Cable Shielding Misunderstandings
 

Accumulated from various threads and contributions that demonstrates
anxiety, not research:

As far I can tell from advice on HF, the thin foil doesn't shield as well at
HF as a thicker braid with good physical coverage.

It sounds like you should stop listening to advice on HF.

I found an RG6 at low cost with copper braid and Al foil (more likely
metalised plastic film)

....and hence mostly likely NOT RG6. RG-anything is barely more than a
public domain trademark. This been hammered to death already so any
appeal to nomenclature should be confined solely to the physical
attributes of wire radius and shield inner radius; and NOT the number
of shields, NOT the coverage of the shield, NOT the property of the
wire being stranded or solid, NOT the property of the shield being al
vs. cu. Everyone of those prohibited-for-discussion characteristics
varies between manufacturers sharing the same nomenclature.

The standard for cable tv and satellite instalations is RG6 "quad shield",
which has a less dense braid, but a (almost) 100% aluminum foil shield.

Foil shield is a gap filler, NOT a conductor in the conventional sense
of long runs. There are no coaxial cables that have only a foil
shield (a plastic carrier of a metal deposition) that are useful for
any antenna work. Hence, the property of foil alone does not bring
any useful quality to the discussion.

As far I can tell from advice on HF, the thin foil doesn't shield as well at
HF as a thicker braid with good physical coverage.

Abysmal sources of information should not be returned to. "Thin foil"
is a gap filler, not a shield.

Besides, most advice out there implies I
have to buy it and try it to be sure, which is stupid because it's cheaper
and faster to get a better cable! RG6 is specified for UHF, I want HF.

RG6 does not have a specification for frequency ranges outside of
loss. In that regard, RG6 is eminently preferable for use at HF over
UHF for that one consideration alone. There are cables of other
physical geometries (about the only thing that counts in this
discussion) that exceed the performance of generic RG6.

http://www.abccables.com/info-rg59-vs-rg6.html is one of the more descriptive
texts I read.

Interesting? Quite banal, in fact, when one stumbles over such
statements as:
"A basic rule of thumb is to use RG6 for any
Rapid Frequencies, and use RG59 for
video frequencies."
Now there's an authoritative standard you can take to the bank (if it
is AIG). Perhaps they meant "Vapid Frequencies."

I must admit I do not understand the theory that foil is worse than
braid at lower frequencies, foil gives 100% coverage and is usually in
addition to braid. Even if it the thickness of the foil that is in
question, I don't see how, according to the article that you linked to,
it " don't(sic) have the proper type of shielding ".

Foil, as pointed out, is in addition to standard shielding. Foil
bridges the gaps between the wires composing the weave of the shield.
Those bridges are highly conductive over the very short distance
between adjacent wires, but as a conductor, foil is miserable as a
sole conductor. That is why foil shields that are the sole shield
have what is called a "drain wire" running the length of the cable. It
is quite obvious that such cables have enormous loss per foot in
transverse mode, but these shielded cables do not operate in that
fashion as they are almost exclusively supporting paired conductors
(twisted pairs that are the signal carriers). Hence, these
applications of foil/drain-wire are limited to low signal use where
the shield will encounter small fields. Even then, they can be
marginal.

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.

Even with this shortfall, one has to consider. On the one hand you
have a 90% coverage cable that gets signal into it. You add a poor
conductor like a metal deposition plastic covering (aka foil) and it
reduces that specific leakage by 3dB. To buy that 3dB in additional
conventional wire coverage may boost the product cost 10% whereas
adding a foil boosts cost only 1%. By reputation around the pickle
barrel, the foil is still a poor solution, but in a particular
application it bought you 3dB that you might have walked away from.

This, of course, is a fantasy scenario to illustrate how a technical
decision is weighed against cost and need. Unfortunately this fantasy
scenario exceeds the technical discussion found in:

I like that BT2002 with the double copper braid, but I'm not yet sure
if the difference justifies the cost

Sole cost based decisions for technical problems rarely prove useful.
You are going to have to decide whether you can accept the performance
you thought you paid for, or pay for the performance you need. As you
have not actually specified any quantitative characteristic, you are
facing either disappointment or illusion.

