Current through coils
 

wrote:
I received an email from someone (an attempted reply mail bounced) who
thinks his mobile antenna is "90-degree resonant". The the upper
portion of the antenna is physically 5 degrees long, the lower section
10 degrees, and the loading coil placed between the two about 1 degree
long. He has tied himself in a knot picturing the system as being
"90-degree" resonant, and thinking the inductor must make up the
missing 74 degrees of antenna height.

Here's a question for you, Tom. Assume for reference that the forward
current phase is at 90 deg and the reflected current phase is at -90
deg at the tip of the antenna and then backtrack the two phasors to
the feedpoint. The phasor currents are known to rotate in opposite
directions. The question for you is: What is the phase of the forward
current originating at the feedpoint? What is the phase of the reflected
current returning to the feedpoint?

Seems you are trying to tell us that the phase of the forward current
at the feedpoint is at a phase angle of 90-16 = 74 degrees and the phase
of the reflected current is -90+16 = -74 degrees at the feedpoint since,
as you say, the antenna is 16 degrees long.

Assuming angle F is the feedpoint phase of the forward current and
angle R is the feedpoint phase of the reflected current:

We know that net current at the feedpoint is Ifor*cos(F) + Iref*cos(R)

According to your theory the net current would be

Ifeedpoint = Ifor*cos(74) + Iref*cos(-74) = .028(Ifor+Iref)

Since the feedpoint impedance is inversely proportional to the feedpoint
current, the feedpoint impedance for your loaded vertical would be a lot
higher than the feedpoint impedance for a 1/4WL vertical but we know
it actually goes in the opposite direction. How do you resolve that
contradiction?

He also, as many people do, visualizes an inductror model where current
winds its way through the copper from end to end. His exact words
being: "Also, in colis(sic) of significant wire length the propagation
time of current in the wire is still approximately 1 E-9
seconds/foot."

For an ideal inductance, the voltage propagates through the coil
at the speed of light. The current lags the voltage by as much
as 90 degrees in an ideal inductance.

For an ideal capacitance, the current propagates through the coil
at the speed of light. The voltage lags the current by as much as
90 degrees in an ideal capacitance.

It seems
some have reached an unbendable conclusion without even understanding
how an inductor works.

It is not your understanding of how an inductor works that is the
problem. It is your misunderstand of how standing wave current
works that is the problem. Let's discuss the problem on the other
thread in progress.

We need to talk about the inductor, or this will go nowhere and in
another three years pop right back up.

You are looking for your keys under the street lamp instead of where
you lost them, because the light is better. The inductor is NOT the
problem. Your misunderstanding of standing waves is the problem.

You fully understand how a coil works in the presence of a traveling
wave. You do not understand how a coil works in the presence of
standing waves, not because you lack understanding of a coil, but
because you lack understanding of standing wave current. If you
keep avoiding your area of misunderstanding, this will go nowhere.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Has it ever occurred to you guys that a coil is a coil wherever it is
used and always behaves in the same way.

Current through coils
 

wrote:
It seems
some have reached an unbendable conclusion without even understanding
how an inductor works.

It certainly seems you have reached an unbendable conclusion without even
understanding how an inductor works in a standing wave environment. The
following reference, emailed to me by a kind reader of this newsgroup,
says exactly what I have been trying to say. I propose that a 75m
bugcatcher loading coil is a "velocity inhibited slow-wave helical
transmission line resonator". The only difference between it and a
1/4WL resonant Tesla coil is the radiating part of the antenna.
I'll quote a few excerpts.

http://www.ttr.com/corum/index.htm

[begin quote]
Tesla Coils and the Failure of Lumped-Element Circuit Theory
by Kenneth L. Corum and James F. Corum, Ph.D.
© 1999 by K.L. Corum and J.F. Corum

In all of those [lumped] circuit models the current is analytically
presupposed to be uniformly distributed along the wire in the coil ...
There are no standing waves on a lumped element circuit component.

