RadioBanter - Current through coils

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Current through coils

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

Current through coils

Yuri Blanarovich wrote:

Tom.
You tell that to the RF ammeters installed on the vertical, W9UCW's pictures
on my page!

1.) I can build an antenna that has greatly uneven currents at the ends
of the loading coil, but the antenna rea above the inductor is wasted
and the system will be less efficient than a properly designed system.

2.) The meters are large and have a good deal of self-capacitance
compared to the capacitance at the point where they are connected, and
are directly connected to the antenna. Bad idea to base a whole concept
of how an antenna works on something like that.

You can mumbo-jumbo all the theory, you can dream of, but reality shows that
in the say, quarter wave vertical, with loading coil the current at both
ends of the coil is different.

It can be different, but in a well designed system it is essentially
the same. The only difference is caused by displacement currents, and
that is a result of stray capacitance. Wind a good coil that has low
self-C to the outside world compared to the antenna hanging above the
coil, and the problem of large uneven current goes away.

Cecil explained the various situation
depending where the coil is placed within the radiator and at overall
antenna curve.

I doubt that. If he explained it in those terms he was missing some
important points.

Try this test, no meters necessary (perhaps the aquarium strip thermometer):
Take your 80m Hustler antenna with Hustler loading coil and whip. At the
resonant frequency put about 600 Watts to it for a while. Stop transmitting
and go feel (or read the temperature on the strips) the coil, bottom end and
the top end. Same temperature? Temperature is proportional to the current
flow (same diameter wire) - warmer end - more current.

Are you saying thermal effects have no bearing?

It's getting pretty dangerous to write a theory based only on a Hustler
mobile coil with almost no stinger above the coil. One of the reasons
the Hustler works so poorly is the distributed capacitance in the coil
is large compared to the tiny stinger above the coil.

The Hustler has narrow bandwidth and poor efficiency because of the
coil design.

Then test two: Keep the RF flowing until heat shrink tubing on the coil
starts melting. Where does it melt first? Bottom of the coil or nicely
uniformly as you claim it should?

I never claimed uniformly in ALL coils. I set boundaries as to the
conditions. I can replace that Hustler coil with another coil and ruin
your theory about standing waves and missing antenna degrees.

73 Tom

Current through coils

wrote in message
oups.com...

Yuri Blanarovich wrote:

Tom.
You tell that to the RF ammeters installed on the vertical, W9UCW's
pictures
on my page!

1.) I can build an antenna that has greatly uneven currents at the ends
of the loading coil, but the antenna rea above the inductor is wasted
and the system will be less efficient than a properly designed system.

2.) The meters are large and have a good deal of self-capacitance
compared to the capacitance at the point where they are connected, and
are directly connected to the antenna. Bad idea to base a whole concept
of how an antenna works on something like that.

You can mumbo-jumbo all the theory, you can dream of, but reality shows
that
in the say, quarter wave vertical, with loading coil the current at both
ends of the coil is different.

It can be different, but in a well designed system it is essentially
the same. The only difference is caused by displacement currents, and
that is a result of stray capacitance. Wind a good coil that has low
self-C to the outside world compared to the antenna hanging above the
coil, and the problem of large uneven current goes away.

Cecil explained the various situation
depending where the coil is placed within the radiator and at overall
antenna curve.

I doubt that. If he explained it in those terms he was missing some
important points.

Try this test, no meters necessary (perhaps the aquarium strip
thermometer):
Take your 80m Hustler antenna with Hustler loading coil and whip. At the
resonant frequency put about 600 Watts to it for a while. Stop
transmitting
and go feel (or read the temperature on the strips) the coil, bottom end
and
the top end. Same temperature? Temperature is proportional to the current
flow (same diameter wire) - warmer end - more current.

Are you saying thermal effects have no bearing?

It's getting pretty dangerous to write a theory based only on a Hustler
mobile coil with almost no stinger above the coil. One of the reasons
the Hustler works so poorly is the distributed capacitance in the coil
is large compared to the tiny stinger above the coil.

The Hustler has narrow bandwidth and poor efficiency because of the
coil design.

Then test two: Keep the RF flowing until heat shrink tubing on the coil
starts melting. Where does it melt first? Bottom of the coil or nicely
uniformly as you claim it should?

I never claimed uniformly in ALL coils. I set boundaries as to the
conditions. I can replace that Hustler coil with another coil and ruin
your theory about standing waves and missing antenna degrees.

73 Tom

Yea Tom, it all started with ALL coils, it is MY theory and you can ruin MY
theory. Riiiight! It's getting pathetic. Yea, meters are too big, Hustler is
crapy, Cecil is wrong, and you never claimed uniformly in all coils, just
those that you have. Reality is wrong, your "theory" is right! Rrrrright!!!
But what a coincidence that what W9UCW measured, jives with what Cecil
calculated. Hmmm!

ANSWER Cecil's question about his modeled example. I guess when someone is
stuck on something and dunt gitit, its tough!
I am just waiting how you will come around, dancing around in mumbo-jumbo
circles and then will become guru on how current IS different in the loading
coils. Happened in the past, will happen again. You are WRONG, reality
proves it, regardless of your detours.

BTW, what engineering degree, from what university do you have or PE that
gives you right to put labels like "JI Engineering" on your products?

bada BUm

Current through coils

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

Current through coils

Tom, W8JI wrote:
"What you are missing is the flux inside the coil links all the turns at
light speed. When it does that, current appears at nearly the same
instant of time (light speed over the spatial distance of the inductor)
in all areas that are linked by flux."

Are any famous authors protagonists of that theory?

One author, Bill Orr, W6SAI writes in the 22nd edition of "Radio
Handbook" on page 5.11:
"Spaced closely around the beam (in a TWT) 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 but, because of
the helical path, the energy progresses along the length of the tube at
a considerably lower velocity that is determined by the pitch of the
helix.

Maybe Varian has a paper on this (just my speculation).

Best regards, Richard Harrison, KB5WZI

Current through coils

Yuri Blanarovich wrote:

Yea Tom, it all started with ALL coils, it is MY theory and you can ruin MY
theory.

What is your theory Yuri?

You didn't explain it.

73 Tom

Current through coils

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

Current through coils

Richard Harrison wrote:

Are any famous authors protagonists of that theory?

In "Fields and Waves in Modern Radio", Ramo and Whinnery, 2nd
edition, there is a section titled: "9-16 The Idealized Helix
and Other Slow-Wave Structures". Quoting: "A rough picture
would convince one that the wave should follow the *wire* with
about the velocity of light, ..."

From the IEEE Dictionary: "slow-wave circuit - A circuit whose
phase velocity is much slower than the velocity of light. For
example, for suitably chosen helixes the wave can be considered
to travel on the *wire* at the velocity of light but the phase
velocity is less than the velocity of light by the factor that
the pitch is less than the circumference."

a 75m bugcatcher loading coil is a slow wave structure with
a velocity factor around 0.017 (calculated and measured).
--
73, Cecil http://www.qsl.net/w5dxp