73's
Richard Clark, KB7QHC

Cable Shielding Misunderstandings
 

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?

Cable Shielding Misunderstandings
 

Richard has debunked much of the FUD about the foil/braid outer conductor.

Not wanting to hijack the thread, but for all the paranoia about whether
the outer conductor works properly and at what freqeuncies, the discussion
has ignored the risk of poorer performance at low HF for such cables with
CCS inner conductors if the cladding is inadequate.

This effect does not seem to worry most ham users of CCS ladder line, so
perhaps ignoring it for RG6 or RG59 is in keeping with that.

I use RG6 selectively, selected cable and selected connectors on selected
applications. There is potentially a good match, prospect of low cost and
good performance.

Inspect a sample of the cable you are considering, whether it is RG59 or
RG6, or any no-name cable for that matter. If through braid leakage is
critical to your application, you are more likely to find no-name RG59 with
higher through braid leakage than RG6.

Owen

Cable Shielding Misunderstandings
 

Owen Duffy wrote in
:

Not wanting to hijack the thread, but for all the paranoia about whether
the outer conductor works properly and at what freqeuncies, the discussion
has ignored the risk of poorer performance at low HF for such cables with
CCS inner conductors if the cladding is inadequate.


I just wanted to focus on one thing at a time, given that types of RG6 seem
to proliferate like types of dog. One thing I'm asking sellers, if it's not
clearly described, is if the core is magnetic or otherwise obviously steel.
I've seen cables that are, and I already intend to avoid them. I restricted
my questions to those where I sas NOT sure of subsequent action.

Cable Shielding Misunderstandings
 

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!

Cable Shielding Misunderstandings
 

Richard Clark wrote in
:

Abysmal sources of information should not be returned to.

Why return when we can find more and more new ones each day? :)

Right now I'm looking he
http://www.bluejeanscable.com/articles/index.htm
If there is any particular reason NOT to return there, please let me know. So
far it looks good. 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'.

Cable Shielding Misunderstandings
 

On Sun, 31 Jan 2010 15:47:41 -0600, Lostgallifreyan
wrote:

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.

You still haven't offered the quantification of one characteristic you
want to achieve other than cost. Why is this? Clearly a cost basis
is wildly off the rails and you offer nothing else to compete against
its failure.

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.

73's
Richard Clark, KB7QHC

Cable Shielding Misunderstandings
 

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

Cable Shielding Misunderstandings
 

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

Cable Shielding Misunderstandings
 

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.

Cheap cable meant for satellite, which IS wht I'll get if I take your
suggestion of buying the cheapest cable called RG6, is a sure recipe for
crappage. Cheap satellite signal cable isn't meant to perform beyond its
specific purpose, and I never expect it to. Of course I'd end up
disillusioned AND disappointed if I chose to use it as general purpose RF
cable. But what did you really want? To help? Or to set me up for failure as
part of some bizarre exercise? I guess only you can know the answer to that,
I don't really care. Considering the cost of any 'RG6' that really qualifies
as adequate, i.e. solid metal foil wrapped by one braid of tightly covering
copper, there's little choice between that and the BT2002 I found, and the
latter will take punishment better, if punishment is the order of the day.

I'll be choosing a double-braided copper, each braid of the '95% coverage'
type, close and compact. I don't care that it costs twice as much, I can
trust it to have decent screening for any circumstances I'm likely to meet
from AF to UHF, and it's thin and flexible, and I can expect it to take
weather and rough handling and be fit for reuse when I want to do that. And
because BT use so much of it I can hitch a ride on the economy of scale that
drives the price down. For what it is, it's better value than the cheapest.

End of discussion.

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.

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

Cable Shielding Misunderstandings
 

Jeff wrote in :

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.


