W7EL's Food for Thought: Forward and Reverse Power
 

Keith Dysart wrote:

Keith, I am preparing a web page on this subject.
Here are a couple of the associated graphics for
the earlier simple example.

http://www.w5dxp.com/easis1.GIF
http://www.w5dxp.com/easis2.GIF
--
73, Cecil http://www.w5dxp.com

Forward and Reverse Power
 

Keith Dysart wrote:

Hey Keith, how about this one?

Rs Pfor=50w--
+----/\/\/-----+----------------------+
| 50 ohm --Pref |
| |
Vs 45 degrees RLoad+j0
100v RMS 50 ohm line |
| |
| |
+--------------+----------------------+

The dissipation in the source resistor is:

P(Rs) = 50w + Pref

How can anyone possibly argue that reflected power
is *never* dissipated in the source resistor? :-)
--
73, Cecil http://www.w5dxp.com

Forward and Reverse Power
 

On Feb 21, 9:53*pm, Cecil Moore wrote:
Keith Dysart wrote:

Hey Keith, how about this one?

* * * * * * * *Rs * * * * * * Pfor=50w--
* * * * *+----/\/\/-----+----------------------+
* * * * *| * *50 ohm * * * * * *--Pref * * * *|
* * * * *| * * * * * * * * * * * * * * * * * * |
* * * * Vs * * * * * * * * * 45 degrees * * RLoad+j0
* * * 100v RMS * * * * * * * 50 ohm line * * * |
* * * * *| * * * * * * * * * * * * * * * * * * |
* * * * *| * * * * * * * * * * * * * * * * * * |
* * * * *+--------------+----------------------+

The dissipation in the source resistor is:

P(Rs) = 50w + Pref

How can anyone possibly argue that reflected power
is *never* dissipated in the source resistor? :-)
--
73, Cecil *http://www.w5dxp.com

Two Saturdays ago I was on a road trip and used
9 litres/100km. How can anyone argue that the fuel
consumption is never equal to the day of the month?
Numerical coincidences can be much fun.

But you will like the generator below even better.
The power dissipated in the generator resistors is
always equal to 50 + Pref, regardless of the load
and line length, thereby always accounting for Pref.

Pfor=50w--
+--/\/\/---+----------------+-------------------+
| 100 ohm | --Pref |
| | 100 ohm |
| +--/\/\/--+ any length |
| ^ | 50 ohm line any load
Vs Is | |
100v RMS 1A RMS | |
| | | |
+----------+---------+------+-------------------+
The generator output impedance is 50 ohms.
Dissipation in the generator resistors is always
50 Watts plus Pref.
With a shorted or open load, the power dissipation
in the generator is 100 W.

Numerical coincidence as proof that Pref is always
dissipated in the generator. :-)

On a more serious note, how would you analyze this
generator using reflected power and constructive
and destructive interference?

...Keith

Forward and Reverse Power
 

Keith Dysart wrote:
Numerical coincidence as proof that Pref is always
dissipated in the generator. :-)

It's no coincidence. The special case example was
carefully selected to make the angle 'A' between
the forward and reflected waves equal to 90 degrees.
Since cos(A) exists in the interference term and
cos(90) = 0, it makes the interference term equal
to zero.

When there is no interference at the source resistor,
100% of the reflected power is *always* dissipated in
the source resistor. This is a chosen special case
condition, NOT a coincidence. In the absence of
interference, there is simply no other place for the
reflected energy to go, i.e. it cannot be redistributed
back toward the load.

There are any number of special cases that will cause
the forward wave and reflected wave to be 90 degrees
out of phase at the source resistor. One of Roy's cases
was just such a case - 1/2WL of 50 ohm feedline with a
0 +/- j50 ohm load.

For any length feedline, there exists a load that
will cause the reflected wave to be 90 degrees out of
phase with the forward wave at the source resistor.
For any of those infinite number of cases, the
reflected energy will be dissipated in the source
resistor.

On a more serious note, how would you analyze this
generator using reflected power and constructive
and destructive interference?

Without analyzing it, based on our previous discussion
of voltage sources and current sources, I would say
that any constructive interference in the voltage
source is offset by an equal magnitude of destructive
interference in the current source and vice versa, i.e.
the net interference inside the source is always zero.
--
73, Cecil http://www.w5dxp.com

Forward and Reverse Power
 

On Feb 22, 5:37 am, Keith Dysart wrote:
On Feb 21, 9:53 pm, Cecil Moore wrote:



Keith Dysart wrote:

Hey Keith, how about this one?

