Owen Duffy wrote:
On Sun, 19 Mar 2006 04:57:54 GMT, Cecil Moore
wrote:

Please see http://www.qsl.net/w5dxp/current.htm

I refer to the diagram in the section entitled "What EZNEC Says About
Current Distribution Using Inductive Loading Stubs"

You use the diagram to assert that there is "not a lot of difference
between inductive loading stubs and loading coils" by comparing the
current distribution with another case.

You show graphically the current on each side of the stub. You do not
show the current in each wire of the stub or the sum of the currents
in the stub.

EZNEC calculates the currents in each wire of the stub? Aren't those
currents a relevant detail that you have omitted from the diagram.

I don't quite follow the theory on the web page, but what does it
predict should happen if there were no antenna at all, and the inductor
were connected to a simple series RC circuit instead of the whip?

I've taken the EZNEC model available there and modified it by replacing
the whip with a wire to ground from the top of the coil
(http://eznec.com/misc/test316_modified.EZ). I added a lumped impedance
in that wire to represent the impedance of the vertical wire I
deleted(*). The feedpoint impedance is the same as for the original
model, and the currents at the top and bottom of the inductor are almost
exactly the same as for the original model. Can the traveling wave
analysis be used to explain the inductor currents in this model? Is
traveling wave analysis necessary to explain them?

(*) The impedance inserted in the new wire isn't equal to the impedance
of the top wire driven against ground. The reason is that the new wire
to ground does radiate some, does have significant impedance itself, and
does interact with the inductor. The modified system, however, is quite
obviously very different in radiating properties from the original, and
isn't too different from a lumped RC load.

Roy Lewallen, W7EL

Roy Lewallen wrote:
. . .
I've taken the EZNEC model available there and modified it by replacing
the whip with a wire to ground from the top of the coil
(http://eznec.com/misc/test316_modified.EZ). I added a lumped impedance
in that wire to represent the impedance of the vertical wire I
deleted(*). The feedpoint impedance is the same as for the original
model, and the currents at the top and bottom of the inductor are almost
exactly the same as for the original model. Can the traveling wave
analysis be used to explain the inductor currents in this model? Is
traveling wave analysis necessary to explain them?

(*) The impedance inserted in the new wire isn't equal to the impedance
of the top wire driven against ground. The reason is that the new wire
to ground does radiate some, does have significant impedance itself, and
does interact with the inductor. The modified system, however, is quite
obviously very different in radiating properties from the original, and
isn't too different from a lumped RC load.

Notice that the current into the grounded wire at the bottom of the coil
is about 1 amp, and the current going into ground at the grounded end of
the added wire is about 0.56 amp. So where is the extra current for the
coil bottom wire coming from? The answer is displacement current from
the coil. That is, the coil is capacitively coupled to ground, and this
causes displacement current from the coil to ground. The effect is
greatest at the end of the coil which is farthest from the source. A
decent model of the coil is an L network, with a series L, and a shunt C
to ground from the far end. This is all that's necessary to explain the
drop in current from the bottom to the top; no current waves, standing
or traveling, no transmission line analysis are required.

If you're not convinced, try this. Change the ground type to free space.
Then connect the bottoms of the two formerly grounded wires together
with another wire. You'll see that the current at the top of the coil is
now very nearly the same as at the bottom. We haven't changed any waves,
antenna lengths, or anything else related to antennas or waves. All
we've done is to eliminate the other side of the capacitor -- we've
removed the C in the equivalent lumped L network.

A simple lumped component model explains the difference between grounded
and free space models just fine. How well does the traveling wave theory
do at it?

Roy Lewallen, W7EL

Roy wrote, "... That is, the coil is capacitively coupled to ground,
and this
causes displacement current from the coil to ground."

In fact, if there were no such current -- if there were no capacitance
from the coil to the world outside the coil -- then the time delay
through the coil, calculated from tau = sqrt(L*C), would be zero. It
is exactly this current that allows there to be a transmission-line
behaviour and a corresponding time delay.

That's not to say, however, that a physically very small loading coil
with practically no capacitance to ground would not work as a loading
coil. It just wouldn't have a transmission line behaviour worth
mentioning.

It is also exactly this displacement current from a large coil that
allows the current at one end of the coil to be substantially different
from the current at the other end.

Cheers,
Tom

K7ITM wrote:
In fact, if there were no such current -- if there were no capacitance
from the coil to the world outside the coil -- then the time delay
through the coil, calculated from tau = sqrt(L*C), would be zero. It
is exactly this current that allows there to be a transmission-line
behaviour and a corresponding time delay.

Tom, have you read what Dr. Corum had to say about that on
page 8 of http://www.ttr.com/corum/index.htm? Here's a partial
quote: "The problem has been that many experimenters working
self-resonant helices have pursued the concept of coil self-
capacitance without really understanding where the notion
comes from or why it was ever invoked by engineers."
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
K7ITM wrote:
In fact, if there were no such current -- if there were no capacitance
from the coil to the world outside the coil -- then the time delay
through the coil, calculated from tau = sqrt(L*C), would be zero. It
is exactly this current that allows there to be a transmission-line
behaviour and a corresponding time delay.

Tom, have you read what Dr. Corum had to say about that on
page 8 of http://www.ttr.com/corum/index.htm? Here's a partial
quote: "The problem has been that many experimenters working
self-resonant helices have pursued the concept of coil self-
capacitance without really understanding where the notion
comes from or why it was ever invoked by engineers."

Cecil,

You keep trying to drag something from a self-resonant helice into a
loading coil discussion.

The two are nearly at opposite extremes in behavior, but even at that
the self-resonant helice can be analyzed with standar L/C analysis.

It's just another way to analyze things, and it's just one way of doing
it.

