Current through coils
 

Richard Harrison wrote:

ON4UN has a graph, Fig 9-22 on page 9-15 of "Low-Band DX-ing" which
shows current distribution of a base-loaded whip, In his example, the
whip is 45-degrees long.. The loading coil provides the extra
45-degrees required for resonance.

Those diagrams were highly misleading, and have been withdrawn from the
current 4th edition. That whole introduction to short verticals has been
completely rewritten and revised.

The problem was that the current profiles of the full-size and loaded
quarter-wave antennas were both drawn against a linear vertical scale of
*phase*. This not only misrepresented the physical lengths of the lower
and upper sections, it also misrepresented the length and effect of the
coil. This presentation is highly misleading, and the first victim was
the person who drew it, and then overlaid current profiles on it.

(I don't think this was ON4UN. When someone sets out to produce a
500-page book, they have to quote some things on trust from other
people; and it may take a few editions to iron out all the kinks. Much
more to the point, ON4UN is right at the leading edge of his subject,
and each edition contains something new and important.)

The replacement diagrams in the 4th edition are much better. They show
current profiles against *physical* height, and help bring out what's
really happening.

Current at the base of ON4UN`s whip is one amp times the cosine of
45-degrees, or 0.707 amp. The loading coil has an input of one amp.

With 1 amp into the loading coil and 0.707 amp out of the loading coil,
the coil definitely does not have the same current at both ends.

You can't quite those figures in evidence, because they were never more
than a speculation based on misunderstandings. When ON4UN came to think
about it, he quite rightly changed his mind.



This brings up another point that hasn't been mentioned so far in this
discussion: there is an important difference between purely inductive
loading, and the kind of loading you can get from any practical
inductOR.

Note the difference. An inductOR is a real-life electronic component, a
coil of wire. InductANCE is its main electrical property - but it isn't
the only one.

When a shortened antenna is loaded by pure inductance, you find the
following:

1. The magnitude and phase of the current flowing into the loading
inductance are both the same as that of the current flowing out (this is
a fundamental property of pure inductANCE). This means the current
distributions in the sections above and below the loading inductance
join up in a sharp kink.

2. What does go through a step change is the *voltage* across the
inductance. This changes in both magnitude and phase. For a typical
centre-loaded whip, the RF voltage is low in all of the bottom section,
but above the loading inductance it's very high indeed.

To repeat: this is the situation for loading with pure inductANCE.

(Sorry to keep shouting "ANCE!" and "OR!" like that, but I'll bet
someone still comes back with a reply that proves they didn't read what
I actually wrote.)

The diagram in ON4UN's 4th edition (Fig 9-44) shows these effects much
better than I can describe them in words. The diagram came from an
article by W7XC in QST for March 1990. I strongly recommend everyone to
look at these diagrams... but please don't treat them as 'bible text'.
Do your own thinking about it.

A different viewpoint on loading inductANCE is given in an article by
Boyer in 'Ham Radio'. This uses the 'antenna-transmission line analog'
theory... and comes to exactly the same conclusions about the effects of
loading inductANCE: it is simply there to resonate the capacitive
reactance that arises from having physically shortened the antenna.
(Ironically, ON4UN's worked examples and computer programs to calculate
loading inductance have always been based on this approach; the
conceptual error in early editions was only in that one diagram.)

With a firm grip on the way that pure inductANCE loads an antenna, you
are then in a good position to look at the differences that appear when
you use a practical inductOR.

With a real-life inductOR, you don't have pure inductANCE any more. It
is embedded in a component that is made from some length of wire, wound
into a coil that has a physical length and diameter, has capacitance
between its own turns, and also has capacitance to the straight sections
of antenna above and below it. The effects of the coil's inductANCE will
still be there, but you can certainly expect to see a lot of detailed
differences.

But the practical differences can't possibly be understood without that
basic understanding about inductANCE as a foundation. Without it, you're
building your house on sand.



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

Current through coils
 

Ian White GM3SEK wrote:
1. The magnitude and phase of the current flowing into the loading
inductance are both the same as that of the current flowing out (this is
a fundamental property of pure inductANCE).

That is a fundamental property of a pure inductance in a lumped
circuit analysis which assumes a DC current or a pure traveling-
wave current. It is NOT a fundamental property of a pure inductance
if the current you are talking about is a net standing wave
current. Your stated principle is simply false for a standing
wave environment. In a transmission line, it is easy to install
a coil that has zero current at one end and an amp of current
at the other end.

It simply doesn't apply in a standing wave environment - and a
75m bugcatcher loaded mobile antenna is a standing wave antenna.
Please take a look at my example and questionaire to understand
what is wrong with your above statement.

