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

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.

dansawyeror wrote:
Roy,

Thank you. It is a quick experiment to build a test coax coil and
measure the Q. That should produce enough evidence to test a counterpoise.

In the mean time the research to build a coil out of copper tubing
continues. So far the only alternative I can conceive is to make a
wooden form and wrap the coil on the outside.

Thanks again - Dan

A copper tube will definitely produce an improved Q. If you do make
comparative measurements of ones made from tubing and from coax, please
post the results.

A real problem in maintaining the Q of coils outside in the weather is
keeping water from getting between the turns. Water is very lossy stuff
at HF, and it has a very high dielectric constant. The two combine to
make it a real Q killer if it gets into any region of high electric
field strength. A bit of accumulated dust mixed with the water makes it
worse yet. So if you anticipate leaving the coil on a form and exposed
to the weather, also check the Q when the coil is wet. See
http://www.eznec.com/Amateur/Article...Feed_Lines.pdf for
results of measurements of wet and dry 300 ohm twin lead. It's not quite
the same situation, but the loss mechanism is essentially the same.

I recommend that you do some modeling or just simple calculating, if you
haven't done so already, to see just how high the Q has to be in order
to keep overall loss acceptable.

One final thing to keep in mind -- I've heard reports of poor
performance of elevated verticals being tracked down to badly imbalanced
currents in the radials. Apparently even small physical differences
among the radials can cause one or two to hog all the current. If this
is so, it seems to me that making them more sharply tuned by inductive
loading might make this effect even worse. So when you get the thing up,
I suggest measuring the current in each radial. This is easily done with
a toroid core with a few turns for the secondary and a fairly low R
across the secondary. It's been discussed a number of times on this
newsgroup, the last time quite recently.

Good luck!

Roy Lewallen, W7EL

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

Just my 29 cents worth... I don't think using real thick
copper tube, coax shield, etc, will really be worth the trouble,
vs using a standard wire wound coil on a form. There is some
increase in performance , but overall it will be fairly small unless
the
wire used in the standard coil is very thin. Once you get to about 1mm
thickness, you will have fairly decent performance. 2mm is even better,

and any increase using a thick tubing will basically be a waste of
time.
I think anyway... The spacing of the wires, and keeping water, etc
from between the coil windings is more important. You can wind a
coil using 14 gauge wire and have a very high Q coil, if wound right.
Even 18 or 16 gauge won't be too bad as long as there is the proper
spacing between windings. Again, just my opinion from building various
mobile antenna coils... I'm pretty picky about my mobile antennas, but
I don't bother using "fat" coil conductors. To me, not worth the extra
trouble, weight, etc. I don't think the extra performance is that great
vs any regular wound coil , that has the proper pitch. With the thinner
wire coils, it's using a close winding pitch, with the wires nearly
touching that makes for excess loss. Not really the thin wire in
itself
unless it's super thin like magnet wire. And yes, I avoid braid for
anything
carrying rf. Even my grounding straps are solid.
MK

The devil is in the details. Modeling shows large coils with 1 mm wire have a Q
in the range of a few hundred. On the other hand a coil with 12 mm tubing has a
Q of about 2000. The R of the 1 mm coil is about 6 Ohms while the 12 mm coil is
on the order of 1 Ohm.

Given these model results it says there is a significant difference between 1 mm
and 12 mm coils.

wrote:
Just my 29 cents worth... I don't think using real thick
copper tube, coax shield, etc, will really be worth the trouble,
vs using a standard wire wound coil on a form. There is some
increase in performance , but overall it will be fairly small unless
the
wire used in the standard coil is very thin. Once you get to about 1mm
thickness, you will have fairly decent performance. 2mm is even better,

and any increase using a thick tubing will basically be a waste of
time.
I think anyway... The spacing of the wires, and keeping water, etc
from between the coil windings is more important. You can wind a
coil using 14 gauge wire and have a very high Q coil, if wound right.
Even 18 or 16 gauge won't be too bad as long as there is the proper
spacing between windings. Again, just my opinion from building various
mobile antenna coils... I'm pretty picky about my mobile antennas, but
I don't bother using "fat" coil conductors. To me, not worth the extra
trouble, weight, etc. I don't think the extra performance is that great
vs any regular wound coil , that has the proper pitch. With the thinner
wire coils, it's using a close winding pitch, with the wires nearly
touching that makes for excess loss. Not really the thin wire in
itself
unless it's super thin like magnet wire. And yes, I avoid braid for
anything
carrying rf. Even my grounding straps are solid.
MK

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'.

On Sat, 18 Feb 2006 08:20:38 -0800, dansawyeror
wrote:

The devil is in the details. Modeling shows large coils with 1 mm wire have a Q
in the range of a few hundred. On the other hand a coil with 12 mm tubing has a
Q of about 2000. The R of the 1 mm coil is about 6 Ohms while the 12 mm coil is
on the order of 1 Ohm.

Given these model results it says there is a significant difference between 1 mm
and 12 mm coils.

Hi Dan,

In the details, indeed.

What is the LENGTH of wire in this 6 Ohm resistor? What is the LENGTH
of wire in this 1 Ohm resistor? How many turns are in these "large
coils?" What is their diameter? What is their solenoid length?

Without these details, there is nothing said that is significant.

73's
Richard Clark, KB7QHC

Hm. Just searched the FCC database for a SPEED, MI... and nothing pops.
QRZ.COM gives quite a few (39) hits for SPEED, but none with the first name
or middle initial that you can make a "mike" from.

Hm.

Jim



"Mike Speed" wrote in message
oups.com...

A visit to the kitchen supplies of your local department store will
reveal a large assortment of coil forms and covers. Until recently I
have limited my antenna work to vhf-uhf. In that circumstance I have
frequently punched a snug hole in a plastic container bottom and
placed it in a bell shaped fashion over lumped LC components. It has
worked best for me to leave the bottom open for ventilation.

I have not done it yet but preliminary plans for a base loaded 160
meter vertical have me considering an inverted plastic garbage can as
a weather shield. The EZNEC models may spare me the effort on that
one!

On Sat, 18 Feb 2006 06:26:14 -0800, Roy Lewallen
wrote:

dansawyeror wrote:
Roy,

Thank you. It is a quick experiment to build a test coax coil and
measure the Q. That should produce enough evidence to test a counterpoise.

In the mean time the research to build a coil out of copper tubing
continues. So far the only alternative I can conceive is to make a
wooden form and wrap the coil on the outside.
John Ferrell W8CCW