Current through coils
 

wrote:
Your measurements are probably wrong.

Not much accuracy is needed to measure a constant phase of zero.
Anyone is invited to duplicate those phase measurements.

When did you measure that? After we resolve the error in current, we
can move on.

I rigged up a 6m dipole yesterday with current pickups driving equal
lengths of coax. Remember, we are not discussing the accuracy of
my magnitude measurements, only of my phase measurements.

After we resolve exactly who made the measurement error, we can
move on. My measurements agree with Figure 14-4 in Kraus'
"Antennas for All Applications", 3rd edition.

Question: would it be legal for me to scan that graphic and post it
on my web page if I give Kraus full credit?
--
73, Cecil, W5DXP

Current through coils
 

Indeed, Ian. Just so.

It's amazing to me that this thread has gone on for so many postings.
(And here I am contributing to it! ;-) But I go back to the very first
posting in the thread, where Reg said it all. Well, maybe not QUITE
all, but close. I'd invite folk to go back and look at that posting.
I believe Reg noted that a real physical (single-layer solenoid) coil
has: inductance, capacitance (with a radial electric field), wire
resistance and radiation resistance. One additional item I'd like to
note is turn-to-turn capacitance; Reg may have been thinking of that
one too, but I didn't get it explicitly from his posting.

Measuring currents doesn't present too much of a challenge if one is
careful about it, but measuring voltage is an entirely different
matter, since it's in the presence of a time-varying magnetic field if
there is any current in the coil. But it is possible to measure the
electric field and the magnetic field if one wishes.

I have not been following this thread very closely, because I really do
NOT expect "the answer" to be any different than what, as Reg noted in
that initial posting, is predicted by an ACCURATE model. People who
designed travelling wave tubes understood very well the properties of a
helix of wire with respect to propagating a wave. The software I've
been using for years now to predict single-layer solenoid coil
behaviour takes into account all the interesting effects, and will
predict quite accurately the first parallel resonance and the first
series resonance, the Q, the inductance, and fundamental transmission
line characteristics below resonance. I have other resources that let
me predict the change in behaviour when a coil is inside a shield.
I've never been surprised by any of the results: taken to correct
limits, they all join up, as Ian notes, with conventional circuit
theory. In fact, even the complex models match conventional circuit
theory, just with more elements in the model.

I do note that one must be careful about exactly what conventional
circuit theory actually says. For example, many people seem to think
that Kirchoff's Voltage Law is something like, "voltages around a loop
add up to zero." That is an abbreviated statement of the law, and is
in general not correct.

Cheers,
Tom

Current through coils
 

Bill Turner wrote:
I've been following this mind-numbing discussion for days now and still
don't have the answer to the original question:

Is the current the same at both ends of a mobile whip's loading coil?

Bill, W6WRT

No, it isn't, provided it's a solenoidal coil of reasonable length. I
don't think anyone disagrees with this. The disagreements are to why,
and what would happen to the current if the coil were made very short.

Roy Lewallen, W7EL

Current through coils
 

Cecil,

One more time.

In a standing wave antenna problem, such as the one you describe, there
is no remaining phase information. Any specific phase characteristics of
the traveling waves died out when the startup transients died out.

Phase is gone. Kaput. Vanished. Cannot be recovered. Never to be seen
again.

The only "phase" remaining is the cos (kz) term, which is really an
amplitude description, not a phase. The so-called "phase reversal" in
longer antennas is not really about phase either. It is merely a
representation of the periodic sign reversal seen in a cosine function.
(This is one more definition of phase to add to the confusion.)

Of course, all of this depends on an ideal system with no losses, etc.
The real world is not ideal, but your posed problem does not appear to
contain any of those nasty realities. You have undoubtedly seen small
phase offsets reported in EZNEC for this sort of antenna. Those phase
offsets represent the impact of real-world effects, such as radiation
and ground effects.

The applicability of linear superposition and the assumption of
steady-state conditions means that the resulting standing wave contains
ALL of the possible information about the system in steady-state mode.
Yes, you can divide the problem back into two traveling waves, in the
manner that Kraus, Balanis, and the entire world understand. But you
won't gain any new information by doing so, because any unique traveling
wave information is permanently lost.


73,
Gene
W4SZ



Cecil Moore wrote:

[snip]

These are not the results predicted by my neighboring ham friend.
I'm confused but here are the things I know for sure.

1. The resonant frequency changed when I installed the coil so
the coil is not a perfect replacement for the wire.

2. The feedpoint impedance decreased from 60 ohms to 45
ohms. Since 45 ohms is closer to 50 ohms than is 60 ohms,
I'm not too interested in knowing why..