The measured current at the bottom of a loading coil is primarily
standing wave current. IT IS NOT FLOWING.

The measured current at the top of a loading coil is primarily
standing wave current. IT IS NOT FLOWING.

Since neither of these two currents are flowing, they don't
have to be equal. They just stand there.

If I present to you a black box with zero amps at one terminal
and one amp at the other terminal, what can we conclude? One
possibility is 1/4 wavelength of coiled up coax with an
infinite SWR. Please ponder that and apply it to your coil
assertion above.

The currents that are doing the flowing are the underlying
current components, the forward current and the reflected
current and they are close to equal. Everything you say
about a coil is true for the forward current and the
reflected current. It is simply not true for the standing
wave current which is just a conceptual construct and not
a flowing phasor at all.

If you really want to accurately apply the principles you are
asserting, you must treat the forward current and reflected
current separately and then superpose the results. Applying
your above principle to standing wave current is akin to
superposing power and that's a no-no.

I have never seen such a wide-spread blind spot.

Take the transmission line example.

---------------------------X----------------------------
Ifor=1.0amp -- --Iref=1.0amp

There's a black box at 'X'. Inside the black box is 1/4WL
of coiled up transmission line. The current measured at
left of the black box is zero amps. The current measured
at the right of the black box is 2 amps. That doesn't
violate any laws of physics. That obeys the laws of physics
for a transmission line with reflections. You are measuring
the currents at a current node and at a current loop. It's
absolutely no big deal.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Cecil,

A few questions.

Do you have a reference that explains this "net standing wave current
that does not flow"?

Does this magical current have any other interesting properties besides
the lack of charge flow?

Do ordinary properties such as inductance and capacitance cease to
function when dealing with "net standing wave current"?

Do you really think it is a good idea to base detailed numerical
analysis on these "conceptual constructs" as you call them?

Do you have a convenient listing of your "conceptual contructs" so that
we can avoid these battles in the future?


8-) 8-)

73,
Gene
W4SZ

Current through coils
 

Cecil Moore wrote:
Ian White GM3SEK wrote:
1. The magnitude and phase of the current flowing into the loading
inductance are both the same as that of the current flowing out (this
a fundamental property of pure inductANCE).

That is a fundamental property of a pure inductance in a lumped
circuit analysis which assumes a DC current or a pure traveling-
wave current. It is NOT a fundamental property of a pure inductance
if the current you are talking about is a net standing wave
current. Your stated principle is simply false for a standing
wave environment. In a transmission line, it is easy to install
a coil that has zero current at one end and an amp of current
at the other end.


Be very careful here. We're talking about the effect of cutting the
physically shortened wire antenna, and inserting a loading device. This
therefore has to be a TWO-terminal device.


It simply doesn't apply in a standing wave environment - and a
75m bugcatcher loaded mobile antenna is a standing wave antenna.
Please take a look at my example and questionaire to understand
what is wrong with your above statement.

The measured current at the bottom of a loading coil is primarily
standing wave current. IT IS NOT FLOWING.

The measured current at the top of a loading coil is primarily
standing wave current. IT IS NOT FLOWING.

Since neither of these two currents are flowing, they don't
have to be equal. They just stand there.


I'm sorry, but those last three paragraphs are simply contradictions in
terms, which demonstrate their own illogicality.

Electrical current is defined as a net rate of transfer of electrons, so
by the very definition of the term there is literally no such thing as a
non-flowing current (except when the current is exactly zero and the
definition becomes moot).

I seriously wonder if you understand what a standing wave is. It is
simply a pattern of variation in current along the length of a
transmission line, which is stable in time.

If you pick any point along the transmission line or antenna wire,
there is a simple net current characterized by one amplitude and one
phase, relative to some other reference point. (In this whole discussion
we discount the normal cyclic sinusoidal variation of instantaneous RF
current which is happening everywhere in the system.)

In our minds, we may choose to explain the causes of the standing wave
by resolving the net physical current into conceptual forward and
reverse components; but the physical system doesn't know what you are
thinking. To be valid, your concept must do nothing more than explain
what's seen to be happening; it cannot seek to affect it.

At the point where you have to say that a measured (and therefore
measurable) current does not flow, your concept is in trouble.


If I present to you a black box with zero amps at one terminal
and one amp at the other terminal, what can we conclude? One
possibility is 1/4 wavelength of coiled up coax with an
infinite SWR. Please ponder that and apply it to your coil
assertion above.

