When a bundle of wires ducks under another in the direction of current flow, the current has to migrate to the outside again, snip

There's no question that it happens


Books, and to a lesser extent the web. Information about this is
snip
I have a great deal of respect for his experience, measurements, and opinions

Again, interesting, but what's been outlined so far is not scientific.
For something of this nature to be of any utility, it must be grounded
in science.

Mike Speed wrote:

Roy Lewallen had written:
When a bundle of wires ducks under another in the direction of
current flow, the current has to migrate to the outside again, snip

There's no question that it happens


Books, and to a lesser extent the web. Information about this is
snip
I have a great deal of respect for his experience, measurements, and opinions

Again, interesting, but what's been outlined so far is not scientific.
For something of this nature to be of any utility, it must be grounded
in science.


The skin effect is most thoroughly grounded in science. What you seem to
be unaware of is that it's *so* well-known that, in any discussion about
RF engineering, the scientific proof of its existence can be 'taken as
read'.

For a detailed scientific proof of the skin effect, try:
http://tinyurl.com/brpq6

That proof is more general - and hence more powerful - than the ones you
find in most engineering texts such as Terman. It demonstrates that, if
an RF current is flowing across *any* conducting surface (not restricted
to any particular shape or cross-section) and also for *any reason* (not
limited to any particular kind of circuit or device) then there will be
a skin effect.

That's the science of it; now back to the engineering.

What Roy said was quite correct. Braid is a kind of composite conducting
surface, made up of the exposed surfaces of the individual strands. The
skin effect means that the outside of the composite surface must always
carry the highest RF current density (amperes per square micron of
cross-sectional area). So whenever the weave of the braid makes an
exposed strand dive below the surface, the RF current must cross over to
the next touching strand that is still exposed. A little way further
along the braid, it will have to cross over again... and again, and
again.

It is hard to visualize exactly how these crossovers happen on a
microscopic scale, but the physics of the skin effect dictate that it
*must* happen somehow. Obviously physical and electrical contact between
the two strands is required. We also know that electrical contact works
better when there is a strong force pushing the two conductors together,
because the force deforms the two surfaces into each other, to give a
greater contact area.

The key fact is that the contact forces between strands in a braid are
very small and unreliable. That means the RF resistance of a length of
braid will be significantly higher than for a smooth conductor with the
same external surface area.

Then it gets worse. Even the thinnest film of corrosion can disrupt the
contact between copper strands in a braid. Unless the current density is
large enough to break down this film, it means the RF current is forced
to flow into the interior of the braid. Again the exact geometry is hard
to visualize, but again the physics dictate that if an isolated
'filament' of current is forced to flow beneath a conducting surface,
the voltage drop per unit length must increase - in other words, the RF
resistance must increase.

Scientific deduction has told us that all these effects must exist.
Whatit cannot tell us is how big they are in real braid, or how
important they are in practice. For that we'll need some measured
numbers.

You have two choices he either look for existing measurements from
people who have demonstrated their competence and scientific approach;
or do it yourself.



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

To extend from Ian's remarks:

In some critical applications, the use of coax cable with braided outer Cu
conductor can cause problems.

RF charge flow (current) in the braid experiences a non-linear circuit
resulting in harmonic distortion or IM or both. Just made coax can have a
very low level of non-linearity with the effect increasing with age (and
probable corrosion). Ag plated Cu braid seems to have less of the
non-linear effect - perhaps because of a poorer mechanism for current to
move from one wire to another.
The effects are small, but can be important in certain applications.
Solid Cu outer conductors have advantages beyond mechanical and
power-handling.
73 Mac N8TT
--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Ian White GM3SEK" wrote in message
snip


Then it gets worse. Even the thinnest film of corrosion can disrupt the
contact between copper strands in a braid. Unless the current density is
large enough to break down this film, it means the RF current is forced
to flow into the interior of the braid. Again the exact geometry is hard
to visualize, but again the physics dictate that if an isolated
'filament' of current is forced to flow beneath a conducting surface,
the voltage drop per unit length must increase - in other words, the RF
resistance must increase.

Scientific deduction has told us that all these effects must exist.
Whatit cannot tell us is how big they are in real braid, or how
important they are in practice. For that we'll need some measured
numbers.

You have two choices he either look for existing measurements from
people who have demonstrated their competence and scientific approach;
or do it yourself.



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

It is hard to visualize exactly how these crossovers happen on a
microscopic scale, but the physics of the skin effect dictate that it
*must* happen somehow.

Theory: "...the physics of skin effect dictate that [it] *must* happen[*]somehow[*]...

Now, compute the consequences of the theory to see if it is right what
it would imply.

Compare those computation results to experiment. If they disagree, the
theory is wrong.

If you can't apply this procedure, your statement cannot be verified
scientifically.

Mike Speed wrote:
It is hard to visualize exactly how these crossovers happen on a
microscopic scale, but the physics of the skin effect dictate that it
*must* happen somehow.

Theory: "...the physics of skin effect dictate that [it] *must* happen
[*]somehow[*]...

Now, compute the consequences of the theory to see if it is right what
it would imply.

Compare those computation results to experiment. If they disagree, the
theory is wrong.

If you can't apply this procedure, your statement cannot be verified
scientifically.


Rubbish!

The word 'theory' has two different meanings - so different, they are
almost the opposite of each other. And you are using the wrong one.