However, a true Tesla coil (circa 1894) is a velocity inhibited slow-wave
helical transmission line resonator: ... one needs transmission line
analysis (or Maxwell's equations) to model these electrically distributed
structures. Lumped circuit theory fails because it's a theory whose
presuppositions are inadequate. Every EE in the world was warned of this
in their first sophomore circuits course.

This phenomenon is decisive. It occurs only on distributed resonators:
it is impossible with any lumped circuit element! (The current has the
same value at every point along a lumped-element.) To understand what
is happening, consider a cylindrical helical coil of height H. The base
is always forced to be a voltage node (it's grounded). The top is always
a relative voltage loop at the odd quarter-wave resonances and a voltage
node at the even (half-wave) resonances. These boundary conditions
constrain the mode patterns on the structure (called spatial harmonics).
We assert that velocity inhibited partially coherent forward and reflected
RF traveling waves form interference patterns on the coil.
[end quote]

"Lumped circuit theory fails because it's a theory whose presuppositions
are inadequate." Seems some EEs have forgotten that sophmore year warning.
Using lumped circuit theory in the presence of standing waves is a form of
"assuming the proof" or "begging the question" and is simply invalid.

Quoting relevant material from Balanis: "The current and voltage
distributions on open-ended wire antennas are similar to the standing
wave patterns on open-ended transmission lines. ... Standing wave antennas,
such as the dipole, can be analyzed as traveling wave antennas with waves
propagating in opposite directions (forward and backward) and represented
by traveling wave currents If and Ib ..."
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Reg Edwards wrote:
Has it ever occurred to you guys that a coil is a coil wherever it is
used and always behaves in the same way.

The coil always behaves in the same way. Unfortunately, the models
used to explain the operation of the coil don't work in the same
way. Please see:

http://www.ttr.com/corum/index.htm
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Cecil Moore wrote:

Reg Edwards wrote:
Has it ever occurred to you guys that a coil is a coil wherever it is
used and always behaves in the same way.

The coil always behaves in the same way. Unfortunately, the models
used to explain the operation of the coil don't work in the same
way. Please see:

http://www.ttr.com/corum/index.htm

Dang Reg, I forgot to post the quote from that web page. Here it is:

"Lumped element representations for coils require that the current
is uniformly distributed along the coil - no wave interference and
no standing waves can be present on lumped elements."

Therefore, lumped element representations for coil CANNOT be
used to analyze standing wave antennas. I wasn't the first
to say that. Using a lumped element representation for a
coil in a standing wave environment is "assuming the proof".

Here's more information along those same lines.

http://www.ttr.com/TELSIKS2001-MASTER-1.pdf
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Cecil Moore wrote:
Ian White GM3SEK wrote:
And also, if the inductively loaded antenna is designed by the
"antenna as transmission line" method (as used by Boyer and ON4UN for
example) it clearly shows that the loading inductance is simply there
to cancel the net capacitive reactance - in other words, it behaves
in exactly the same way as you would in any other circuit.

If that is true, you guys shouldn't have any difficulty proving me wrong
and sending me back to the woodshed once and for all. If the above is not
entirely true, please don't put me under house arrest until I present
the truth as I see it. And certainly, call me on anything that is wrong.


The only problem for the rest of us is that you seem to have unlimited
energy and time :-)


If you will keep listening with an open mind, I think I can show you
that the words, "clearly", "simply", and "exactly", in your above
statement are not entirely correct.

There is one typo in that statement: I posted it part-way through
changing from "In other words, you use the inductance in exactly the
same way as you would in any other circuit" to "In other words, it [the
inductance] behaves in exactly the same way as it does in any other
circuit". I stand by both of those statements.

--
Ian, no one has explained the antenna currents reported by EZNEC at:

http://www.qsl.net/w5dxp/qrzgif35.gif

How can 0.1+ amp of current be 'flowing' into the bottom of the coil
and 0.7+ amp of current be 'flowing' out of the top of the coil. It's
been days now and no one has offered an explanation.

