Cable Shielding Misunderstandings
 

joe wrote:

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.

The Sangean ATS-909 appears to operate no higher than 30 MHz. In the HF
range, antenna efficiency and transmission line mismatch have no
significant effect on the signal/noise ratio (unless the system is
exceptionally lossy and/or the receiver exceptionally noisy, neither
very likely), hence they don't affect your ability to hear stations.

Roy Lewallen, W7EL

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.

It seems very strange that you are taking things to the ultimate when
considering coax cable, whilst considering using a very inferior
portable radio for your reception!!

Jeff

Cable Shielding Misunderstandings
 

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

Cable Shielding Misunderstandings
 

Jeff wrote in :

It seems very strange that you are taking things to the ultimate when
considering coax cable, whilst considering using a very inferior
portable radio for your reception!!


Not really. A few tens of metres of cable whose cost is not more than 3 times
the cheapest of satellite coaxes, and whose total cost is less than half the
lowest cost of that radio when found second-hand, is hardly overdoing it.

Cable Shielding Misunderstandings
 

Lostgallifreyan wrote:
Jeff wrote in :

It seems very strange that you are taking things to the ultimate when
considering coax cable, whilst considering using a very inferior
portable radio for your reception!!


Not really. A few tens of metres of cable whose cost is not more than 3 times
the cheapest of satellite coaxes, and whose total cost is less than half the
lowest cost of that radio when found second-hand, is hardly overdoing it.

I think the point is that radio is very much under-doing it!!!

Jeff

Cable Shielding Misunderstandings
 

Jeff wrote in
:

Lostgallifreyan wrote:
Jeff wrote in
:

It seems very strange that you are taking things to the ultimate when
considering coax cable, whilst considering using a very inferior
portable radio for your reception!!


Not really. A few tens of metres of cable whose cost is not more than 3
times the cheapest of satellite coaxes, and whose total cost is less
than half the lowest cost of that radio when found second-hand, is
hardly overdoing it.

I think the point is that radio is very much under-doing it!!!


Why are you so set against that radio? A lot of people like it (some of them
enough to modify it rather than replace it). What do you recommend? And how
much would it cost? This thread wasn't about that radio but this is worth
pursuing, you seem to have a strong feeling about it. I just bought it
because it seemed like a good cheap base to start from. (Not cheap if I'd had
to buy new, but I purposely avoided that).

Cable Shielding Misunderstandings
 

Lostgallifreyan wrote in
:

Jeff wrote in
:

Lostgallifreyan wrote:
Jeff wrote in
:

It seems very strange that you are taking things to the ultimate when
considering coax cable, whilst considering using a very inferior
portable radio for your reception!!


Not really. A few tens of metres of cable whose cost is not more than
3 times the cheapest of satellite coaxes, and whose total cost is less
than half the lowest cost of that radio when found second-hand, is
hardly overdoing it.

I think the point is that radio is very much under-doing it!!!


Why are you so set against that radio? A lot of people like it (some of
them enough to modify it rather than replace it). What do you recommend?
And how much would it cost? This thread wasn't about that radio but this
is worth pursuing, you seem to have a strong feeling about it. I just
bought it because it seemed like a good cheap base to start from. (Not
cheap if I'd had to buy new, but I purposely avoided that).


Further, the ATS-909 is a fairly old design. Not many appear used on eBay,
and new ones still sell for what I think are excessive prices, from Germany,
Japan and elsewhere. Bad radios surely get sold on as fast as people can
pass them off on someone else. They're unlikely to be in shorter supply
secondhand than new, when they're as old a design as this one is, and very
few second-hand ones remain unsold when an auction ends. I'm not trying to
correlate buyers opinions with the finer points of radio engineering, but it
remains a fact that people would rather keep them and use them than sell them
on, which is fairly convincing as an argument to get one if the price is
good, so I got one. Had to wait a few months too, for an auction that had low
competition, but I think it was worth it. A lot of people documented
modifications, suggesting an enthusiastic technically adept following. That
is one of the things that helped me decide to get one. It means I'm not
reliant on one supplier for info or advice if I need to fix or modify it.

Really poor radios don't go through what I just decribed, they sink without
trace instead of surviving for over a decade with such deep involvement from
so many of their users.

Cable Shielding Misunderstandings
 

Lostgallifreyan wrote:
Lostgallifreyan wrote in
:

Jeff wrote in
:

Lostgallifreyan wrote:
Jeff wrote in
:

It seems very strange that you are taking things to the ultimate when
considering coax cable, whilst considering using a very inferior
portable radio for your reception!!

Not really. A few tens of metres of cable whose cost is not more than
3 times the cheapest of satellite coaxes, and whose total cost is less
than half the lowest cost of that radio when found second-hand, is
hardly overdoing it.
I think the point is that radio is very much under-doing it!!!

Why are you so set against that radio? A lot of people like it (some of
them enough to modify it rather than replace it). What do you recommend?
And how much would it cost? This thread wasn't about that radio but this
is worth pursuing, you seem to have a strong feeling about it. I just
bought it because it seemed like a good cheap base to start from. (Not
cheap if I'd had to buy new, but I purposely avoided that).


Further, the ATS-909 is a fairly old design. Not many appear used on eBay,
and new ones still sell for what I think are excessive prices, from Germany,
Japan and elsewhere. Bad radios surely get sold on as fast as people can
pass them off on someone else. They're unlikely to be in shorter supply
secondhand than new, when they're as old a design as this one is, and very
few second-hand ones remain unsold when an auction ends. I'm not trying to
correlate buyers opinions with the finer points of radio engineering, but it
remains a fact that people would rather keep them and use them than sell them
on,

OR throw them away when they die, or put them is a box in a closet. You
really don't know.


which is fairly convincing as an argument to get one if the price is
good, so I got one. Had to wait a few months too, for an auction that had low
competition, but I think it was worth it. A lot of people documented
modifications, suggesting an enthusiastic technically adept following.

