Loading coils: was Dish reflector
 

On Apr 23, 6:59*pm, K7ITM wrote:
On Apr 23, 2:32*pm, Roy Lewallen wrote:

Jim Lux wrote:
Roy Lewallen wrote:

Let's see how well the principles involved are understood.

What is the delay through a physically very small toroidal coil with
the same inductance as the solenoidal coil? Why?
As in a coil wound on a toroidal magnetic core? or a air cored solenoid
bent in a circle?

I'll say one wound on a magnetic core, simply to keep the size small,
the coupling tight, and the field confined. I don't, however, care how
long a piece of wire it's wound with.

Roy Lewallen, W7EL

There are some other configurations that I personally think are
interesting to ponder. *You might never actually build one this way,
but you'll probably gain some insights considering it: *an antenna,
say a nominally 1/4 wave vertical for 40 meters made from 4" diameter
aluminum tube (irrigation pipe) twenty feet long, resonated with a
loading coil placed inside the tube across a gap of two or three
inches in the pipe. *Capacitance from the coil is almost entirely to
the pipe in this case, not to the world outside the pipe, so the
effect is capacitance in parallel with the coil, not as in a
transmission line where the capacitance is to ground. *That's a
different situation than one where a coil with a diameter much larger
than the antenna conductor is used, where the coil has significant
capacitance to the outside world (e.g. to ground).

It's also worth considering that the charge distribution on an antenna
is dynamic, so it's probably not a good idea to try to analyze the
antenna as if there was the same capacitance to ground from the coil
as there would be if the charge distribution on the antenna wire were
static (that is, the DC case, or at a frequency that's a tiny fraction
of the lowest natural resonance of the antenna system).

Analyzing exactly how even a simple wire antenna works in detail is
far from trivial, and when you add in a coil that has significant
physical size, it further complicates things. *If you use a simplified
model, it can be useful to gain insights into what's going on, but
don't expect the details to be correct. *Be wary about gaining
insights that aren't actually true.

Cheers,
Tom

Tom you are correct in not making a descision on the basis of one
observation but consideration of all the observable facts. The analogy
of this is only with the use of alln segtments of a jigsaw puzzle can
there be confidance of the ensuing picture.
Roy stated that his conclusion was only based on one observable and
where his statement said he did not care beyond that single observable
which matched a predetermined picture. An accurate production of the
ensuing picture comes about only with a matching relationship between
all the parts of the jigsaw and certainly not based on the visual of
one. Your last sentence speaks volumes regarding Roy;'s responce
Regards
Art

Loading coils: was Dish reflector
 

Jim Lux wrote:
Roy Lewallen wrote:

Absolutely true. And it can't handle things like patch antennas or
antennas printed on a PCB.

NEC does OK at microstrip patches with air dielectric (or foam with very
low permittivity). I've used it to model an array of 9 patches and the
port to port coupling calculated by NEC and measured by a VNA were
pretty close (within measurement uncertainty).

Yes, I meant patch antennas with common dielectrics, which are far more
common. It's of course the dielectric that NEC can't account for. I've
designed quite a few antennas on PCB material, but use a fudge factor
based on comparison between measured and EZNEC results of a simple
antenna near the same frequency. This gets me pretty close, but even
this approach wouldn't be adequate if field coupling through the
dielectric is significant.

It's pretty darn slow at this, though (lots and lots of wires in each
patch), I used lumped loads for the matching network model (capacitive
probe feed)

For large models, calculation time goes up as the cube of the number of
segments, so big models can get slow all right. However, EZNEC has
undergone a pretty dramatic speed improvement over time as various code
substitutions and updated compilers have been used, and it's much, much
faster than older NEC compilations. And some versions of NEC have been
similarly updated, so people using different NEC compilations can
experience pretty different calculation speeds.

Roy Lewallen, W7EL

Loading coils: was Dish reflector
 

On Apr 23, 7:42*pm, "Dave" wrote:
"Art Unwin" wrote in message

...

