Current through coils
 

Myron A. Calhoun wrote:
"--I`m not learning much."

Alas, I`d wager I don`t have anything new for Professor Calhoun, but
from some of the postings, what I write may be new in part to someone.

Current is defined as movement of electrons through a conductor.
Obviously, an incomplete definition. As we know, there is beam current
in the vacuum of a CRT, and that at r-f, skin-effect forces most
electrons to the surface so that most electron movement is on or very
near the surface of the conductor rather than "through" it.

When currents of equal amplitude and opposite direction meet at various
points along a perfect transmission line, as they might after a perfect
reflection of the incident wave, one might measure zero amps at
cancellation points along the line. These would be distributed
periodically as the waves, in fixed phase relation and equal and
opposite currents, coincide. The measurement of zero amps occurs because
the ammeter is measuring both currents simultaneously. If a directional
coupler is used to measure the current in each direction separately, it
will be found that the currents traveling in opposite directions are
passing through each other without effect. Standing waves are more
manifestation than anything else.

There was a PBS TV Channel appeal tonight. It Quoted Einstein as saying:
"Nothing happens until something moves".

Incident and reflected waves move on a transmission line but standing
waves are stationary and don`t move. So, it is the incident and
reflected waves that make something happen. Not the stationary waves.

Circulating energy within a transmission line causes standing waves.
Without a reflection, they don`t exist. Energy must be accepted by an
antenna to be radiated. It is best if it is accepted on the first pass
so that there are no more losses on additional passes. A perfect match
at the antenna accepts energy on the first pass. A mismatched antenna
produces an SWR. Magnitude of the SWR is an indication of how much
mismatch there is.

Best regards, Richard Harrison, KB5WZI

Current through coils
 

wrote:
My obviously-overloaded must-be-pea-sized brain sure has trouble thinking
of current which is NOT flowing, since my basic internal definition of
"current" is something like "electrons flowing past a point".

Forward current is a traveling wave and is flowing. Reflected
current is a traveling wave and is flowing. Both can be
represented by phasors. But since they are traveling in
opposite directions, their phasors are rotating in opposite
directions and their superposed sum always add up to the
same constant phase angle = zero (in a thin wire).

Given a 1/2WL (-90 deg to +90 deg) thin wire dipole fed in
the center (at 0 deg) with 1.0 amps, the standing wave current
magnitude on the antenna is cos(theta) and the phase angle
is a constant zero degrees from end to end. Reference: Figure
14-2, page 464 of "Antennas for All Applications", by Kraus
and Marhefka, 3rd edition.

If the phasor sum of two currents is not rotating, it is not
flowing. Since phasors, by definition, change phase at a rate
of (2*pi*f) and the standing wave current doesn't change
phase at all, I don't think standing wave current is a phasor.
Standing wave current is what we measure on a standing-wave
antenna like a 75m mobile center-loaded bugcatcher.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Richard Harrison wrote:
If a directional
coupler is used to measure the current in each direction separately, it
will be found that the currents traveling in opposite directions are
passing through each other without effect. Standing waves are more
manifestation than anything else.

And this is equally true of standing wave antennas. Kraus goes
as far as assuming the forward and reflected currents on a
1/2 wavelength thin-wire dipole are equal in magnitude so
they must be close to equal in magnitude. I have estimated that
the magnitude of the reflected current after the round trip
to the end of the dipole and back to be in the ballpark of
90% of the forward current at the feedpoint.

This entire episode of constant current through a loading coil
is the result of thinking standing wave current flows. As you
say it is a manifestation of our thinking process and our
measurements. It doesn't flow at all.

Incidentally, a horizontal dipole above earth can be analyzed
as a single-wire transmission line system with the earth being
the ground return path. Balanis says, "Standing wave antennas,
such as the dipole, can be analyzed as traveling wave antennas
with waves propagating in opposite directions (forwards and
backwards) and represented by traveling wave currents If and Ib
in Figure 10.1(a)."

