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Those diagrams were highly misleading, and have been withdrawn from the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

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

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

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

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

....[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)

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.

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

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

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