Cecil, what formula do you use for the velocity factor of a coil of
diameter D, length L, and N number of turns, in metric units if its
convenient.

Do you have a formula for the self-resonant frequency?
----
Reg.

Reg Edwards wrote:
Cecil, what formula do you use for the velocity factor of a coil of
diameter D, length L, and N number of turns, in metric units if its
convenient.

Reg, it's equation (32) from Dr. Corum's paper at:

http://www.ttr.com/TELSIKS2001-MASTER-1.pdf

There is a test in the preceeding paragraph to see if
that equation is appropriate for a particular coil.
Equation (32) is derived from empirical data collected
on coils that pass that test.

Just be sure the diameter, pitch, and wavelength are
all in meters and it will be metric.

I'll send you a .gif file of that page of Dr. Corum's
paper. The graph in Fig. 1 is for equation (32).

While you are at it, take a look at equation (47) for
the characteristic impedance of the coil and let us
know what you think.

Do you have a formula for the self-resonant frequency?

Here's what I have been doing lately:

1. Using as close as EZNEC can come to my 75m bugcatcher
coil stock, create enough turns for the modeled coil to
be self-resonant on 4 MHz. My 75m bugcatcher coil stock
is ~0.5 ft diameter and 48 turns per foot.

2. Delete enough turns to make it look like my real-
world bugcatcher coil. Use that coil for EZNEC modeling
at 4 MHz.

3. Assume the velocity factor didn't change appreciably
when deleting those turns.

4. Calculate the number of linear feet occupied by the
coil by dividing the length of the coil by the velocity
factor.

5. Calculate the percentage of a wavelength occupied by
the coil by dividing the results of (4.) above, by 246
feet, a wavelength at 4 MHz.

Of all the measurements and modeling so far, this is what
I have come up with as the most accurate estimate of the
percentage of a wavelength occupied by the coil.

And no, it is not 90 degrees minus the rest of the antenna.
The requirement for a purely resistive feedpoint impedance
is that the superposition of the forward and reflected voltages
have the same phase angle as the superposition of the forward
and reflected currents - nothing more.
--
73, Cecil http://www.qsl.net/w5dxp

Dear Cec,

After 3/4 of a bottle of Australian Cabernet Sauvignon, I plucked up
sufficient courage to present my printer with Corum's paper.

Lo and behold, it worked perfectly. Even the small amount of color was
accurately reproduced.

After speed-reading it I came to the conclusion it is unnecessarily
over-complicated. What on Earth does "Voltage Magnification by
Coherent Spatial Modes" mean?

For years, my approach to loading coils at HF has been to calculate
the inductance and capacitance per unit length of coil from DC
principles. And then calculate the velocity factor and Zo from
transmission line principles. Which gives results in the right ball
park according to what few experiments I have made with actual anennas
and helices on the 160 and 80 meter bands.

Then there was G3YXM who deliberately put more turns on the coil on
the grounds it was easier to remove them than add to them in case
pruning was required. Pruning was required and he ended up by removing
all the excess turns.

Have you compared VF's (a critical parameter) in my programs with
Corum's values for close-wound coils of usual proportions? I must try
to find time to do it myself.

Thanks very much for posting me Corum's paper. I am pleased to see
the University of Nis has not been seriously affected by the bombing
and guided missiles during the US Yugoslavian attacks. Have the
bridges across the Danube been replaced yet?
----
Reg.

Reg Edwards wrote:
After speed-reading it I came to the conclusion it is unnecessarily
over-complicated.

Maybe making it over-complicated also makes it over-accurate? :-)

What on Earth does "Voltage Magnification by
Coherent Spatial Modes" mean?

It means that super high SWRs result in super high voltages.
It's the usual VSWR = Vmax/Vmin for coherent signals.

Have you compared VF's (a critical parameter) in my programs with
Corum's values for close-wound coils of usual proportions?

I haven't yet figured out the English unit to Metric unit
conversion procedure for turns on a coil. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:
. . .
For years, my approach to loading coils at HF has been to calculate
the inductance and capacitance per unit length of coil from DC
principles. And then calculate the velocity factor and Zo from
transmission line principles. Which gives results in the right ball
park according to what few experiments I have made with actual anennas
and helices on the 160 and 80 meter bands.

How do you calculate the coil C to use in the transmission line formulas?

Roy Lewallen, W7EL

How do you calculate the coil C to use in the transmission line
formulas?

Roy Lewallen, W7EL
===================================

I'm surprised a person of your knowledge asked.

Go to Terman's or other bibles, I'm sure you'll find it somewhere, and
find the formula to calculate the DC capacitance to its surroundings
of a cylinder of length L and diameter D.

Then do the obvious and distribute the capacitance uniformly along its
length.

The formula will very likely be found in the same chapter as the
inductance of a wire of given length and diameter.

I have the capacitance formula I derived myself somewhere in my
ancient tattered notes but I can't remember which of the A to S
volumes it is in.

I'm 3/4 ot the way down a bottle of French Red plonk. But Terman et
al should be be quite good enough for your purposes.

And its just the principle of the thing which matters. It's simple
enough. I don't suppose you will make use of a formula if and when
you find one.
----
Reg.

I did a search quite some time ago and failed completely in finding the
formula you describe, in Terman or any other "bible". The formula for
the capacitance of an isolated sphere is common, but not a cylinder. The
formula for a coaxial capacitor is common also, but the capacitance
calculated from it approaches zero as the outer cylinder diameter gets
infinite.

Maybe you could take a look after the wine wears off, and see if you can
locate the formula. By your earlier posting, it sounds like you've used
it frequently, so it shouldn't be too hard to find. I'd appreciate it
greatly if you would. And yes, I would make use of the formula -- I'm
very curious about how well a coil can be simulated as a transmission
line. The formula you use would be valid only in isolation, so
capacitance to other wires, current carrying conductors, and so forth
would have an appreciable effect. I showed not long ago that capacitance
from a base loading coil to ground has a very noticeable effect. Do you
have a way of taking that into account also?

Roy Lewallen, W7EL

Reg Edwards wrote:
How do you calculate the coil C to use in the transmission line
formulas?
Roy Lewallen, W7EL
===================================

I'm surprised a person of your knowledge asked.

Go to Terman's or other bibles, I'm sure you'll find it somewhere, and
find the formula to calculate the DC capacitance to its surroundings
of a cylinder of length L and diameter D.

Then do the obvious and distribute the capacitance uniformly along its
length.

The formula will very likely be found in the same chapter as the
inductance of a wire of given length and diameter.

I have the capacitance formula I derived myself somewhere in my
ancient tattered notes but I can't remember which of the A to S
volumes it is in.

I'm 3/4 ot the way down a bottle of French Red plonk. But Terman et
al should be be quite good enough for your purposes.

And its just the principle of the thing which matters. It's simple
enough. I don't suppose you will make use of a formula if and when
you find one.
----
Reg.