Loading coils: was Dish reflector
 

Art Unwin wrote:
You based your proof of a magnetic wave in a vacuum but it is an
accelerating charge
which obviously must have mass, that is radiation ala the particle.

The accelerating charges are slow-moving electrons.
The RF current moves at the speed of light in the
(conductive) medium. Therefore, the RF current is
associated with photons emitted by the electrons.
Photons have zero rest mass and zero electric charge.
Photons are the particles associated with RF waves.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

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Roy Lewallen wrote:
What is the delay through a physically very small toroidal coil with the
same inductance as the solenoidal coil? Why?

A toroidal coil cannot be modeled using the Dr. Corum
formulas. But I will take a stab at the answer.

In a physically very small toroidal coil, all the
turns are tightly coupled, i.e. the flux caused by
one coil links all of the other windings so the
delay should be quite small. In any case, one
cannot use a current with unchanging phase (referenced
to the source phase) to calculate the delay through
anything. The only phase information left in a
standing wave is in the magnitude. If the current
magnitude at the bottom of the coil is 1.0, the phase
shift is the ARCCOSine of the current magnitude at
the top of the coil for a base-loaded resonant
antenna. Actual phase measurements on the current
in standing-wave antennas is meaningless. We already
know it hardly changes at all with length. EZNEC
confirms that statement.

In an air-core solenoidal coil, like the one w8ji
used, the flux linkage tends to be associated with
adjacent turns so all the flux does not link all
the coils. Tom's coil was 100 turns, 10 TPI, 2"
diameter. The first turn was 10 inches away from
the last turn. The delay through that coil calculates
out to be about 25 nS.

If we setup a 2" transmit coil and a 2" receive
coil 10 inches away in air, the energy transfer
efficiency would be very small. I don't have a
formula for such but I assume one (or more) exists.

Bottom line: There are now formulas for calculating
the Z0 and VF of large air-core loading coils which
are known to be in the family of *slow-wave* devices.

I doubt that a toroidal coil is in the family of
slow-wave devices.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

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steveeh131047 wrote:


Tom,

Yes I have EZNEC and recognise what a great tool it is. Its
predictions were the benchmark against which I tested the various coil
models I read about, and no-one has yet suggested that it can't be
trusted for modelling a helix.

I'm not on some "philosophical quest" - I'm just an old, retired, guy
who still likes learning and wants to understand more about how things
work; I hope that never leaves me! I stumbled on this discussion quite
by chance and tried to understand the various "positions" being taken.
Perhaps I'm over-simplifying, but it seemed to me there was a group
who favoured the transmission-line model and a group against it. I've
tried dispassionately to understand the various arguments and to form
my own conclusions.

Now here's my problem:

* The results I get using a model based on transmission-line analysis
are very close to my EZNEC predictions - not perfect, but way better
than any lumped-element analysis results
* I don't see quantitative, non-empirical, arguments being put forward
to support lumped-element analysis
* I see numeric arguments being put forward by Cecil to support a
transmission-line approach - they look convincing to me and, although
I see a lot of unpleasant personal attacks on him, I don't see any
scientific challenge to his figures
* On the other hand I see folk whose work I rate highly, seemingly
willfully to misunderstand some of the points which Cecil puts forward

Please don't think I'm trying to defend Cecil - I wouldn't be so
presumptuous, and anyway he's old enough to look after himself! I'm
just trying to understand why, what seems to me to be such a
persuasive argument, generates such opposition. Either there's some
glaring technical error here which I haven't yet spotted, or perhaps
there's a long "history" between various "personalities" of which I'm
ignorant?

Still confused,

Steve G3TXQ

There aren't many people who would support a lumped-element analysis on
this newsgroup. Most people know the limitations of using network theory
in these circumstances. The technical arguments against Cecil's approach
were offered a long time ago. This latest is just a flareup that will
soon die down. You shouldn't be confused. The transmission line model of
antennas is well accepted and hoary with age, particularly for
bi-conical antennas (see Schelkunoff). There are a couple of other types
of models with equal validity. If you really want to know the physical
score, though, you have to get an electromagnetics text that discusses
the integral equations that govern antenna behavior. Pay particular
attention to the parts that explain why numerical methods like EZNEC
have to be used for solutions rather than the symbolic math most people
would expect and want.