Your length of coiled up coax is a FOUR-terminal device, like Richard's
transformer was. It isn't an applicable solution for this problem.


The currents that are doing the flowing are the underlying
current components, the forward current and the reflected
current and they are close to equal. Everything you say
about a coil is true for the forward current and the
reflected current. It is simply not true for the standing
wave current which is just a conceptual construct and not
a flowing phasor at all.

If you really want to accurately apply the principles you are
asserting, you must treat the forward current and reflected
current separately and then superpose the results.

It is entirely *your* responsibility to ensure that your postulated
forward and reflected currents obey the same circuit laws as the
physical net current. If you cannot do that, your concept fails.


Applying
your above principle to standing wave current is akin to
superposing power and that's a no-no.

I have never seen such a wide-spread blind spot.

Take the transmission line example.

---------------------------X----------------------------
Ifor=1.0amp -- --Iref=1.0amp

There's a black box at 'X'. Inside the black box is 1/4WL
of coiled up transmission line. The current measured at
left of the black box is zero amps. The current measured
at the right of the black box is 2 amps. That doesn't
violate any laws of physics.

The laws it violates are those of logic. Your black box is not allowed
to sometimes have two terminals and sometimes need four.

That obeys the laws of physics
for a transmission line with reflections. You are measuring
the currents at a current node and at a current loop. It's
absolutely no big deal.

Sorry, I just don't see it. But what I do see are the contradictions and
inconsistencies of logic that you are forced to resort to, in order to
arrive at the conclusion you've already decided upon. I think that
proves the exact opposite.



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

Current through coils
 

Gene Fuller wrote:
Do you have a reference that explains this "net standing wave current
that does not flow"?

No, this is one of those topics that is supposed to be "obvious
to the most casual observer" and not worthy of further comment.
Why it is not obvious to you and others is beyond belief. Some-
thing terrible has happened to the educational system since 1957
when I learned all these principles at Texas A&M. I assume that
this is just a part of the "dumbing down" of the educational
system that I keep hearing so much about. Incidentally, these
concepts are obvious to Walter Maxwell.

Does this magical current have any other interesting properties besides
the lack of charge flow?

It's not magical but yes, it's phasor value (if it has one)
doesn't rotate which is prima facie evidence that it doesn't
flow. Any flowing phasor has a rotation of omega (2*pi*f).
The fact that standing waves don't have a rotation is proof
that they don't flow. To tell the truth, standing waves are
a product of the human mind. The forward and reflected waves
couldn't care less about standing waves. Last time I talked
to them, they didn't even know that each other existed except
for the points in the transmission line where their constructive
interference causes the wire to heat up. :-)

Surely you understand that standing waves in a transmission line
don't flow - they just stand there, which is why they are called
"standing waves". Exactly the same principle applies to standing
wave antennas.

Do ordinary properties such as inductance and capacitance cease to
function when dealing with "net standing wave current"?

No, but one has to exercise caution and invoke the superposition
principle to ascertain what is happening. The two traveling waves
have to be analyzed separately and then superposed to obtain valid
results. If you analyze net current without superposition, you are
doing the same thing as superposing powers, which is a known no-no.

Do you really think it is a good idea to base detailed numerical
analysis on these "conceptual constructs" as you call them?

All human thought is based on conceptual constructs. That's what
makes us different from the rest of the animals. If you don't
like "conceptual constructs", then go swim in a zoo pool with
the alligators. :-)

Do you have a convenient listing of your "conceptual contructs" so that
we can avoid these battles in the future?

Only dealing with one at the moment, Gene. The lumped circuit
model falls apart unless you first apply it to the component
currents and then superpose. The distributed network analysis
was developed to avoid that very problem. So the only valid
choices are to either use the lumped circuit analysis on the
component currents and then superpose or use the distributed
network analysis which, in the end, boil down to the same thing.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Ian White GM3SEK wrote:
Be very careful here. We're talking about the effect of cutting the
physically shortened wire antenna, and inserting a loading device. This
therefore has to be a TWO-terminal device.

Yes, I realize that. Do you realize that the characteristic
impedance of a single #14 wire 30 feet above ground is 600
ohms? That pesky ground return path raises its ugly head once
again. It's impossible to install a two-terminal system 30
feet above *ground* and have it remain a two-terminal system.
It's only a two-terminal system in your mind.

Electrical current is defined as a net rate of transfer of electrons, so
by the very definition of the term there is literally no such thing as a
non-flowing current (except when the current is exactly zero and the
definition becomes moot).

