Current through coils
 

Current through coils
 

Reg Edwards wrote:
A 100 turn coil, 10 inches long, 2 inches in diameter, has an
inductance of 102 microhenrys, a Q of aproximately 380 at F = 1.9 MHz,
and the self-resonant frequency is 12.0 MHz.

I'll bet the measured self-resonant frequency would be lower
if mounted as a base-loading coil on my pickup.

Seems the VF of the coil is 0.041 based on 10" being 1/4WL
at 12 MHz. Assuming that VF holds down to 1.9 MHz we
can calculate the electrical length of the coil on 1.9 MHz
which will be the same as the phase shift through the coil.

So I get about ~14 degrees of phase shift through that coil
at 1.9 MHz assuming the self-resonant frequency really
is 12 MHz at the spot where the coil is mounted.

If the coil were used on 3.8 MHz, the phase shift would
be ~28 degrees.

But my 75m bugcatcher coil shows to be self-resonant at
6.6 MHz while sitting there on my pickup being driven
by an MFJ-259B. It is 6.5" long. When 6.5" is 1/4WL
at 6.6 MHz, the VF = 0.0145, considerably lower than
the coil above and operating much closer to its self-
resonant frequency.

A length of 6.5" coil with a VF of 0.145 on 4 MHz is
~55 degrees of phase shift. And indeed the net current
at the top of the coil drops to about 2/3 of what it
is at the bottom.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

John Popelish wrote:
Dang! I was looking forward to your test results, and a description of
your test method. I think your 2" diameter coil is a good example of an
inductor that is neither a perfect "lump" nor a pure transmission line.

That's what my back of the napkin calculations would indicate.
I get ~14 degrees at 1.9 MHz or ~28 degrees at 3.8 MHz based
on 90 degrees at 12 MHz.

But based on what these guys measured before, anyone would be
a fool to predict the results without knowing what the test
setup looks like. In fact, the prediction challenge was a
blatently obvious attempt to lead the unsuspecting down
a primrose path without a roadmap. Do you think W7EL would
ever make a prediction based on the meager amount of
information provided? :-) A few years ago he provided some
information but kept changing parameters daily until I got
tired and withdrew my estimate. But it turned out in the
end that I was pretty close.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Reg wrote:
"Now, now gentlemen, there is no need to re-start the Civil War."

In Texas we call it the War Between the States!

The following is from my daughter Linda Edwards (not related to Reg) who
lives in London:
The scene is on a mexican golf course and the "Federales" are
investigating an apparent homicide. Investigator asks: "What was the
murder weapon?" Reply is: "A golf gun." Investigator asks "What`s a golf
gun?" Reply is:
"Don`t know but it sure made a hole in Juan!"

best regards, Richard Harrison, KB5WZI

Current through coils
 

Richard Harrison wrote:
J.D. Kraus wrote on page 185 of his 1950 edition of "Antennas":
"Thus, a helix with circumference too small for the axial mode of
radiation (circumferennce less than 2/3 wavelength) has a nearly
sinusoidal current distribution, caused by alternate reinforcement and
cancellation of two oppositely directed traveling waves on the helix of
nearly equal amplitude Izero as suggested in Fig. 7-13c. Both traveling
waves are of the Tzero transmission mode type."

Over on qrz.com, Tom was trying to prove Kraus wrong when he said in
"Antennas for All Applications", 3rd edition: "A coil (or trap) can
also act as a 180 degree phase shifter as in the collinear array
of 4 in-phase 1/2WL elements in Fig. 23-21B. Here the elements
present a high impedance to the coil which may be resonated without
an external capacitor due to its distributed capacitance. The coil
may also be thought of as a coiled-up 1/2WL element."

In trying to prove one could not obtain Kraus' 180 degree phase shift
with a coil [because everyone knows the phase shift is always zero],
Tom accidentally let slip the following - quoted from qrz.com:

W8JI wrote:
"By the way, I swept S12 phase with my network analyzer on a
100uH inductor a few hours ago while working on a phasing
system. The phase shift through that series inductor was about
-60 or -70 degrees on 1 MHz, ... "

Say what? Tom reporting a phase shift through an inductor? Will
miracles never cease?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

I think Reg's estimate of both inductance and first self-resonance (or
is that self-anti-resonance?? ;-) are a bit high. Methods I trust all
predict about 97uH and self-resonance at very close to 8MHz, for a coil
wound with 16AWG to a 2-inch inner diameter. (Inductance would be a
bit lower if that's the wire center-to-center diameter.) Winding and
measuring one to see which estimate is closer is left as an exercise
for the reader, but I wouldn't trust either Reg's or my estimates on
this for building an inductance standard (and besides, I'd build such a
standard with a lower L/D).

