Current through coils
 

Richard Clark wrote:
What are they when you raise the assembly (I distinctly note that this
is NOT the resonance of the COIL you are speaking of) two feet higher?

Isolating the magmount would be disconnecting the
ground plane from a 1/4WL antenna and would contribute
nothing of value to the discussion.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Richard Harrison wrote:
Distributed network theory is newer than lumped network theory but both
have been around plenty long enough to be well established.

Neumann didn't prove that the current through a coil is uniform.
He *assumed* it to be true as a simplification, so he could factor
out the current term from under the integral sign in one of Maxwell's
equations. Now Neumann's mathematical shortcut is being reported
here as a fact of physics. That's how well established it is.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Roy Lewallen wrote:
For that case, it's been theoretically and demonstratively shown to be
equal at both ends.

Only one of the reported measurements showed the magnitudes of
the currents to be equal and that was a small toroidal coil
quite unlike a 75m bugcatcher coil. Wes's web page shows the
currents to be unequal at the ends of the coil. Figure 3 shows
1.03 amps at the bottom of the coil and 0.66 amps at the top
of the coil. That's not equal.

http://www.k6mhe.com/n7ws/Loaded%20antennas.htm

--

Here's an example from EZNEC where the currents are not
only unequal, but if one considers to standing wave current
to be flowing, more current is flowing into the bottom of
the coil than out the top. What does the lumped-circuit model
have to say about that? This technical question goes unanswered.

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

--

As for phase shift using standing wave current, consider the
following half of a dipole:

FP----------x----------y----------

'x' is at the 30 degree point and 'y' is at the 60 degree
point. What is the standing wave phase shift between point
'x' and point 'y'? We can see from Figure 14-4 in Kraus'
"Antennas for All Applications", 3rd edition, that the
measured phase shift would be zero between 'x' and 'y'.
So why is it a surprise that if the wire between 'x' and
'y' is replaced by a coil, the phase shift remains zero?
The measured phase shift in the wire and the coil are the
same. This technical question goes unanswered.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Cecil Moore wrote:
Last night I posted 1/4WL, 3/4WL, and 5/4WL measured resonant
points using the same coil stock as W8JI but with a shorter
50 uH coil. This coil 2" dia, 8 tpi, 8.5" long. It was
sitting on a mag mount on my metal desk.

I think what we have here is Cecil has bought into the idea that
radically different behavior on one or more self-resonant frequencies
means we can apply an analogy of that behavior to all other
frequencies.

Most of us probably want a theory that works all the time, and we
probably think only one situation that proves the theory wrong is
required to dismiss the theory as being incorrect.

What Cecil is asking us all to do is is agree to a theory that only
works in special cases and ignore the times it does not.

73 Tom

Current through coils
 

Cecil Moore wrote:
Richard Harrison wrote:
Distributed network theory is newer than lumped network theory but both
have been around plenty long enough to be well established.

Neumann didn't prove that the current through a coil is uniform.
He *assumed* it to be true as a simplification, so he could factor
out the current term from under the integral sign in one of Maxwell's
equations. Now Neumann's mathematical shortcut is being reported
here as a fact of physics. That's how well established it is.

At the most basic level, Cecil does not understand what scientific
theories are all about.

That was not a "shortcut" or approximation. It was a rigorous test for
the limiting case where the coil has no other properties except pure
inductance, so no electromagnetic radiation is taking place. In that
limiting case, Maxwell's equations MUST join up with conventional
circuit theory... and indeed they DO.

In that limiting case, the current at the two terminals of a pure
inductance must be the same in both magnitude and phase. By "current" we
mean the simple, straightforward movement of charge. If you count the
electrons in and out at the two terminals, there can be no difference in
either magnitude or phase because that would require electrons to be
stored or lost from somewhere - which inductance cannot do. Kirchhoff's
current law recognises the logic of this.

This is how inductance always works in every type of non-radiating
circuit, both in theory and in real life.


When developing a new theory, it is normal, standard required practice
to test it for simplified, limiting cases that we already understand.
The new theory MUST work for all these test cases; it MUST connect
seamlessly with everything we already know.

At his point, some heckler pipes up: "Ah, but what about Einstein?"
Thank you, sir - the perfect example to prove my point! If Einstein's
equations of relativity are tested for the limiting case where
velocities are very low, they connect seamlessly into Newton's laws of
motion. If they hadn't, Einstein would have thrown them out and gone
back to think again.

Another point: antennas are the home territory of classical physics,
where everything is consistent with everything else. Classical physics
is plain physical reality, verified every day, a billion times over.
All of our validated knowledge joins together completely seamlessly -
and that's how we test anything new. In the home territory of classical
physics, reality is hard, sharp and clear. It allows no gaps and no
excuses. "What don't fit, ain't true."

Trained scientists accept the discipline of that, and turn it into a
useful tool. Engineers aren't always trained to think that way, but the
best are of a mind to do it anyway. They're creating new ideas all the
time, and the ones that "don't fit" or "don't make sense" are simply
abandoned (sometimes they're abandoned before they even make it into
fully conscious thought). One way or another, lots of ideas get thrown
away; no big deal, it's just part of the process of *having* ideas.

So...

When Cecil's theory is tested for the simple limiting case of pure
inductance, it MUST join up seamlessly with conventional circuit theory.
If it requires anything that "don't fit", such as a phase shift in
current through a pure inductance, or special kinds of "current" that
are different from the simple, straightforward movement of charge
(electrons), then the theory fails.



