenna impedances will need to be
matched back to 50ohms anyway. How much loss you will encounter by using
75ohm cable will depend on the actual set up.


Apparently no-one knows the impedance of a Sangean ATS-909 radio, I've asked
several people, at least one of whom specialises in modifying that radio.
Looks like 1K is best guess based on schematic. Loss won't bother me so much
as SNR. Several people advised that a 15 foot vertical whip is likely to
overload the input so loss is not my main concern.

Cable Shielding Misunderstandings
 

Jeff wrote in
:

As a matter of interest why are you looking at 75ohm cable, when most
people and equipment use 50ohm.

On the basis of that logic, what possible use could there be for 400 ohm
transmission line, and why then is it so very popular?

In the case of the OP's questions, IIRC they relate to a RO application.

To enlighten you, high performance noise optimised ham receivers for
microwave bands are oftenm if not usually designed for a specific input
impedance that is quite different to 50 ohms... yet we use them with 50 ohm
transmission linees.

If you think the choice of 50 ohm line is a no-brainer, you are probably
right.

Owen

Cable Shielding Misunderstandings
 

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.

Cable Shielding Misunderstandings
 

Lostgallifreyan wrote:
Jeff wrote in :

Any receiving equipment you get will almost certainly have a 50 ohm
(nominal!) input, so any higher antenna impedances will need to be
matched back to 50ohms anyway. How much loss you will encounter by using
75ohm cable will depend on the actual set up.


Apparently no-one knows the impedance of a Sangean ATS-909 radio, I've asked
several people, at least one of whom specialises in modifying that radio.
Looks like 1K is best guess based on schematic. Loss won't bother me so much
as SNR. Several people advised that a 15 foot vertical whip is likely to
overload the input so loss is not my main concern.

1K is just a guess as it is just one component in the antenna input
circuit.

Also, nobody observed that the input impedance of the radio can vary
significantly with the setting of the "RF Gain" control.

Crude measurements on a DX-398 show the impedance near 85 ohms at 'max'
gain and near 280 ohms at 'min' gain. The measurements were crude and
the error could be 20%. Use these numbers with caution.

There are no guarantees that the input impedance does not change with
frequency, either.