The forward current (If) through a loading coil is very close
to constant magnitude. The reflected current (Ib) back through
a loading coil is very close to constant magnitude. Their
phasor sum is the standing wave current that we measure. There
is no physics requirement that the phasor sum of the forward
and reflected currents be of equal magnitude on both ends of
the loading coil.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Reg Edwards wrote:
By the way, reflections and standing waves are irrelevant and don't
enter the argument. Sorry Cecil!

I'm curious, Reg. With traveling wave energy flowing in both
directions through a loading coil, how do you analyze it
without taking the forward and reflected energy into account?
All my reference equations have a term with a positive exponent
for forward current and another term with a negative exponent
for reflected current.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Reg Edwards wrote:

I'm afraid Cecil has a fixation about standing waves and reflections.

They are part and parcel of a distributed network analysis
and are embedded in many transmission line equations.

As far as the constant current through the coil goes:

Neglecting losses, the forward current through a coil is
constant magnitude with changing phase. The reflected
current through a coil is constant magnitude with changing
phase. Those conditions satisfy the necessary and sufficient
conditions of the laws of physics concerning a coil.

The laws of physics do not require the phasor sum of the
forward current and reflected current to be constant
magnitude. In fact, the laws of physics prohibit the
standing wave current from being equal magnitude except
at equidistant points from a current maximum or minimum.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Cecil Moore incorrectly posted:

W8JI measured a 60 degree phase shift through a 100uH coil
at 1 MHz. Consider that at one end of that coil the
forward and reflected currents may be:
Ifor = 0.55 amps at zero deg, Iref = 0.45 amps at zero deg.

Inet = Ifor + Iref = 1 amp at zero degrees.

At the other end of the coil, the forward and reflected
currents may be:

Ifor = 0.55 amps at +60 deg, Iref = 0.45 amps at -60 deg
Inet = Ifor*cos(60) + Iref*cos(-60)

Inet = 0.275 + 0.225 = 0.5 amps at zero deg

The above analysis clearly shows how wrong Cecil is, and how he invents
reasons to support things.

I NEVER measured a current shift of 60 degrees, and I never said I
measured a current difference of 60 degrees.

The phase shift I measured was in VOLTAGE. It simply shows the voltage
is out of step with the current. It doesn't indicate current is shifted
60 degrees between each inductor terminal at all, and I never said it
did.

The current level at each end of the inductor was, as far as I can
measure with test equipment, equal.

This is another clear case of Cecil taking things out of context and
mixing them with his idea that an inductor treats current differently,
depending on what direction it "flows" (or whatever he is claiming, I
certainly can't make sense of his arguments).

It is always better to let people directly post what they say, and not
have it run through a "Cecil Moore" filter.

73 Tom

Current through coils
 

wrote:
I NEVER measured a current shift of 60 degrees, and I never said I
measured a current difference of 60 degrees.

The phase shift I measured was in VOLTAGE. It simply shows the voltage
is out of step with the current. It doesn't indicate current is shifted
60 degrees between each inductor terminal at all, and I never said it
did.

The subject was Kraus' 180 degree current phase shifting coils. Here
are you exact words (and all of your words about that measurement)
quoted from qrz.com:

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, crossing ZERO phase at self resonance (where loss became very
high) near 18 MHz, and gradually increasing leading phase above 18MHz
reaching 90 degrees and staying there well above resonance.

I apologize for missing the small detail that S12 was a voltage
measurement rather than a current measurement but I'm sure you can
see how that was an honest mistake and easy to make. You didn't
mention "voltage" at all in your posting and the context was current.
I didn't recall until your objection here today that S12 is a voltage
parameter measurement.

But that leads to a question. Why were you using voltage measurements
to try to disprove Kraus' statement about 180 degree current phase
shifting coils. Quoting from: "Antennas for All Applications", Kraus
and Marhefka, 3rd edition, page 824: "A coil (or trap) can also act
as a 180 deg (current) phase shifter as in the collinear array ...
The coil may also be thought of as a coiled-up 1/2WL element."

