Current through coils
 

"Gene Fuller" wrote:
I did not say anything about W8JI's measurements. He had a completely
different setup, and I had nothing to do with it.

Didn't say you did and it's good that the two were unrelated -
just wanted to point out the contradictions between your
EZNEC results and W8JI's 3 nS measurements.

I have uncovered a slight conceptual error in my traveling wave
antenna simulation. I took care to eliminate reflections between
the top of the coil and the load on the traveling wave wire. But
I didn't do anything to eliminate reflections from the bottom of
the coil. So the current phase at the load at the bottom of the
coil is not from a traveling wave. It is instead from a standing
wave or a combination of the two waves.

The bottom section is one foot long. Knowing the frequency,
e.g. 4 MHz, allows us to calculate the delay in that one foot
of wire, i.e. 0.0041 WL = 1.5 degrees. So the current
phase at the bottom of the coil is -1.5 degrees on 4 MHz.
With the current phase at the top of the coil being 10.72
degrees, that gives a phase shift through the coil of
9.22 degrees which is equivalent to 6.4 nS, more than
double W8JI's measured value still posted to his web page.
--
73, Cecil, W5DXP

Current through coils
 

Cec, do you think that knowledge of reflected waves and phase angles
and propagation delays will enable an antenna designer to construct
something that will win contests every time? ;o)

I'd rather place my confidence in screwing an extra length on the top
end of the loaded whip and damn the extra propagation delay.

I nearly didn't post this.
----
Reg

Current through coils
 

Cecil,

The numbers you quote below have no relationship to the numbers from the
model I sent you. This is the third time you have "accidentally" screwed
with the model.

73,
Gene
W4SZ

Cecil Moore wrote:


The bottom section is one foot long. Knowing the frequency,
e.g. 4 MHz, allows us to calculate the delay in that one foot
of wire, i.e. 0.0041 WL = 1.5 degrees. So the current
phase at the bottom of the coil is -1.5 degrees on 4 MHz.
With the current phase at the top of the coil being 10.72
degrees, that gives a phase shift through the coil of
9.22 degrees which is equivalent to 6.4 nS, more than
double W8JI's measured value still posted to his web page.
--
73, Cecil, W5DXP

Current through coils
 

Gene Fuller wrote:
The numbers you quote below have no relationship to the numbers from the
model I sent you. This is the third time you have "accidentally" screwed
with the model.

It was no accident. Those numbers are from your model modified
to an 8.5 ft. tall antenna. *Our original agreement was an 8 ft.
tall antenna.* Your antenna was almost 50% longer, and that was a
violation of the agreed upon boundary conditions. If you made
it 50 feet tall the delay through the coil would be even smaller.
I'll send you the modified files.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Roy, W7EL wrote:
"I maintain there`s no such group as "other coils", but that coils act
quite differently depending on their physical sizes and the amount of
coupling between turns."

I wrote:
"That`s how the experts say the coil in a TWT works, and is no different
from other coils."

All coils aren`t inside TWTs, but all coils do create inductance.

Bill Orr wrote this concerning the coil in a Traveling Wave Tube:
"Figure 25 is a simplified sketch of a basic helix-type TWT tube. Spaced
closely around the beam is a circuit, in this case a helix of tightly
wound wire, capable of propagating a slow wave. The r-f energy travels
along the wire at the velocity of light, because of the helical path,
the energy progresses along the length of the tube at a considerable
slower veloity than is determined primarily by the pitch of the helix."

Terman wrote this concerning the coil in a TWT:
"The beam is shot through a long, loosely wound helix, and is collected
by an electrode at anode potential as shown."

Lenkurt wrote:
"The RF signal travels as a surface wave around the turns of the helix,
toward the collector, at about the velocity of light. The forward or
axial velocity is slower, of course, because of the pitch and diameter
of the helix."

Orr`s example was a helix of tightly wound wire.

Terman`s example was a long, loosely wound helix, and Lenkurt did not
specify how tight or loosely the coil was wound. In all cases the coil
retarded the signal well below the velocity of light along the axis that
the electron beam traveled so that the beam could keep up with the
signal along the path. The beam needs to be speeded as well as slowed
for velocity modulation.

Point is that group velocity does not exceed the velocity of light even
in W8JI`s coil no matter how he makes it. There is no way to coerce
actual energy to exceed the velocity of light. It would turn into a
pumpkin or something.