You have hit the nail squarely on the head without realizing it.
A non-flowing current doesn't exist in reality but that is exactly
what you are measuring when you measure standing-wave current.
The only things that exist in reality are the forward and reflected
current. So you guys are basing your high and mighty concepts on
something that doesn't even exist in reality. No wonder you are
confused. You are measuring two currents flowing in opposite
directions at the same time and don't realize it.

I seriously wonder if you understand what a standing wave is. It is
simply a pattern of variation in current along the length of a
transmission line, which is stable in time.

Nope, that's not what it is. For example, a current standing
wave on a particular transmission line is the sum of one amp
flowing in one direction and one amp flowing in the opposite
direction. Exactly what is the net charge flow when identical
currents are flowing in opposite directions? Let's see now,
this is a really tough one. One amp flowing in one direction
minus one amp flowing in the opposite direction. What could
the result possibly be? :-) Hint: think DC to see what the
net charge flow would be.

If you pick any point along the transmission line or antenna wire, there
is a simple net current characterized by one amplitude and one phase,
relative to some other reference point. (In this whole discussion we
discount the normal cyclic sinusoidal variation of instantaneous RF
current which is happening everywhere in the system.)

I suggest you review traveling wave phasors which rotate at omega
(2*pi*f). A standing wave 'phasor' doesn't rotate at all so a standing
wave current is not moving. I'm not even sure it is technically valid
to call a standing wave current a "phasor" since it doesn't even possess
a frequency characteristic. Please think about a perfectly stationary,
non-revolving 'phasor' and then comment. Wouldn't a non-revolving
phasor be DC?

To be valid, your concept must do nothing more than explain
what's seen to be happening; it cannot seek to affect it.

The same thing applies to your concepts. So what do your concepts
say about a phasor with an omega(2*pi*f) equal to zero as is the
case for standing waves? Are standing waves really DC? Do they
exist at all anywhere besides the human mind?

At the point where you have to say that a measured (and therefore
measurable) current does not flow, your concept is in trouble.

Sorry, I have absolutely no idea what that means. Surely you have
measured zero current at a standing wave current minimum where the
forward current equals one amp and the reflected current equals
one amp. Is that zero amps in the act of flowing?

Your length of coiled up coax is a FOUR-terminal device, like Richard's
transformer was. It isn't an applicable solution for this problem.

If you include that pesky ground under antennas, it is. My electronics
equation book contains a formula for the characteristic impedance
of a single wire transmission line over ground. Is that invalid?
Doesn't that sound very much like a dipole wire in the air?

The laws it violates are those of logic. Your black box is not allowed
to sometimes have two terminals and sometimes need four.

An antenna system installed on this earth is always a four terminal
system whether you like it or not. Haven't you ever seen those diagrams
of the current return to ground from an antenna system?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Ian White GM3SEK wrote:
Electrical current is defined as a net rate of transfer of electrons, so
by the very definition of the term there is literally no such thing as a
non-flowing current ...

I'm sorry, I forgot to provide a reference for your
non-existant non-flowing current. On page 464 of
"Antennas for All Applications", by Kraus and Marhefka,
3rd edition, it shows the current on a 1/2WL dipole along
with its phase. The phase is *fixed* at zero degrees over
the entire 1/2 wavelength. So what does an RF current
with a fixed phase of zero degrees really mean? It means
that the 'phasor', if it is indeed a phasor, doesn't
flow. How could an RF current with a fixed phase of zero
degrees manage to flow? e^wt would be zero.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

....[snip]....
The measured current at the bottom of a loading coil is primarily
standing wave current. IT IS NOT FLOWING.
....[snip]....

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".

I'm enjoying reading this thread, but, what with all the difficulties
my brain is having with such subtle points, I'm NOT learning much!

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

Current through coils
 

A coil inevitably occupies space.

In particular, one of its dimensions is length.

Therefore it can be, and indeed for accurate modelling always should
be, treated as a component having distributed L, C and R.

It just makes the mathematics somewhat more complicated. Hyperbolic
functions can be involved.

Like a transmission line, a coil possesses Zo, phase-shift,
attenuation and Q.

It is why my coil-loaded antenna programs provide answers in the right
ball-park although I havn't a clue about the rules which govern the
American ball game.

By the way, reflections and standing waves are irrelevant and don't
enter the argument. Sorry Cecil!
----
Reg, G4FGQ.

Current through coils
 

Myron,

I'm afraid Cecil has a fixation about standing waves and reflections.
He brings them into arguments on every possible occasion.
Nevertheless he is very convincing and manages to drag most people in.

Give your brain a rest. Visit your nearest barber shop and tune up.
You will feel much better.
----
Reg.