I state again

and again, and again, adding bafflegab and gobbledygook with every
iteration... I love it art, how much deeper can you go with this?

Well I am following the laws of physics. Since you were asleep during
part of the lecture When the examination comes along you can only
rely on baffle gab rather than ALLthe facts given during the lecture.
Fortunately, one can graduate by providing some incorrect answers up
to a limit where as total attention is the only route to a 100 percent
score. You would be wise to accept that your knowledge on physics is
not totally without error because of the baffle gab that you provided
instead
Regards
Art

Loading coils: was Dish reflector
 

On Apr 23, 7:49*pm, Jim Lux wrote:
Tom Donaly wrote:
Jim Lux wrote:
Tom Donaly wrote:
Finally, a modest question: if you have EZNEC, why would you
be wasting time with something inferior? The gold standard is the gold
standard.
Perhaps more the silver or electrum standard.
EZNEC doesn't do dielectric loading, for instance. (unless you get the
Nec4 engine from Roy)
And, it's a MoM code, so things not well represented by collections of
wires aren't necessarily modeled well.

Nothing is perfect, but which is better, EZNEC or the Cecil-Corum method
of modeling antennas?

Depends on what your modeling needs are. *NEC and it's ilk are more
generalized, but take more computational effort. *The Corums have an
analytical approximation that is reasonably good for a certain class of
configurations, although I have to say that for the original Corum
application of Tesla Coils, a lumped approximation gets you almost as
close, at much less work, considering the usual construction tolerances
in a tesla coil. *Modern Tesla Coil modeling is typically done with
either a lumped model or a FEM code that assumes it's axially symmetric
and often an assumed voltage distribution. The assumed distribution the
result of a combination of more detailed analytical modeling and some
experimental measurements on real coils, and speeds up the computation
drastically, while not adversely affecting the accuracy of the results
(that is, the changes are less than a few percent, comparable to
construction tolerances on these things).

73,
Tom Donaly, KA6RUH

You are correct in pointing out that a Tesla coil is a lumped
inductance. A "preponderance" of a lumped load disqualifies the use of
Maxwell's statements
The only metrics he supplied to justify the presence of equilibrium
were distributed loads and no more.
Regards
Art

Loading coils: was Dish reflector
 

On Apr 23, 7:40*pm, Jim Lux wrote:
Roy Lewallen wrote:

Regards,
Steve G3TXQ

Yes, you would see this in the real world. EZNEC does a very good job of
modeling a wire antenna with a loading coil, provided that you model the
coil as a wire helix rather than lumped "load", and you can trust the
results. As I've implied, a lumped load is quite a good model for a
physically small, essentially non-radiating loading coil like a toroid
on a magnetic core.

Roy Lewallen, W7EL

which makes perfect sense...
NEC is a MoM code and is ideally suited to calculating the current
induced in one wire by the currents in other wires. I should think it
would do an excellent job modeling a air core solenoid, especially if
the wire diameter is small compared to the spacing between turns,etc,
assuming that you don't get into numerical precision problems.

If eznec does not take into account dielectric loading then the
application is not in equilibrium and thus Maxwells laws are not
applicable. Maxwells laws are based solely on the presence of
equilibrium or accountability of all loads applied which when all are
added equals zero per Newtons laws.
Regards
Art

Loading coils: was Dish reflector
 

On Apr 23, 8:43*pm, Roy Lewallen wrote:
Jim Lux wrote:
Roy Lewallen wrote:

Absolutely true. And it can't handle things like patch antennas or
antennas printed on a PCB.

NEC does OK at microstrip patches with air dielectric (or foam with very
low permittivity). *I've used it to model an array of 9 patches and the
port to port coupling calculated by NEC and measured by a VNA were
pretty close (within measurement uncertainty).