It's because you modeled a real-life coil, whose length and diameter are
each a significant fraction of the size of the whole antenna. Nobody
disputes that the currents at the two ends of a real-life coil are going
to be different... but the reason is because of its other properties
besides pure inductance.

You are hung up on something far more fundamental. You are
misrepresenting the fundamental electrical properties of inductance to
make them fit your theory.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Current through coils
 

Reg Edwards wrote:
Has it ever occurred to you guys that a coil is a coil wherever it is
used and always behaves in the same way.


That is only true if you say it about pure inductance.

But a "coil" is a real-life component that has other properties like
physical size, number of turns, self-capacitance and leakage inductance.
A coil interacts electromagnetically with the circuit in which it finds
itself, so it doesn't always behave in the same way. But pure inductance
does.

The difference between inductANCE and an inductOR - a real-life coil -
is not just playing with words.

There is a reason for having those two different words... and that
reason is the key to this whole debate.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Current through coils
 

Reg Edwards wrote:
"Has it ever occurred to you guys that a coil is a coil however it is
used and always behaves in the same way?"

Yes! But when it is part of an antenna system, the system imposes energy
upon the coil in ways which the coil does not control. "The system is
the solution", AT&T used to say.

John D. Kraus writes on page 176 of his 1950 edition of "Antennas":
"The term transmission mode is used to describe the manner in which an
electromagnetic wave is propagated along an infinite helix (that`s a
coil, right?) as though the helix constituted an infinite transmission
line or wave guide."

Wave guides and transmission lines are subject to reflections. These
produce the standing wave patterns exhibited in many text books. Kraus
uses the helix very generally. To him it can collapse to a single loop
or be stretched to a straight wire.

When a "normal mode" helix (coil) is used as part of a antenna system,
It radiates normal to the axis of the coil, similar to the manner it
would were it stretched out to a straight wire. A reflection within the
antenna system would return energy toward the generator, similar to the
manner it would with straight wires. The same sort of interaction
between incident and reflected waves must occur. There is no other way.
These produce variatiations in both current and voltage in a periodic
manner along the helix as described for transmission lines which should
be familiar to all.

The whole section of helical antennas in Kraus is interesting. Kraus is
the inventor of the Axial mode helical antenna. I think he tells the
story in his 3rd edition of how he went home and wound one up and tested
it after being told by an expert of the times that such an antenna was
impossible.

Best regards, Richard Harrison, KB5WZI

Current through coils
 

Ian White GM3SEK wrote:

SNIPPED A LOT


You are hung up on something far more fundamental. You are
misrepresenting the fundamental electrical properties of inductance to
make them fit your theory.



I agree with Cecil.

An Inductor in a DC circuit under transient conditions has a classic L/R
response.

An inductor in a AC power line [60 Hz] acts as a classic inductor.

An inductor in a LF antenna system acts as a classic inductor when the
physical AND electrical dimensions are very small compared to a wavelength.

An Inductor in a HF shortened antenna does NOT act like a classic
inductor. It is a significant portion of the HF circuit and must be
treated as such.

My 60 meter mobile antenna is 90 degrees long, 1/4 wavelength resonant
at 18 +j0 ohms [MFJ analyzer]. It is 10 degrees long from feedpoint to
base of coil. Current into the coil is 98% of feedpoint current [cos 10
degrees]. The antenna is 5 degrees long from top of coil to top of
antenna. The current at the top of coil calculates to 9% of feed current
[sin 5 degrees].

Conclusion: the coil, at 75 degrees of the circuit, has to be treated
differently from DC or LF models.

Current through coils
 

"Ian White GM3SEK" wrote
Reg Edwards wrote:
Has it ever occurred to you guys that a coil is a coil wherever it
is
used and always behaves in the same way.


That is only true if you say it about pure inductance.

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

Ian, old boy, you are no better than the rest of the gaggle !

There's not one of the clever buggers who can design a coil-loaded
whip for a given frequency using a pencil, paper and a pocket
calculator.

They have to copy an already existing, pre-tested, model after
searching through the antenna comics.
---
Reg.