The current level at each end of the inductor was, as far as I can
measure with test equipment, equal.

That sure doesn't make technical sense. If there was no phase shift
in the current, then the voltage was lagging the current. But we know
the current lags the voltage through an inductor by as much as 90
degrees in the ideal case. If the voltage is delayed by 60 degrees,
then the current must necessarily be delayed by 60 degrees plus the
lag to satisfy the laws of physics. If you will run the experiment
using current probes, I assure you that the current will experience
more of a phase shift than the voltage, just as the laws of physics
predict. Which means there was more than a 60 deg current phase
shift through the coil which makes my argument even stronger.

Now, if you are talking only about the magnitude of the current then
of course, the current was equal at both ends of the coil because
reflected energy was absent for that measurement. IT IS THE PRESENCE
OF REFLECTED ENERGY THAT MAKES FALSE YOUR ASSERTION ABOUT NET CURRENT.
I think we are in perfect agreement about systems without reflections.

This is another clear case of Cecil taking things out of context ...

Not out of context, Tom. The entire quote is just above. The mistake
was an honest one and easy to make. I'm only human. :-)

and
mixing them with his idea that an inductor treats current differently,
depending on what direction it "flows" ...

Sorry, I never said that. An inductor treats forward waves and
reflected waves exactly the same according to the laws of physics.
Your statement is more akin to your idea that standing wave current
flows into the bottom of a coil and out the top. The coil treats all
traveling waves exactly according to the laws of physics and exactly
as you and I understand those laws of physics. I accept everything
you say about traveling wave current through a coil. My argument with
you is that a standing wave current is not a traveling wave current
and doesn't behave like a traveling wave current. That seems rather
obvious to me.

1. Just as you say, the forward traveling-wave current through a coil
is of constant magnitude. Here's what Walter Maxwell says: "If an
inductance is in series with a line that has no reflections, the current
will be the same at both ends of the inductor." All three of us agree on
that statment.

2. As in (1) above and just as you say, the reflected current through
a coil is of constant magnitude.

3. There is no law of physics that requires the standing wave
current to be equal at the top and bottom of a coil. In fact,
such a requirement violates those laws of physics. Here's what
Walter Maxwell says: "If an inductance is
in series with a line that has reflections, the current will NOT be
the same at both ends of the inductor. Consequently, circuit analysis
will not work when both forward and reflected currents are present
in a lumped circuit. When reflections are present, a current node
and a current loop can appear at separate points on an inductor
simultaneously." Judging from what he has posted earlier, Richard
Harrison agrees with those statements.

There's an EZNEC graphic at: http://www.qsl.net/w5dxp/qrzgif35.gif

Would you have us believe that 0.1+ amps is flowing into the bottom
of the coil and 0.7+ amps is flowing out of the top of the coil?
It is your concept that standing wave current flows that is the
problem. Please explain how a current with a zero phase angle
from tip to tip on a 1/2WL thin wire dipole can possibly flow without
a rotating phase angle.

It is always better to let people directly post what they say, and not
have it run through a "Cecil Moore" filter.

I agree, Tom, but you were not posting here so I quoted what I honestly
thought you said over on qrz.com. I quoted the same thing on qrz.com over
a number of days. You could have pointed out my mistake a lot sooner and
saved me from making it here.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Walter Maxwell, W2DU wrote:
Hi Cecil, I just today found the (QRZ.com) discussion. I agree
with your position 100%. It's as simple as this:

If an inductance is in series with a line that has no reflections,
the current will be the same at both ends of the inductor.

If an inductance is in series with a line that has reflections,
the current will NOT be the same at both ends of the inductor.

Consequently, circuit analysis will not work when both forward
and reflected currents are present in a lumped circuit.