Yes, I meant patch antennas with common dielectrics, which are far more
common. It's of course the dielectric that NEC can't account for. I've
designed quite a few antennas on PCB material, but use a fudge factor
based on comparison between measured and EZNEC results of a simple
antenna near the same frequency. This gets me pretty close, but even
this approach wouldn't be adequate if field coupling through the
dielectric is significant.
Exactly. Maxwells law application is solely on the condition of all
forces be accounted for such that the summation is equal zero.
Omission of consideration of a force that is present prevents the
summation from equaling zero which means the creation of an error.
Very simple my dear Watson. This is tantamount to creating an abitrary
border where one omits recording the full amount of flux created.
Regards
Art

It's pretty darn slow at this, though (lots and lots of wires in each
patch), I used lumped loads for the matching network model (capacitive
probe feed)

For large models, calculation time goes up as the cube of the number of
segments, so big models can get slow all right. However, EZNEC has
undergone a pretty dramatic speed improvement over time as various code
substitutions and updated compilers have been used, and it's much, much
faster than older NEC compilations. And some versions of NEC have been
similarly updated, so people using different NEC compilations can
experience pretty different calculation speeds.

Roy Lewallen, W7EL

Loading coils: was Dish reflector
 

K7ITM wrote:
On Apr 23, 2:32 pm, Roy Lewallen wrote:
Jim Lux wrote:
Roy Lewallen wrote:
Let's see how well the principles involved are understood.
What is the delay through a physically very small toroidal coil with
the same inductance as the solenoidal coil? Why?
As in a coil wound on a toroidal magnetic core? or a air cored solenoid
bent in a circle?
I'll say one wound on a magnetic core, simply to keep the size small,
the coupling tight, and the field confined. I don't, however, care how
long a piece of wire it's wound with.

Roy Lewallen, W7EL

There are some other configurations that I personally think are
interesting to ponder. You might never actually build one this way,
but you'll probably gain some insights considering it: an antenna,
say a nominally 1/4 wave vertical for 40 meters made from 4" diameter
aluminum tube (irrigation pipe) twenty feet long, resonated with a
loading coil placed inside the tube across a gap of two or three
inches in the pipe. Capacitance from the coil is almost entirely to
the pipe in this case, not to the world outside the pipe, so the
effect is capacitance in parallel with the coil, not as in a
transmission line where the capacitance is to ground. That's a
different situation than one where a coil with a diameter much larger
than the antenna conductor is used, where the coil has significant
capacitance to the outside world (e.g. to ground).

It's also worth considering that the charge distribution on an antenna
is dynamic, so it's probably not a good idea to try to analyze the
antenna as if there was the same capacitance to ground from the coil
as there would be if the charge distribution on the antenna wire were
static (that is, the DC case, or at a frequency that's a tiny fraction
of the lowest natural resonance of the antenna system).

Analyzing exactly how even a simple wire antenna works in detail is
far from trivial, and when you add in a coil that has significant
physical size, it further complicates things. If you use a simplified
model, it can be useful to gain insights into what's going on, but
don't expect the details to be correct. Be wary about gaining
insights that aren't actually true.

Cheers,
Tom

I didn't do exactly as you say, Tom, but I did take a homemade coil
(140mm long, 155mm in diameter) and wrapped it with a sheet of .005 inch
copper foil separated by a couple of strips of double sided foam tape.
It acted more like a transmission line than the bare coil, but it had
some peculiarities that made its behavior puzzling to say the least.
73,
Tom Donaly, KA6RUH

Loading coils: was Dish reflector
 

On Apr 23, 8:04*pm, Cecil Moore wrote:
Tom Donaly wrote:
Nothing is perfect, but which is better, EZNEC or the Cecil-Corum method
of modeling antennas?