73 Tom

wrote:
You keep trying to drag something from a self-resonant helice into a
loading coil discussion.

My 75m bugcatcher coil is self-resonant around 6.7 MHz so it
meets the minimum requirement for a Tesla coil at 6.7 MHz.
4 MHz is 60% of the Tesla coil self-resonant frequency. The
coil is known to possess a 90 degree delay at 6.7 MHz. That
would make the delay ~60 degrees at 4 MHz. Dr. Corum suggests
a 15 degree limit for the lumped-circuit model.

The two are nearly at opposite extremes in behavior, but even at that
the self-resonant helice can be analyzed with standar L/C analysis.

Unfortunately, that's not true. One cannot assume the
presuppositions of one's model without proof. The "standar L/C
analysis " assumes the delay through a coil is zero, i.e.
faster than light. The delay through a self-resonant coil
is known to be 90 degrees. That "standar L/C" model is
invalid at the self-resonant frequency where the coil
is acting like a 1/4WL open-circuit transmission line
stub.
--
73, Cecil http://www.qsl.net/w5dxp

K7ITM wrote:
Roy wrote, "... That is, the coil is capacitively coupled to ground,
and this
causes displacement current from the coil to ground."

In fact, if there were no such current -- if there were no capacitance
from the coil to the world outside the coil -- then the time delay
through the coil, calculated from tau = sqrt(L*C), would be zero. It
is exactly this current that allows there to be a transmission-line
behaviour and a corresponding time delay.

Yes. And this, not the C across the coil, is what should be used for
transmission line formulas when treating an inductor as a transmission
line. When the ground was removed and replaced by a wire, the
transmission line properties of the coil changed dramatically, while the
C across the coil didn't change significantly.

That's not to say, however, that a physically very small loading coil
with practically no capacitance to ground would not work as a loading
coil. It just wouldn't have a transmission line behaviour worth
mentioning.

It is also exactly this displacement current from a large coil that
allows the current at one end of the coil to be substantially different
from the current at the other end.

Yes again, with one slight modification. You'll note from the EZNEC
models that the current actually increases some as you go up from the
bottom of the inductor. This is the effect noted by King which is due to
imperfect coupling between turns. It results in currents at both ends
being less than at the center.

A transmission line can be represented by a series of L networks with
series L and shunt C. You can achieve any desired accuracy by breaking
the total L and C into enough L network sections. The requirement for
validity is that the length of line represented by each section must be
very small relative to a wavelength. For the example coil, a single
section is entirely adequate at the 5.89 MHz frequency of analysis.
However, at some higher frequency this model won't be adequate, and
either more L sections or a distributed model is necessary. If the
reasons for this aren't obvious, many texts cover it quite well. No
special "traveling wave" analysis is required.

I spent several years of my career designing very high speed TDR and
sampling circuits, which involved a great deal of modeling. At the tens
of GHz equivalent bandwidths of the circuitry, even very small
structures such as chip capacitors and short connecting runs often had
to be treated as transmission lines. One of the skills important to
building an accurate model which would run in a reasonable amount of
time, particularly on the much slower machines being used in the earlier
part of that period, is determining when a lumped L, pi, or tee model is
adequate and when a full-blown transmission line model has to be
used(*). My models were used in the development of quite a number of
circuits that were successfully produced in large numbers.

(*) One of the characteristics of the SPICE programs at the time was
that the time step was never longer than the delay of the shortest
transmission line in the model. So if you willy-nilly modeled everything
as a transmission line, you'd end up with an excruciatingly short time
step and consequently unnecessarily long calculation time.

Roy Lewallen, W7EL

Correction:

Roy Lewallen wrote:
K7ITM wrote:
. . .
It is also exactly this displacement current from a large coil that
allows the current at one end of the coil to be substantially different
from the current at the other end.

[I wrote:]
Yes again, with one slight modification. You'll note from the EZNEC
models that the current actually increases some as you go up from the
bottom of the inductor. This is the effect noted by King which is due to
imperfect coupling between turns. It results in currents at both ends
being less than at the center.

Tom's statement doesn't need modification, it's correct as written.
Imperfect coupling between turns causes current which is different at
the ends than in the middle. Tom said, correctly, that displacement
current is the cause of the currents at the ends being different from
each other.

Roy Lewallen, W7EL

Roy Lewallen wrote:
When the ground was removed and replaced by a wire, the
transmission line properties of the coil changed dramatically, while the
C across the coil didn't change significantly.

Moral: The self-resonant frequency of a loading-coil needs to
be measured in the mobile antenna system, no on the bench.

Yes again, with one slight modification. You'll note from the EZNEC
models that the current actually increases some as you go up from the
bottom of the inductor. This is the effect noted by King which is due to
imperfect coupling between turns. It results in currents at both ends
being less than at the center.

It results in a deviation away from the perfect cosine envelope
exhibited by a 1/2WL thin-wire dipole. In any case, the delay
through a 75m bugcatcher coil is tens of degrees, not 3 nS.

If the
reasons for this aren't obvious, many texts cover it quite well. No
special "traveling wave" analysis is required.

The self-resonant frequency of that modeled coil is around 9 MHz.
Since the coil is 90 degrees at 9 MHz, it would be ~59 degrees
at 5.9 MHz. Dr. Corum suggests a 15 degree limit at which the
lumped-circuit model needs to be abandoned in favor of the
distributed-network model or Maxwell's equations.
--
73, Cecil http://www.qsl.net/w5dxp

Cec, who the heck is Dr Corum?

Is he yet another Bible writer?

Nobody's ever heard of him.

What makes you think he is right?

Is it just because you think he agrees with YOU?

And you may have taken him out of context and misquoted him anyway.
----
Reg.