You are using the layman's meaning of 'an unproven speculation' - but in
science, the word means almost the exact opposite. A scientific theory
is "a well-substantiated explanation of some aspect of the natural
world; an organized system of accepted knowledge that applies in a
variety of circumstances to explain a specific set of phenomena" [1].
(In scientific language, an unproven speculation is called a hypothesis
or a conjecture. It is specifically *not* called a theory.)

A *scientific* theory explains the underlying reasons for a huge number
of different experimental and practical observations, so that they all
mesh together and support each other. Theory supports observation; and
observation supports theory. Equally important, the outside edge join up
seamlessly with the theory and observations about related areas of
science. This means that scientific 'theory' is vastly more than mere
speculation: it has the power to predict what will happen in cases we
haven't even looked at yet.

As I said, the crossovers between strands of braid are hard to visualize
and predict in detail - but that is entirely our problem. Our lack of
understanding doesn't change the way things work.

There is no absolute proof that the skin effect will apply to braided
strands, but this is only a very small gap in our knowledge. At both
sides of that gap are situations where we're completely certain it does
apply. Moreover, there is no rational reason to suppose the skin affect
might fail to apply to braided strands.

Based on that solid body of theoretical and practical knowledge about
the skin effect, it only needs a very small amount of additional
speculation to bridge the gap in our knowledge about braid. Applying
what we do know to what we don't, it immediately gives us a clear and
simple explanation why the RF resistance of braid is greater than a
smooth surface, and why it increases dramatically when the braid is
corroded.

This is a perfectly normal application of scientific logic to bridge
small gaps in our knowledge. Since nothing can ever be proved in
absolute terms, I must philosophically decline your challenge to waste
time on modeling it in detail :-)

In terms of strict logic, the onus is on you to find a way to disprove
it and to offer something else in its place. Good luck with that.




[1] http://wordnet.princeton.edu/perl/webwn?s=theory


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

Ian White GM3SEK wrote:
Our lack of understanding doesn't change the way things work.

Neither does our understanding. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Mike, leakage and inductive and gap effects in woven braid has been in
the science literature for over 40 years. To keep this discussion
focused I have emailed one specific reference to Roy. Hopefully Roy has
the time and resources to do the research and make the results available
to us in terms we can understand.

Mike Speed wrote:

When a bundle of wires ducks under another in the direction of current flow, the current has to migrate to the outside again, snip


There's no question that it happens



Books, and to a lesser extent the web. Information about this is

snip

I have a great deal of respect for his experience, measurements, and opinions


Again, interesting, but what's been outlined so far is not scientific.
For something of this nature to be of any utility, it must be grounded
in science.

Mike Speed wrote:

Again, interesting, but what's been outlined so far is not scientific.
For something of this nature to be of any utility, it must be grounded
in science.

I can assure the readers that all the effects I've discussed are soundly
based on very well known principles. Anyone truly interested in the
topic can find ample confirmation of what I've said, although it might
take a bit of digging. The Johnson and Graham text is an excellent place
to start.

What's lacking is good measured data for typical shields, and even
that's going to have limitations because of the wide variations among
cables and manufacturers. But even some rules of thumb will be useful.

But you've shown an interest in the topic. Why don't you make some
measurements of coils made from tubing and from coax shields and report
back?

Roy Lewallen, W7EL

Roy Lewallen wrote:

SNIPPED

What's lacking is good measured data for typical shields, and even
that's going to have limitations because of the wide variations among
cables and manufacturers. But even some rules of thumb will be useful.

But you've shown an interest in the topic. Why don't you make some
measurements of coils made from tubing and from coax shields and report
back?

Roy Lewallen, W7EL

I've been away from that measurement field [integrity of coaxial braid
shields] for almost 15 years. Measured data does exist for the effects
of shield parameters [weave angle, optical coverage, # strands {picks},
wire gauge, etc.] based on using traveling wave excitation of the braid
itself in either quadraxial test fixtures or over a controlled ground
plane with known Zo between the cable and the plane. Test methods are
IEEE defined [after 15+ years I can't recall a specific IEEE Test Method
reference]. These quantify a 'leakage inductance'.

"Roy Lewallen" wrote
I can assure the readers that all the effects I've discussed are
soundly
based on very well known principles.

====================================

Roy, you seem to have forgotten proximity effect.

If one calculates the Q of a coil from HF skin resistance of the wire
and from coil inductance, one gets ridiculously high values of Q.

Other producers of coil calculators appear to have forgotten this too.
That's if they were ever aware of it.

I have a coil, about 4 inches long, about 1.7 inches in diameter, with
about 90 close-wound turns of 1mm diameter wire, which has an
inductance of about 100 micro-henrys. The measured value of Q at 1.9
MHz is about 240.

This makes the proximity effect about 3.5 or 4 times the effect of
simple HF wire skin resistance. This is a large amount.

This is the first time such information has been appeared on a
newsgroup or published in bibles anywhere else. They didn't have Q
meters 120 years ago, in Heaviside's time, when such factors were
first considered.

My findings are incorporated in program SOLNOID3 which estimates Q
(and other characteristics) for coils of various dimensions. There
are, of course, other factors which influence Q which is a relatively
unimportant coil characteristic.

What do you do with Q once you have taken the trouble to find it? The
other more important things will already be apparent.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........