When reflections are present, a current node and a current loop
can appear at separate points on an inductor simultaneously.

Walt

I think it would be better if Walt represented himself, unless he ASKED
you to post that Cecil.

Why not let Walt post what he thinks rather than someone else telling
us what Walt thinks? We each should speak for ourselves and let other
people speak for themselves. If Walt has something to say, I'm sure he
will chime in.

Drumming up support by writing what we **think** others say or mean
seems pretty desperate.

All of that aside, what does it matter who agrees or disagrees? Things
work they way they work. All reliable measurements I've seen disagree
with Cecil's theory.

I can't believe anyone with experience in RF systems would think a
physically small inductor (small in terms of wavelength) would have
significantly unequal currents in the inductor or in the terminals of
the inductor, especially any difference caused by "reflected waves".

73 Tom

Current through coils
 

wrote:
I think it would be better if Walt represented himself, unless he ASKED
you to post that Cecil.

Walt isn't presently posting for reasons of his own.
He certainly gave me permission to quote his email. I
will ask him if he wants to defend his statements here.

If Walt has something to say, I'm sure he will chime in.

I'm sure he will if he feels so inclined.

Drumming up support by writing what we **think** others say or mean
seems pretty desperate.

Those were Walt's words, not mine. (Your statement reeks of
desperation, i.e. "Please don't quote any authorities who
disagree with me.")

All reliable measurements I've seen disagree with Cecil's theory.

Simply not true, Tom, proving once again that you don't understand
what I am saying. My "theory" explains exactly why neither you
nor W7EL could get the current at the ends of the coil to be
equal without creating a current maximum point inside the coil.

If you would just take time to understand what I am saying,
you wouldn't need to utter falsehoods about what I am saying.

The standing wave net current at each end of the coil is indeed
the same magnitude if a current maximum or current minimum point
is located inside the coil.

Walter Maxwell says: "When reflections are present, a current node
and a current loop can appear at separate points on an inductor
simultaneously." Does that sound like constant net current through
a coil? Exactly what is it about Walt's statement that you don't
understand?

I can't believe anyone with experience in RF systems would think a
physically small inductor (small in terms of wavelength) would have
significantly unequal currents in the inductor or in the terminals of
the inductor, especially any difference caused by "reflected waves".

A 75m bugcatcher coil is about 0.2% of a wavelength and so qualifies
as "physically small". The current at each end of that coil used
with an eight foot whip is nowhere near equal. I cannot believe
that anyone with experience in RF systems could utter such nonsense.
(That logic is a double-edged sword.)

I think you and others simply didn't realize that the current in
a 75m mobile bugcatcher system is almost 100% a standing wave
current and now you guys are desperately trying to cover up your
mistake. Others have stopped responding instead of admitting their
mistake. So I ask you again: How do you explain the flow of a current
whose phase is unchanging (fixed at zero degrees in a thin wire
dipole). Doesn't it take a rotating phasor for current to flow?
That's a simple yes/no question. What do you want to bet that it
goes unanswered?

Incidentally, I posted a little questionaire a couple of days ago.
The response to that questionaire has been zero. Why do you reckon
all the gurus are afraid to respond?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

wrote:
I think it would be better ...

I forgot to ask you, Tom, why you trimmed out all the
technical content from my posting and whinned about
what was left? So I ask you once again:

In the graphic at: http://www.qsl.net/w5dxp/qrzgif35.gif

do you really expect anyone to believe that 0.1+ amp
is flowing into the bottom of that coil and 0.7+ amp
is flowing out of the top of the coil. If, as you say,
net standing wave current actually flows, then miracles
must be possible and the coil is creating energy.

If, OTOH, net standing wave current just stands there
then such results as reported by EZNEC are just the
facts of physics. It is the forward current and
reflected current that are doing the flowing and
their magnitudes are indeed individually equal through
the coil.
--
73, Cecil http://www.qsl.net/w5dxp