Lacking what, specifically? I wanted a general purpose radio with full AM
coverage to 30 MHz, and I wanted it to be cheap and portable. Then I wanted
to give it a decent chance of getting signals when I'm not carrying it
around.

As for a radio that that is only fit for getting BBC World Service, are you
sure you're not confusing the ATS-909 with whatever Sangean's original was,
as cloned by Roberts with model R9921? That really IS a basic radio designed
for that purpose, the ATS-909 does more.

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


Not the point. It's easier to attenuate than to do almost anything else. Even
the radio itself can do that.

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.


If you know of any that fit my description above, please name them.

Cable Shielding Misunderstandings
 

Lostgallifreyan wrote:
Jeff wrote in :

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.


Lacking what, specifically? I wanted a general purpose radio with full AM
coverage to 30 MHz, and I wanted it to be cheap and portable. Then I wanted
to give it a decent chance of getting signals when I'm not carrying it
around.

Chiming in late on this one.

The antenna isn't usually the limiting factor on modern radios. You'll
likely do as well with a random wire as a seriously engineered system.
Since you're only receiving, this is the case.

If you are wanting 500 KHz to 30 MHz, and you want full coverage, you'll
be hard pressed to beat a random length dipole and maybe give yourself a
little tuning cap on your end if you like. Just put up as much wire as
your space will permit, and there you go. This assumes that you use
ladder line to feed, not coax. For such a wide range antenna, ladder
line is the way to go.

That's going to wring out just about the last bit of performance you can
expect, unless you want to go to the bitter edge and construct
directional antennas. At the 500 KHz end, that will be a tad difficult.

Now for your application, the performance difference between a chunk of
wire, my random length dipole, and some directional gastraphagus will be
surprisingly little.

Use, or do not use the advice.

- 73 de Mike N3LI -

Cable Shielding Misunderstandings
 

Lostgallifreyan wrote:
Jeff wrote in :

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.


Lacking what, specifically? I wanted a general purpose radio with full AM
coverage to 30 MHz, and I wanted it to be cheap and portable. Then I wanted
to give it a decent chance of getting signals when I'm not carrying it
around.

As for a radio that that is only fit for getting BBC World Service, are you
sure you're not confusing the ATS-909 with whatever Sangean's original was,
as cloned by Roberts with model R9921? That really IS a basic radio designed
for that purpose, the ATS-909 does more.

Yes, but you were not talking about the other bands that it covers, you
only mentioned HF.


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


Not the point. It's easier to attenuate than to do almost anything else. Even
the radio itself can do that.

So why are you so worried about the co-ax and SNR, if you add an
attenuator in order to make the radio work properly you will also
attenuate any interference (and degrade your SNR).


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.


If you know of any that fit my description above, please name them.

Virtually any comms receiver will give you coverage of AM to 30MHz, many
also have Band 2 vhf as well, they are too numerous to mention, but have
a look at this link and pick the ones that actuall have good RF performance:

http://www.eham.net/reviews/products/8

Jeff

Cable Shielding Misunderstandings
 

Jeff wrote in
:

Yes, but you were not talking about the other bands that it covers, you
only mentioned HF.


Fair enough, though I had mentioned it in earlier posts.


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


Not the point. It's easier to attenuate than to do almost anything
else. Even the radio itself can do that.

So why are you so worried about the co-ax and SNR, if you add an
attenuator in order to make the radio work properly you will also
attenuate any interference (and degrade your SNR).


Because I want to reduce the noise from stuff in the bulding compared to
whatever hits the whip antenna. Sure, attenuation might reduce SNR in a noisy
resistance (or subsequent gain stage) but NOT due to due to simple shrinkage
of scale (R = Ratio...), but that's why I want to get the SNR higher to start
with. It's the separation of internal noise signals from external wanted
signals that matters, same as for anyone using coax. Surely it's not suddenly
wrong because I'm doing it? If so, this isn't about science anymore.

If you know of any that fit my description above, please name them.