A transmission line model shows 13.5 electrical degrees of line at
1.9MHz, at about 6140 ohms. You should probably look up an article by
John Mezak that appeared in "RF Design" some years ago before you to
applying that info willy-nilly, but I don't really expect you will.
Have fun.


Cecil Moore wrote:
Reg Edwards wrote:
A 100 turn coil, 10 inches long, 2 inches in diameter, has an
inductance of 102 microhenrys, a Q of aproximately 380 at F = 1.9 MHz,
and the self-resonant frequency is 12.0 MHz. Its radiation resistance
at 1.9 MHz is negligible.

Good stuff Reg. Modeling it as a transmission line, what would
be its Z0 and VF?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Cecil Moore wrote:
John Popelish wrote:

Dang! I was looking forward to your test results, and a description
of your test method. I think your 2" diameter coil is a good example
of an inductor that is neither a perfect "lump" nor a pure
transmission line.


That's what my back of the napkin calculations would indicate.
I get ~14 degrees at 1.9 MHz or ~28 degrees at 3.8 MHz based
on 90 degrees at 12 MHz.

But based on what these guys measured before, anyone would be
a fool to predict the results without knowing what the test
setup looks like. In fact, the prediction challenge was a
blatently obvious attempt to lead the unsuspecting down
a primrose path without a roadmap. Do you think W7EL would
ever make a prediction based on the meager amount of
information provided? :-) A few years ago he provided some
information but kept changing parameters daily until I got
tired and withdrew my estimate. But it turned out in the
end that I was pretty close.

Predictions are little more than an ego trip, unless they are a test
of a specific calculation method used for the prediction. But I was
willing to wait for the test result and an after the fact description
of the test method, to see what understanding might be teased out that
combination of facts. The discussion might also have lead to a better
way to perform such a test.

Baby steps.

Current through coils
 

"Richard Harrison" wrote
Kraus characterizes inductors as helices. At one extreme they are
stretched into straifht wires. At the other they collapse into
single
loops.

=====================================

See program SOLNOID3 which calculates inductance of 1-turn loops, via
multi-turn solenoids, to straight wires. With many other parameters
of interest such as temperature rise for a given applied voltage, as
in tank circuits and antenna loading coils.

So far as the author is aware there are no bugs in it. It's been
around and much used for a few years.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Current through coils
 

John Popelish wrote:
Predictions are little more than an ego trip, unless they are a test of
a specific calculation method used for the prediction. But I was
willing to wait for the test result and an after the fact description of
the test method, to see what understanding might be teased out that
combination of facts. The discussion might also have lead to a better
way to perform such a test.

Still might. But I suspect the test results are already available
and just being withheld because someone doesn't like the results.
I'm a skeptical, suspicious type. I'm sure Tom wishes he had not
published his "-60 to -70 degree phase shift" results.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Reg Edwards wrote:
See program SOLNOID3 which calculates inductance of 1-turn loops, via
multi-turn solenoids, to straight wires.

Reg, at http://www.ttr.com/TELSIKS2001-MASTER-1.pdf equation (43)
gives the helical wave guide effective characteristic impedance.
Unfortunately, that web site is not responding at the moment (probably
because all the gurus here are accessing it wondering what they did
wrong.) :-) But, when the page becomes available would you take a look
at that equation? It goes something like this:

Zc = 60/Vf[I0(tau*alpha)*K0(tau*alpha)]

Is tau the transmission coefficient and alpha the attenuation constant?
Are I0 and K0 functions? Have you ever seen this equation before?

Also, Fig. 1 is a graph of velocity factor vs the diameter of the helix
divided by the wavelength for 10k, 5k, 2.5k, 1k, 500, 250, 100, and 50
turns per wavelength. It says: "Tightly wound coils are slow wave
structures." My 75m bugcatcher coil appears to fall nicely into the
catagory of a "tightly wound slow wave structure".
--
73, Cecil http://www.qsl.net/w5dxp