73,
Tom Donaly, KA6RUH

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steveeh131047 wrote:
. . .
Now here's my problem:

* The results I get using a model based on transmission-line analysis
are very close to my EZNEC predictions - not perfect, but way better
than any lumped-element analysis results
* I don't see quantitative, non-empirical, arguments being put forward
to support lumped-element analysis
* I see numeric arguments being put forward by Cecil to support a
transmission-line approach - they look convincing to me and, although
I see a lot of unpleasant personal attacks on him, I don't see any
scientific challenge to his figures
* On the other hand I see folk whose work I rate highly, seemingly
willfully to misunderstand some of the points which Cecil puts forward

Please don't think I'm trying to defend Cecil - I wouldn't be so
presumptuous, and anyway he's old enough to look after himself! I'm
just trying to understand why, what seems to me to be such a
persuasive argument, generates such opposition. Either there's some
glaring technical error here which I haven't yet spotted, or perhaps
there's a long "history" between various "personalities" of which I'm
ignorant?

Still confused,

Steve G3TXQ

Some of your questions can be answered by doing a google groups search
of this newsgroup for the topic "Current in antenna coils controversy"
in 2003. There are several other lengthy threads at later dates, such as
"Current across the loading coil - from scratch", "Loading coils; was :
Vincent antenna", "Standing-Wave Current vs Traveling-Wave Current", and
a number of threads prior to 2003.

Roy Lewallen, W7EL

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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.

It doesn't take much precision to know that there's
something wrong when two measurements are a magnitude
apart or when someone asserts a 3 nS delay through
a 10 inch long slow-wave solenoid coil with a VF of
0.033. :-)
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

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Tom Donaly wrote:

There aren't many people who would support a lumped-element analysis on
this newsgroup. Most people know the limitations of using network theory
in these circumstances. The technical arguments against Cecil's approach
were offered a long time ago. This latest is just a flareup that will
soon die down. You shouldn't be confused. The transmission line model of
antennas is well accepted and hoary with age, particularly for
bi-conical antennas (see Schelkunoff). There are a couple of other types
of models with equal validity. If you really want to know the physical
score, though, you have to get an electromagnetics text that discusses
the integral equations that govern antenna behavior. Pay particular
attention to the parts that explain why numerical methods like EZNEC
have to be used for solutions rather than the symbolic math most people
would expect and want.

73,
Tom Donaly, KA6RUH

I did and do support lumped element analysis for a very small toroidal
loading inductor, and extensively posted the reasons why in this
newsgroup about six years ago ("Current in antenna coils controversy",
2003). Cecil and Yuri were arguing that the coil would replace some
number of "degrees of antenna" and its current therefore would have a
substantial phase difference between input and output ends. I made and
posted careful measurements to support my statement, after which Cecil
invented his "standing wave current" and went off in various directions.

Roy Lewallen, W7EL

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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)

All program types, including the demo, of EZNEC v. 4.0 and later do
dielectric loading similar to NEC-4. (The method came from sources other
than NEC-4.) Like the NEC-4 implementation, it's of limited accuracy and
usefulness -- it's really good only for thin wire insulation of moderate
permittivity.

And, it's a MoM code, so things not well represented by collections of
wires aren't necessarily modeled well.

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

Roy Lewallen, W7EL

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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

Loading coils: was Dish reflector
 

On Apr 23, 10:10*pm, Roy Lewallen wrote:

Some of your questions can be answered by doing a google groups search
of this newsgroup for the topic "Current in antenna coils controversy"
in 2003.

Roy, I've glanced at some of those references and it looks like
there's years of "catch-up" reading for me :)

It seems like the starting point for one of the earliest discussions
was whether or not there is a variation in current amplitude along the
length of a loading coil, with some pretty strong opinion saying that
there isn't. I don't want to go over old ground, but perhaps you can
give me a simple answer to this one question:

When I use EZNEC to model a 6ft whip above a loading coil (40T, 6"
diameter, 12" long), and look at the current distribution across the
coil at the resonant frequency of the antenna (3.79 MHz), I see 1A at
the base of the coil increasing to 1.07A at the centre of the coil and
then dropping to 0.69A at the top of the coil. My question is: "Can I
believe that I would see a similar current variation in the 'real
world', or is this some failing of EZNEC to model the antenna
properly?"

Regards,
Steve G3TXQ

Loading coils: was Dish reflector
 

steveeh131047 wrote:

Roy, I've glanced at some of those references and it looks like
there's years of "catch-up" reading for me :)

It seems like the starting point for one of the earliest discussions
was whether or not there is a variation in current amplitude along the
length of a loading coil, with some pretty strong opinion saying that
there isn't. I don't want to go over old ground, but perhaps you can
give me a simple answer to this one question:

When I use EZNEC to model a 6ft whip above a loading coil (40T, 6"
diameter, 12" long), and look at the current distribution across the
coil at the resonant frequency of the antenna (3.79 MHz), I see 1A at
the base of the coil increasing to 1.07A at the centre of the coil and
then dropping to 0.69A at the top of the coil. My question is: "Can I
believe that I would see a similar current variation in the 'real
world', or is this some failing of EZNEC to model the antenna
properly?"

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