EZNEC and the Cecil-Corum method are in agreement.
Correct but the solution is in error as equilibrium demands that the
radiator is in equilibrium ie equal to a period or multiples there of.
Both of the above determined that resonance alone without reference
to the period was a reflection of equilibrium.
This may account for Krauss's error in determiming gain via Maxwells
law by determining a half wave was in equilibrium and thus determined
gain was approx
3db more than that was actually attainable. Later measurements mad by
others show that Kraus over estimated the gain by 100 % the equivalent
of 3db or the doubling of gain or energy expenditure.This error is
actually a reflection of pitch of windings which is a fraction of the
possible generated electric field for maximum efficiency.

Lesson
One must account for all actual vectors used within a boundary for
equilibrium
where the addition of all vectors finish at the starting point of the
period. The idea that a coil does not radiate is fallacious since the
charge is still accelerating but to a lesser extent than when current
moves along a straight wire. The total velocity factor is the average
velocity of that of a slow wave plus the velocity without the addition
of lumped loads ie straight radiator addition.This being equal to the
total length of wire
required with tha absence of lumped loading ie straight.
Regards
Art




--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

Loading coils: was Dish reflector now: Delay Lines
 

"Jim Kelley" wrote in message
...

For a more quantitative illustration of how distributed reactance in
transmission lines causes delay see
http://www.rhombus-ind.com/dlcat/app1_pas.pdf

73, ac6xg

In graduate school, more years ago than I care to admit, I scrapped a
surplus computer for parts. The computer had been custom built for the
Savannah River nuclear facility. In addition to the many hundreds of
2N404A germanium transistors, I found the core memory made of ferrite
cores about 0.1 inches in diameter and about 30 mils thick. But the most
unusual thing, at least to me, was a flexible coaxial cable about six
feet long made of a ferrite-loaded rubber core wound with 40 gauge
enameled wire, wrapped in a thin cellulose acetate film (Scotch tape?),
covered with a braid shield with a vinyl covering. Of course, it was a
distributed delay line. I never measured its impedance and delay
properties accurately, but the cable had a significant delay that could
easily be seen on a 5 MHz bandwidth scope. Even with an approximate
termination, the cable's losses were quite high.

This was obviously a commercial cable, but in all of the years since, I
have never seen anything like it.

By the way, after dismantling one panel covered with terminal strips, I
found a typed note inside that said, "Built by pigmies in darkest
Africa."

--
73, Dr. Barry L. Ornitz WA4VZQ

Loading coils: was Dish reflector
 

On Thu, 23 Apr 2009 15:29:04 -0700, Jim Kelley
wrote:

Cecil Moore wrote:
Jim Kelley wrote:
... and as any good dry labber knows, it's a dead giveaway to report a
precision greater than one can actually measure. :-)

I have reported no precision - my 100 MHz scope has
not been calibrated since I retired.

Precision is the number of sig figs. You "might" have calculated three,
rounded up, and reported two.

Precision is NOT accuracy. Resolution is NOT precision. Accuracy is
defined with precision to a resolution.

You can state a value with great precision and be 100% in error.

100 V is quite precise; "about" 100 V is less precise.

100 V has three places of resolution.

If the true value is actually 201.45 V then 100 V is precise, somewhat
resolved, but inaccurate. On the other hand, 201.45 V is very
precise, highly resolved, and accurate to within 0.005 V (if we are to
trust it as a reference) or 25 parts per million.

I can anticipate the objection (to confound my statement above) that
100 V has both resolution and precision. True, but that objection
would miss the point. Some standards are nominal (or cardinal) values
such as an 1 MHz URQ-23 frequency standard: 1 place of resolution, but
highly precise with an accuracy of (from my experience) of 6 parts per
trillion (after calibration against a cesium beam standard).

I can anticipate the fine objection that the nominal value of 1 is
actually 1.00000000000. Again, true, but in a world where you own an
URQ-23 (and no one has access to HP 5071 cesium clocks), then you get
to snub that objection and demand: "It IS exactly 1!"

73's
Richard Clark, KB7QHC