Velocity Factor and resonant frequency
 

Richard Harrison wrote:
. . .
Surely an antenna loading coil resembles Kraus` low-frequency helix. It
has an open-circuit whip producing a reflection into one end. The
circumference is well below 1/2-wavelength, giving a current
distribution such as shown in Fig. 8-3a for a frequency below the axial
mode of operation.
. . .

So does this mean if we put a current into one end of the inductor I
described, it'll take about 40 ns for current to reach the other end? So
we should expect a phase delay in the current of 90 degrees at 6.15 MHz,
or about 15 degrees at 1 MHz, from one end to the other?

What good are all these books if the information can't be used to solve
a simple problem?

Roy Lewallen, W7EL

Velocity Factor and resonant frequency
 

Roy Lewallen wrote:
So does this mean if we put a current into one end of the inductor I
described, it'll take about 40 ns for current to reach the other end? So
we should expect a phase delay in the current of 90 degrees at 6.15 MHz,
or about 15 degrees at 1 MHz, from one end to the other?

Equation (32) at http://www.ttr.com/TELSIKS2001-MASTER-1.pdf
answers that question. The VF is about double the "round and
round the coil" calculated value and the VF changes with
frequency.
--
73, Cecil http://www.qsl.net/w5dxp

Velocity Factor and resonant frequency
 

Cecil Moore wrote:
Roy Lewallen wrote:

So does this mean if we put a current into one end of the inductor I
described, it'll take about 40 ns for current to reach the other end?
So we should expect a phase delay in the current of 90 degrees at 6.15
MHz, or about 15 degrees at 1 MHz, from one end to the other?


Equation (32) at http://www.ttr.com/TELSIKS2001-MASTER-1.pdf
answers that question. The VF is about double the "round and
round the coil" calculated value and the VF changes with
frequency.

Beware of academics who use phrases such as "anisotropically conducting
cylindrical boundary," "helically disposed surface waveguide," and
"voltage magnification by standing waves." These are just figures of
speech. Some academics - fractenna comes to mind - get so carried away
with their ideas, they'll try anything to justify them, including the
use of nounspeak and polysyllabic jargon. Real scientists and engineers
don't have to use such tactics to make a point.
73,
Tom Donaly, KA6RUH

Velocity Factor and resonant frequency
 

Roy, W7EL wrote:
"What good are all the books if the information can`t be used to solve a
simple problem?"

Many problems fit examples in the books. Some don`t. Implications in my
case are sometimes slow to sink in. An example is what Kraus writes on
page 227 of the 3rd edition of "antennas":
"Thus, a helix combines the geometric forms of a straight line, a
circle, and a cylinder."

Cecil says that RF on a helix may take a short-cut. He may be right. Why
would not a wave deviate from the round and round path on a coil and
sweep at least in part directly along the cylindrical length? It may be
a case for experimentation with a variety of lengths, pitches, and
circumferences.

Best regards, Richard Harrison, KB5WZI

Velocity Factor and resonant frequency
 

If the language in the Corum paper bothers you, check the math.

The Corum paper gives a mathematical solution of Maxwell's equations
for a helix that allows you to answer the questions that have been
ongoing in this thread.

I'm not going to claim that I worked through the solution myself. I
did read it and try to understand the process. It looks just like what
I've done over and over again in electromagnetic theory courses. You
write down the wave equation, the boundary conditions, and you solve.

The geometry of the problem leads to lots of bessel functions and the
necessity to numerically solve the resulting equations to find the
solutions in the middle ground between a lumped circuit coil and a TWT
helix.

I feel like this is typical of physical problems that have a limiting
case at either end. If you go to infinite propagation speed, or a coil
that is very very short compared to a wavelength, you get a lumped
circuit.

If you go to a very long helix with respect to a wavelength, the wave
on the helix goes round and round the turns.

In the middle, the Corum paper describes the situation in mathematical
detail.

Both crackpots and respectable scientists can use complicated terms.
The former use them to obscure unverifiable claims, the latter use them
to try to put a concise name on something that has a complicated
mathematical description.

Throw all the words out of the Corum paper, if you like. Let's look
only at the solution. Is there anything wrong with the mathematical
solution to the wave equation on the helix presented there? Is the
solution in fact applicable to a GIVEN ham antenna loading coil? Can
we use it to predict the difference in current at either end of a
loading coil for a given ham antenna loading coil?