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

Current through coils
 

Cecil Moore wrote:
I think what we have here is Cecil has bought into the idea that
radically different behavior on one or more self-resonant frequencies
means we can apply an analogy of that behavior to all other
frequencies.

False. Nobody has said anything about "all other frequencies".

OK. I'm not sure what you are trying to say. It sounds like we all know
and agree a reasonable physial size lumped inductor with good flux
coupling between ends and stray C to the outside world that is small
compared to termination impedance works like a lumped inductor below
self-resonance of the inductor. There is only a tiny transmission line
effect and that effect has more to do with spatial size than conductor
length.

So...

What are you trying to say? Can you give an example in you own words
explaining what you are trying to say and why it is meaningful or
useful to others?

Surely you can put it all into a few words.

73 Tom

Current through coils
 

On Thu, 09 Mar 2006 21:40:40 GMT, "Cecil Moore"
wrote:

Your 100uH coil above exhibits 60 degrees of phase shift
even for the voltage and that's 1/6 wavelength

On Thu, 16 Mar 2006 08:56:19 GMT, Cecil Moore wrote:

When the speculation is that the coil presents a 1:1 replacement for
the delay of the "missing" segment of the resonant antenna, then this
premise stumbles at the starting blocks.

Nobody said anything about a 1:1 replacement. That was just
somebody's strawman.

We all know who "somebody" is. [threadbuster #4]

But if this is news to you, it must have been one of your other
personalities (Hokum's Razor?) at the keyboard who posted the message
at the top.

Current through coils
 

On Thu, 16 Mar 2006 09:08:37 GMT, Cecil Moore wrote:

Richard Clark wrote:
What are they when you raise the assembly (I distinctly note that this
is NOT the resonance of the COIL you are speaking of) two feet higher?

Isolating the magmount would be disconnecting the
ground plane from a 1/4WL antenna and would contribute
nothing of value to the discussion.

Certainly not for you, that is apparent. It would only more closely
conform to your reference material's isolated coils, and that would
jeopardize your "confirmations" n'est pas?

No matter, this thread has been put to bed anyway. It is coasting in
entertainment gear now.

Current through coils
 

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

Current through coils
 

wrote:
OK. I'm not sure what you are trying to say. It sounds like we all know
and agree a reasonable physial size lumped inductor with good flux
coupling between ends and stray C to the outside world that is small
compared to termination impedance works like a lumped inductor below
self-resonance of the inductor.

I absolutely agree about the characteristics of an inductor presupposed
by the lumped-circuit model. But one cannot use the presuppositions
of the model to prove the model is valid for all cases. The dissagreement
here is one of degree. I would have stated the assertion above, it behaves
"... like a lumped inductor *far* below self-resonance of the inductor." Dr.
Corum sets the "far below" range at 15 degrees or 1/6 the self-resonant
frequency.I previously showed that 15 degrees of 450 ohm transmission
line will change a load of 50+j0 into a load of 54+j100 ohms. That may
be accurate enough for Dr. Corum to start using the lumped-circuit model,
but not for me. I would be more inclined to set the limit at 5 degrees.

What are you trying to say? Can you give an example in you own words
explaining what you are trying to say and why it is meaningful or
useful to others?

What I am trying to say can best be illustrated by an example task that
I need to perform. I have a 180 foot dipole that I need to shorten so I
am going to install loading coils. I want the length of the loaded antenna
to be 90 feet. I'm pretty naive at this, so I am just going to build the
coil
out of the wire I remove from the antenna. My neighboring ham friend
says that will work just fine. And in the process of reducing the size of
my antenna, I want to learn something about antennas so I have borrowed
two toroidal current pickups to measure the current.

Note that at the frequency where the dipole is 1/2WL and resonant,
it is 180 feet long and 180 degrees long so the number of feet of
wire is also the number of degrees of antenna. Here is my 1/2WL
dipole with current pickup coils installed at points 'x' and 'y' and
FP is the feedpoint,the impedance of which is 60 ohms.

------------------------------FP-------x---------------y-------

Total length is 180 feet. The distance between 'x' and 'y' is 45 feet.
Since feet = degrees in this case, the number of degrees between
'x' and 'y' is known to be 45 degrees from antenna theory. Those
45 degrees are what I am going to attempt to replace with a coil.

So I adjust the feedpoint current to one amp at a reference phase
angle of zero degrees and measure the current at 'x' and the current
at 'y'. The current at 'x' is 0.92 amp at 0 deg. The current at 'y' is
0.38 amp at 0 deg. Already I am not understanding my measurements.
The electrical length between 'x' and 'y' is obviously 45 deg. Why is
there no phase shift at all in the measured current between 'x' and
'y' and the feedpoint current? But I want to complete this task so
I will wait until later for an explanation to that apparent paradox.

I take the 45 feet of wire from each side of the dipole, wind it
into two coils, and install them in each side. Now the 90 foot
long dipole looks like this.

-------//////-------FP------x-//////-y------

I make some more measurements with the feedpoint current set
to one amp at zero degrees. The resonant frequency of the dipole
has changed from the earlier resonant frequency. I have to adjust
the number of turns on the coil to return to the original frequency.
I discover that the feedpoint impedance has dropped to 45 ohms.
I measure the current at each end of the coil and at one end it is
1.1 amp at 0 deg and at the other end it is 0.6 amp at 0 deg

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