Also, electric current follows the course of maximum potential
difference and that`s along the conductor supplying the electrons. The
wave impels electrons to move in the conductor.

Kraus wrote:
"The helical antenna, which is discussed in this chapter, may be
regarded as the connecting link between the linear antenna and the loop
antenna, discussed in preceeding chapters. The helical antenna is the
general form of antenna of which the linear and the loop are special
cases. Thus, a helix of fixed diameter collapses to a loop as spacing
approaches zero. On the other hand, a helix of fixed spacing between
turns straightens out into a linear conductor as the diameter approaches
zero.

This thread has been about a coil loaded whip. This is a standing wave
antenna. When the signal gets to the antenna tip it has no where else to
go but return over the path which brought it. The coil has an incident
wave impinging from the transmitter and an out-of-phase signal reflected
from its tip. These two waves have the same origin so they are locked in
step to make standing waves in both voltage and current. These determine
the ratios of voltage to current at each point along the signal route.
In this respect the coil behaves as a conductor in the antenna. It has
more opposition to the signals traversing it than a straight wire but
the volts and amps at each of its ends can obviously be very different.
Thus, current in one end of the coil can be very different from the
current at the other end of the same coil.

Best Regards, Richard Harrison, KB5WZI

Current through coils
 

From all that, I gather that your answer is "yes", that you do believe
that the current in a small inductor with close turn spacing (i.e., one
where the fields from the turns couple well) flows around and around
along the wire at near the speed of light, resulting in a delay from end
to end approximately equal to the wire length divided by the speed of light.

Or did I misinterpret what you said, and you don't believe this?

Roy Lewallen, W7EL

Richard Harrison wrote:
Roy, W7EL wrote:
"I maintain there`s no such group as "other coils", but that coils act
quite differently depending on their physical sizes and the amount of
coupling between turns."

I wrote:
"That`s how the experts say the coil in a TWT works, and is no different
from other coils."

All coils aren`t inside TWTs, but all coils do create inductance.
. . .

Current through coils
 

Rot, W7EL wrote:
"From all that, I gather your answer is Yes."

I believe the wave is guided by the wire in its path and takes no
shortcut along the axis of a coil.

Best regards, Richard Harrison, KB5WZI

Current through coils
 

Roy Lewallen wrote:

From all that, I gather that your answer is "yes", that you do believe
that the current in a small inductor ...

There's those buzz words "small inductor" again. We are talking about
75m bugcatcher coils, not "small inductors". Small inductors have a
high self-resonant frequency. We are talking about large inductors
operated relatively near their self-resonant frequencies.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

It'll be easy enough to show that's false. If I set up a simple
measurement with a piece of Air-Dux in series with a resistor, a couple
of calibrated current probes, and a dual-channel scope, will you believe
the results? Or would you rather have someone else make the measurement
or do it yourself?

Roy Lewallen, W7EL

Richard Harrison wrote:
Rot, W7EL wrote:
"From all that, I gather your answer is Yes."

I believe the wave is guided by the wire in its path and takes no
shortcut along the axis of a coil.

Best regards, Richard Harrison, KB5WZI

Current through coils
 

Richard Harrison wrote:

Rot, W7EL wrote:
"From all that, I gather your answer is Yes."

I believe the wave is guided by the wire in its path and takes no
shortcut along the axis of a coil.

If 100% of the flux from each and every coil physically linked
100% of each and every other coil, the current would indeed skip
from one end of the coil to the other without interference. That
is the basic presupposition of the lumped-circuit model.

Quoting Dr. Corum:
"Lumped element circuit theory assumes that there are no wave
interference phenomena present, ...", i.e. no superposition
of forward and reflected waves, i.e. no standing waves.

Continuing the quote:
"This is manifested by two phenomena: 1. The current distribution
function is spatially uniform across each element. 2. The spatial
phase delay between circuit extremities is zero."

One has to imagine that W8JI's 2" dia x 12" length 100 uH coil
links 100% of the flux in coil number 1 with coil number 100
a foot away and vice versa. That's quite an imagination but W8JI
did measure a 3 nS delay, virtually instantaneous, so it must
be true.
--
73, Cecil http://www.qsl.net/w5dxp