Rs Pfor=50w--
+----/\/\/-----+----------------------+
| 50 ohm --Pref |
| |
Vs 45 degrees RLoad+j0
100v RMS 50 ohm line |
| |
| |
+--------------+----------------------+

The dissipation in the source resistor is:

P(Rs) = 50w + Pref

How can anyone possibly argue that reflected power
is *never* dissipated in the source resistor? :-)
--
73, Cecil http://www.w5dxp.com

Two Saturdays ago I was on a road trip and used
9 litres/100km. How can anyone argue that the fuel
consumption is never equal to the day of the month?
Numerical coincidences can be much fun.

But you will like the generator below even better.
The power dissipated in the generator resistors is
always equal to 50 + Pref, regardless of the load
and line length, thereby always accounting for Pref.

Pfor=50w--
+--/\/\/---+----------------+-------------------+
| 100 ohm | --Pref |
| | 100 ohm |
| +--/\/\/--+ any length |
| ^ | 50 ohm line any load
Vs Is | |
100v RMS 1A RMS | |
| | | |
+----------+---------+------+-------------------+
The generator output impedance is 50 ohms.
Dissipation in the generator resistors is always
50 Watts plus Pref.
With a shorted or open load, the power dissipation
in the generator is 100 W.

Numerical coincidence as proof that Pref is always
dissipated in the generator. :-)

On a more serious note, how would you analyze this
generator using reflected power and constructive
and destructive interference?

...Keith

It's easy to lose sight of what's important when you get bogged down
in numerical coincidences and the like. To me, some things are
clearly important with respect to analyzing such systems:

1. If a generator is linear and matched to a line (Zgen = Zline, not
Zgen* = Zline), then no matter where a "reverse" signal comes from,
that signal does not reflect at the source:line junction. The
"reverse" signal can come from a reflection at a load, from another
generator at the other end of the line, from something feed in through
a coupler, from an electric eel biting the line--it doesn't matter.
There is no need for an analysis involving "constructive" or
"destructive" interference.

1a. Just because a "reverse" signal on the line does not reflect at
the generator:line junction, that does NOT mean that additional power
is dissipated inside the source.

2. You MUST have an accurate model of the inside of the source to
know how it will respond to some particular load and to signals that
impinge on its output port. With respect to figuring out what goes on
inside the source and what power may or may not be dissipate there,
there is NO advantage to knowing how the load or signals got there.

3. To correctly analyze conditions on a line that's fed only from one
end, with a load on the other end, there is NO NEED OR ADVANTAGE to
know what goes on inside the generator (beyond knowing the power it
delivers to that effective load, perhaps).

3a. There may be some advantage in knowing the source impedance of a
generator (or transmitter) in calculating the power delivered to a
load at the source's output port, but there is no advantage to knowing
it if you want to determine the standing wave ratio or reflection
coefficient on the line, or what net impedance that line+load presents
to the source; that is all determined solely by the line and the load.

The stuff about constructive/destructive interference with respect to
figuring out what happens inside a source is, to me, just so much
dancing on the head of pins. Welcome to dance if you so wish, but I'd
just as soon sit that one out.

Cheers,
Tom

Forward and Reverse Power
 

K7ITM wrote:
It's easy to lose sight of what's important when you get bogged down
in numerical coincidences and the like. To me, some things are
clearly important with respect to analyzing such systems:

Is it a coincidence when one amp flows through a one ohm
resistor with one volt across it and dissipates one watt?
No, it is the laws of physics in action. The fact that
everything is a unity magnitude is because of the particular
values chosen for the example.

My example was NOT coincidence. I deliberately chose values
that would cause the forward wave and reflected wave to be
90 degrees out of phase at the source resistor. Under those
conditions, there is no interference present and all of the
reflected energy is dissipated in the source resistor. There
are an infinity of such examples and it is true for both
voltage sources and current sources.

The fact that there is even one example discredits the assertion
that reflected energy is *never* dissipated in the source.

1. If a generator is linear and matched to a line (Zgen = Zline, not
Zgen* = Zline), then no matter where a "reverse" signal comes from,
that signal does not reflect at the source:line junction. The
"reverse" signal can come from a reflection at a load, from another
generator at the other end of the line, from something feed in through
a coupler, from an electric eel biting the line--it doesn't matter.
There is no need for an analysis involving "constructive" or
"destructive" interference.

An interference analysis reveals exactly where all the energy is
going and that's what this discussion is all about. It may not
matter to you but it obviously matters to Keith and me.

1a. Just because a "reverse" signal on the line does not reflect at
the generator:line junction, that does NOT mean that additional power
is dissipated inside the source.

That's true. If total destructive interference exists at the
source resistor, then all of the reflected energy is redistributed
back toward the load. For the simple sources we have been using,
predicting how much reflected energy is dissipated in the source
resistor is a piece of cake.