I was going to write "typical" ham antenna loading coil, but I realized
that's a trap. You can't take the solution in the Corum paper, reduce
it to a rule of thumb for the "typical" ham loading coil, and then use
that rule of thumb to make quantitative predictions about a particular
coil in a particular configuration!

Does the solution applicable to all helix sizes and pitches (the
transcendental equation, equation 28 for the constant tau) and the
equation for the velocity of propagation along the axis of the coil
(phase velocity = omega/beta) give the correct delay or not?

I think equation 28 can be solved in Matlab or Mathematica or something
else. I haven't quite figured out if Figure 1 uses approximations or
if it is numerical solutions of equation 28. If it's the latter, you
can just use the figure.

73,
Dan
N3OX

Velocity Factor and resonant frequency
 

So is that a "yes", or "no"?

Roy Lewallen, W7EL

Richard Harrison wrote:
Roy, W7EL wrote:
"What good are all the books if the information can`t be used to solve a
simple problem?"

Many problems fit examples in the books. Some don`t. Implications in my
case are sometimes slow to sink in. An example is what Kraus writes on
page 227 of the 3rd edition of "antennas":
"Thus, a helix combines the geometric forms of a straight line, a
circle, and a cylinder."

Cecil says that RF on a helix may take a short-cut. He may be right. Why
would not a wave deviate from the round and round path on a coil and
sweep at least in part directly along the cylindrical length? It may be
a case for experimentation with a variety of lengths, pitches, and
circumferences.

Best regards, Richard Harrison, KB5WZI

Velocity Factor and resonant frequency
 

wrote:
I think equation 28 can be solved in Matlab or Mathematica or something
else. I haven't quite figured out if Figure 1 uses approximations or
if it is numerical solutions of equation 28. If it's the latter, you
can just use the figure.

Since Fig. 1 occurs after equation (32), I assumed it was for
equation (32). The three variables in equation (32) are coil
pitch, coil diameter, and wavelength. Those are essentially
the same variables plotted in Fig. 1 with the diameter per
wavelength ratio and the turns per wavelength ratio.

Dr. Corum says that for coils passing the litmus equation
test, the error is less than 10% which is perfectly acceptable
for this discussion where W8JI's VF seems to be off by about
5000%. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Velocity Factor and resonant frequency
 

Velocity Factor and resonant frequency
 

Tom Donaly wrote:

Beware of academics who use phrases such as "anisotropically conducting
cylindrical boundary," "helically disposed surface waveguide," and
"voltage magnification by standing waves." These are just figures of
speech. Some academics - fractenna comes to mind - get so carried away
with their ideas, they'll try anything to justify them, including the
use of nounspeak and polysyllabic jargon. Real scientists and engineers
don't have to use such tactics to make a point.
73,
Tom Donaly, KA6RUH

Beware of academics who use "real physical based equations" as they may
mislead you because they are based in reality.

tom
K0TAR

Velocity Factor and resonant frequency
 

Tom Donaly wrote:
Consider your normal mobile antenna with a large loading coil. Now,
in your mind's eye, replace the coil with a cylinder. Now, compute the
cutoff frequency for that cylinder for either a TE or TM mode and see
how close you can get to 3.75 Mhz. Of course, if your waves are slow
enough, you should be able to cram something in there, but you have to
show experimentally both that you can do it, and how you can do it.
Tom Donaly, KA6RUH

TE, TM, or TEM a small loading coil cannot behave very much like a
transmission line.

If you read the Corum paper carefully, you see he clearly states it is
an approximation or solution for a coil under the very special
condition of being self-resonant.

He is working on Tesla coils, not loading coils. He is working with
coils that have essentially no termination and that actuallly behave
like a series of L networks with high reactance shunt C and high
reactance series L.

Worse yet, we have one person who is trying to use a large diameter
helice with wide turn spacing that is large enough to support TEM waves
as a comparison to a tiny fraction of a wavelength dieameter and length
inductor that has relatively close turn spacing and tight coupling from
turn to adjacent turn.

Unless we do something to cause the radial electric field to be very
intense and support significant displacement currents, all the standing
waves in the world external to the won't make a coil behave like a
linear conductor.

Notice also how Cecil misquotes to make a point. The Vf I measured on
80 meters for a large bug-catcher style coil was actually .5 compared
to spatial length, not 1.0

On the other hand Cecil has measured virtually nothing, Yuri has
measured nothing, and Harrison probably hasn't even owned a bug catcher
coil being a technician class license holder.

It's easy to dismiss measurements when you have not done a thing on
your own except talk.

73 Tom