That technically adept following could not provide any useful
information on the input impedance. While actually knowing the impedance
may not be of much value in your endeavors, given the availability of
the schematic, someone could have modeled the input in Spice.


That
is one of the things that helped me decide to get one. It means I'm not
reliant on one supplier for info or advice if I need to fix or modify it.

Really poor radios don't go through what I just described, they sink without
trace instead of surviving for over a decade with such deep involvement from
so many of their users.

A really good radio probably doesn't need a bunch of modifications to
deal with deficiencies.

Some radios hang around because they were built in large volumes.
Quality and performance may mean little.

The point that was made is your radio does not really warrant the effort
your are putting into the antenna. Any variety of quick and easy
antennas may give you adequate results.

So, here is what I see.
1) Worrying about the radio's input impedance is of little value
2) The choice of coax won't make much difference - performance wise, but
copper braid is much easier to solder to.
3) A simple wire antenna at the end of the coax should be sufficient
4) A balun (or un-un for the picky) between the antenna and coax is
probably worthwhile. 9:1 or 10:1 won't make any difference.
5) Figure out what you are going to do about lightning protection.
6) Rather than spend weeks sorting out the details, string up some wire
and listen to the radio.

Cable Shielding Misunderstandings
 

joe wrote in :

The point that was made is your radio does not really warrant the effort
your are putting into the antenna. Any variety of quick and easy
antennas may give you adequate results.


On the other hand, I've been told that a good RF ground and a well-sited
antenna make more difference than anything else. Who cares about the radio, I
can change that. If I don't at least try to get the antenna right, what would
be the point of that change in radio? Where I live I'm unlikely to ever get
much, but spending what amounts to a couple of weeks food money on trying is
worth a go.

So, here is what I see.
1) Worrying about the radio's input impedance is of little value

Of course. Wasn't me who was worrying about it, once I learned a bit about
it, some weeks back. I recently pointed out that striving to use 50 ohms for
an SWL setup that had undetermined impedances didn't matter to me. Am I wrong
both ways?

2) The choice of coax won't make much difference - performance wise, but
copper braid is much easier to solder to.

True. This is something I pointed out, several times. I also pointed out that
given the want to try things, a cable that can be reliably reused is better
than one that can't, so a fragile cheap foil screened cable is more trouble
than its worth.

3) A simple wire antenna at the end of the coax should be sufficient

Maybe. I'll be trying that. There's no room out there to run it without
bringing it close to buldings so to get anything decent it's going to have to
be vertical, so that immediately has a few demands. Can't just shove it up
there, it has to be safe. Tenants tend to have binding conditions for putting
up stuff like that too.

4) A balun (or un-un for the picky) between the antenna and coax is
probably worthwhile. 9:1 or 10:1 won't make any difference.

I intend to try one. The exact winding ratio doesn't bother me that much.
What bothers me is that if I don't mention one someone does, and if I do, I'm
told I shouldn't use one. The degree of contradiction I see suggests I'm not
the only one with some rather vague ideas. I read posts by John Doty that
have persisted a while online in several places since he wrote them. They
make sense, so I'll try them. They basically aim to reduce peaks and nulls in
sensitivity for various points in the HF bands.

5) Figure out what you are going to do about lightning protection.

Already have. It will go direct from antenna to ground through a winding,
there will be no direct current link from antenna to coax. The coax also will
have a 1:1 ferrite transformer at the receiver end.

6) Rather than spend weeks sorting out the details, string up some wire
and listen to the radio.


A simple wire direct to the radio doesn't help here. I'm in a basement, in an
inner city valley. Too much building around me, too much RFI, and too much
scaffolding too, major works being done to the building by the landlords.
Until I can get some undisturbed access to the back yard to wire an antenna,
I have no choice BUT to think of what I can do. The moment actually trying
stuff becomes easier than living with that and reading and posting online,
then I'll do it. I've bought a few things, coax, toroids, a cheap whip mast,
but there's not much I can do out there with anything yet.

One thing I've learned is that for every bit of informed advice, a bit of
informed contradictory advice will be found. Considering how easily people
give vent to it, it seems wise to ask and watch the answers and make up my
own mind. I also prefer to measure three times and cut once. It's usually
cheaper. I have plenty of time and not much money.

Cable Shielding Misunderstandings
 

Not really. A few tens of metres of cable whose cost is not more than 3
times the cheapest of satellite coaxes, and whose total cost is less
than half the lowest cost of that radio when found second-hand, is
hardly overdoing it.
I think the point is that radio is very much under-doing it!!!


Why are you so set against that radio? A lot of people like it (some of them
enough to modify it rather than replace it). What do you recommend? And how
much would it cost? This thread wasn't about that radio but this is worth
pursuing, you seem to have a strong feeling about it. I just bought it
because it seemed like a good cheap base to start from. (Not cheap if I'd had
to buy new, but I purposely avoided that).

It is really more the difference in consideration between the coax and
the radio that strikes me. You are making a huge fuss over the coax, but
appear to have little consideration over the radio which is a far more
important issue. You seem to be set on the Sagen when it is is far from
the best solution, but nit picking over the coax, which in reality most
likely won't make a shred of difference.

It is a perfectly adequate radio for what it was designed to be; a
portable that you take away on holiday to listen to BBC world service
on, but as for using it for anything more it is lacking.

Even you admitted in an earlier post that it was overloaded by anything
more than a whip antenna!!

You would be far better off buying a dedicated HF receiver or a
transceiver with a far superior performance. They are available on Ebay
as well.

Jeff