3. The current at 'x' increased from 0.92 amp at 0 deg in the
wire dipole to 1.1 amp at 0 deg in the loaded dipole. The phase
didn't change.

4.The current at 'y' increased from 0.38 amps at 0 deg in the
wire dipole to 0.6 amp at 0 deg in the loaded dipole. The phase
didn't change.

5. No matter where I measure the current in either system, the
phase always comes up zero degrees between any two points
from tip to tip anywhere on either dipole no matter how far
apart are the measurement points. My neighboring ham friend said the number
of degrees in the coil had to be the number of degrees in the
wire and indeed, both are measured to be zero degrees, but
I wonder if that's really what he had in mind when he said
the delay would be equal. Zero equals zero, but what does
that mean for me?

The change in feedpoint impedance and the different current
magnitudes don't much bother me but I am really bothered
by those phase measurements. The dipole is 180 degrees long
and the current should be changing phase, at least on the wire
if not through the coil. I need some expert to explain how those
phase measurements on the wire are possible on both antennas.
I know my phase measurements are correct but why are they
always zero degrees? And since they are always zero degrees,
what information are they providing?
--
73, Cecil, W5DXP

Current through coils
 

Current through coils
 

"Roy Lewallen" wrote :
No, it isn't, provided it's a solenoidal coil of reasonable length. I
don't think anyone disagrees with this. The disagreements are to why,
and what would happen to the current if the coil were made very short.

The phase shift measurements are being made using a signal with
unchanging phase and thus incapable of providing a phase shift.

The disagreement is what would valid phase measurements actually
look like if a signal capable of changing phase was used to make the
phase measurements.
--
73, Cecil, W5DXP
..

Current through coils
 

Cecil Moore wrote:

I rigged up a 6m dipole yesterday with current pickups driving equal
lengths of coax. Remember, we are not discussing the accuracy of
my magnitude measurements, only of my phase measurements.

What was the indicator? What was the coupling device?

On six meters, it would take a darned small probe and indicator to not
greatly perturb the system.

If I was going to test something like this, I'd use a small indicator
hanging from the antenna and do it on a low frequency.

So, tell us about the probe and indicator.

73 Tom

Current through coils
 

"Gene Fuller" wrote:
Phase is gone. Kaput. Vanished. Cannot be recovered. Never to be seen
again.

So how can a signal, devoid of phase, be used to measure the phase
shift through a loading coil?

The only "phase" remaining is the cos (kz) term, which is really an
amplitude description, not a phase.

How can one make a phase measurement using only the amplitude
of a signal?
--
73, Cecil, W5DXP

Current through coils
 

"Roy Lewallen" wrote:
The measurement looks good to me. The phase is exactly what EZNEC
predicts -- constant along the wire. The ratio in magnitudes we'd expect
depends on the positions along the wire, not just the spacing.

We are not talking about the magnitude measurements right now.
We are talking about the phase measurements. What good does
it do to use a signal, whose phase is fixed, to measure the phase
shift through a coil or through a wire.

The phase is constant along the wire with or without the presence
of a coil. Why then is a phase shift of zero on both sides of the
coil surprising? And of what importance is that measurement?
--
73, Cecil, W5DXP

Current through coils
 

wrote

Cecil Moore wrote:
I rigged up a 6m dipole yesterday with current pickups driving equal
lengths of coax. Remember, we are not discussing the accuracy of
my magnitude measurements, only of my phase measurements.

What was the indicator? What was the coupling device?

I have an assortment of toroids of various materials from Amidon. I'm
at work right now and I don't remember if I used 43 or F material.

On six meters, it would take a darned small probe and indicator to not
greatly perturb the system.

They are small toroids. I chose 6m because the dipole area was
physically small.

If I was going to test something like this, I'd use a small indicator
hanging from the antenna and do it on a low frequency.

Please feel free to make that measurement. W7EL just reported that
EZNEC agrees with my phase measurements. So does Kraus.

So, tell us about the probe and indicator.

Similar to the ones W7EL used. They were calibrated within one turn
of each other. The signals at the ends of the coax lines were calibrated
for equality in magnitude and phase. Magnitudes are a relative measurement
but phase was not. I ran the experiment two ways.

One was Lissajous figures on my 100 MHz Leader. The other was putting
the two samples in opposite phase to each other, i.e. phasor subtraction.
For small angles, the angle is equal to the sine of the angle so the
addition
of two coherent sine waves yields an amplitude proportional to the phase
difference when the phase difference is small. The phase difference was
so small it was virtually undetectable.
--
73, Cecil, W5DXP