Virtually any comms receiver will give you coverage of AM to 30MHz, many
also have Band 2 vhf as well, they are too numerous to mention, but have
a look at this link and pick the ones that actuall have good RF
performance:

http://www.eham.net/reviews/products/8


Thanks, that will be useful. The ATS-909 is just a starting point. I want to
have tried it, even if I just sell it on. (Was why I bought it used, that way
I won't lose out).

Cable Shielding Misunderstandings
 

Lostgallifreyan wrote:

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!

Of course it has no technical merit, it is just words! Some of them are
pretty ambiguous too, like that "video Frequencies" bit. Are the video
frequencies they refer to The frequencies that television signals are
broadcast or are tehy the frequencies that a video signal uses. THere is
a difference.

There's more. From that paragraph above, your not asking us to do your
research for you, are you?

- 73 de Mike N3LI -

Cable Shielding Misunderstandings
 

Richard Clark wrote:

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


I read that as "suppositories" instead of suppositions, Richard. Perhaps
both may be right? 8^)

Cable Shielding Misunderstandings
 

Michael Coslo wrote in
:

Lostgallifreyan wrote:

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!

Of course it has no technical merit, it is just words! Some of them are
pretty ambiguous too, like that "video Frequencies" bit. Are the video
frequencies they refer to The frequencies that television signals are
broadcast or are tehy the frequencies that a video signal uses. THere is
a difference.

There's more. From that paragraph above, your not asking us to do your
research for you, are you?

- 73 de Mike N3LI -


It's not that bad. I've seen far worse. And no, I'm not asking you or anyone
else to do anything. When I'm not checking here and replying I'm reading
other stuff. I'll doing more of that because it doesn't argue so much.

Cable Shielding Misunderstandings
 

Michael Coslo wrote in
:

Richard Clark wrote:

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


I read that as "suppositories" instead of suppositions, Richard. Perhaps
both may be right? 8^)


Careful. Don't worry about mine, worry about the one you might seem to be
licking. :)

Cable Shielding Misunderstandings
 

On Wed, 03 Feb 2010 14:06:05 -0500, Michael Coslo
wrote:

Richard Clark wrote:

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


I read that as "suppositories" instead of suppositions, Richard. Perhaps
both may be right? 8^)

Reminds me of the old joke about the Engineer's problem with
constipation: he worked it out with a slide rule.

Mathematician, pencil
Draftsman, compass
antenna designer, gin pole
..... and so on.
you get the point.

73's
Richard Clark, KB7QHC

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.

Perhaps there is a reason. It isn't terribly important at all.

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.

Not to be overly precise, but the whip won't overload your radio, strong
signals might. Those little radios are pretty sensitive.

So what are you trying to do here, aside from get an external signal
into a radio? If you want to have an audiophile grade antenna system,
you need to go out and get some good hardline coax*. You can also make
some measurements to determine the exact input impedance of your radio,
then construct a balun to match it to the rest of the system. If
measurement isn't your thing, you can probably get by with a 9:1 balun,
as a back of the envelope calculation.

Then if you can put them in, you need around 120 radials that you use
for the ground on your antenna. You can either elevate them, lay them on
the ground, or shallow bury them. If you have a wife the third option is
probably what you want to do.

There are other little tweaks, such as silver contacts, a good quality
tuner, and probably some I haven't thought about yet. Some still hold
out for low oxygen copper.

Do all that, and you can still do pretty close to the results with a
long wire hanging out of your window.

Now if you get a communications grade radio, some of my less tongue in
cheek suggestions might help more. But make no mistake, you are deep
into the world of diminishing returns.

note 1.

Coolest hardline I ever saw was at a TV station. It was about a foot in
diameter on the jacket, and I didn't see the center conductor, but my
best guess is that it was around 2 inches in diameter.

note 2.

I'm not trying to be rude, but you've been getting some good info in
here, you're just not taking it. Reminds me of some of the students
coming out of college these days with a nice fresh bachelor's degree.
They don't accept input, and think they should be promoted for showing
up on time.

- 73 de Mike N3LI -