I took Roy's chart and without calculating a single voltage or
current, not only matched Roy's correct results but I uncovered
an error he had made. That's a pretty good track record considering
that Roy's data went unchallenged for many years.

2. You MUST have an accurate model of the inside of the source to
know how it will respond to some particular load and to signals that
impinge on its output port. With respect to figuring out what goes on
inside the source and what power may or may not be dissipate there,
there is NO advantage to knowing how the load or signals got there.

We are discussing single-source, single transmission line, single
mismatched load systems. Where the energy components come from is
obvious.

3. To correctly analyze conditions on a line that's fed only from one
end, with a load on the other end, there is NO NEED OR ADVANTAGE to
know what goes on inside the generator (beyond knowing the power it
delivers to that effective load, perhaps).

This discussion is all about what is going on inside the source.
If you don't care to engage in that discussion, please feel free
not to.

3a. There may be some advantage in knowing the source impedance of a
generator (or transmitter) in calculating the power delivered to a
load at the source's output port, but there is no advantage to knowing
it if you want to determine the standing wave ratio or reflection
coefficient on the line, or what net impedance that line+load presents
to the source; that is all determined solely by the line and the load.

The argument about what happens inside a source is about 20 years
old now and is still raging. I'm simply trying to contribute
something to that argument.

The stuff about constructive/destructive interference with respect to
figuring out what happens inside a source is, to me, just so much
dancing on the head of pins. Welcome to dance if you so wish, but I'd
just as soon sit that one out.

That's your opinion and that's OK. If you choose not to attempt
to understand interference, you will forever remain ignorant of
its usefulness as an analysis tool. For the simple examples presented
so far, how much reflected power is dissipated in the source
resistor has been accurately predicted for all examples.

The interference phenomenon is well understood in the field of
optical physics and is a very useful tool in that field. The
principles are the same for RF waves. Why not use the tool?
Incidentally, optical physicists are NOT dancing on the head
of a pin when they calculate the irradiance of the bright rings
and dark rings.
--
73, Cecil http://www.w5dxp.com

Forward and Reverse Power
 

Cecil Moore wrote:


The interference phenomenon is well understood in the field of
optical physics and is a very useful tool in that field. The
principles are the same for RF waves. Why not use the tool?
Incidentally, optical physicists are NOT dancing on the head
of a pin when they calculate the irradiance of the bright rings
and dark rings.

Cecil,

I don't recall just how you became such an expert on optics, but your
proposed use of constructive and destructive interference is not the way
to calculate bright and dark rings. Great for handwaving explanations,
or textbook explanations, but close to useless for detailed calculations.

In the real world most most problems of interest are not simple one
dimensional set-ups with ideal lossless components. All of your nice
power equations with cosine cross-terms get completely unwieldy in the
real world.

Do you even wonder why you seem to be the pioneer in trying to apply
constructive and destructive interference to HF problems? Do you suppose
that no other smart folks ever thought along the same path? Do you
suppose there is a reason why essentially all of the textbooks and
scholarly writings on transmission lines virtually ignore constructive
and destructive interference for detailed calculations?

You have recently demonstrated that you can get exactly the same answers
as Keith, Roy, and others. However, beyond satisfying your own needs,
you have demonstrated exactly nothing in addition to the results
available from conventional analysis. That is dancing on the head of a pin.

There is really no particular need to discover "where the power goes".
The equations for ordinary classical physics are self-consistent. If one
gets the fields analyzed correctly, or equivalently the voltages and
currents, then energy and power will take care of themselves. You simply
will not find a case where all of the forces or fields are worked out
correctly but the energy is not conserved.

73,
Gene
W4SZ

Forward and Reverse Power
 

Gene Fuller wrote:
You have recently demonstrated that you can get exactly the same answers
as Keith, Roy, and others.

Which means, that contrary to all the earlier assertions,
an energy+interference analysis works. The analysis that
Roy called "gobbledegook" caught an error in his own data
that he had overlooked for many years and about which he
had made some false technical assumptions which he promoted
on his web page.

If an energy analysis provided nothing more than uncovering
the errors in Roy's math and concepts, it was more than worth
the effort.

There is really no particular need to discover "where the power goes".

Maxwell and Bruene have been arguing about such for 20
years now. The need to discover "where the power goes"
apparently exists for them and others.

If you, like Roy and Tom, don't care where the power
goes, that OK, but please don't try to present yourselves
as experts on a subject you don't care enough about
enough to have ever studied it in detail.
--
73, Cecil http://www.w5dxp.com

Forward and Reverse Power
 

On Feb 23, 2:32 pm, Cecil Moore wrote:
....
If you, like Roy and Tom, don't care where the power
goes,


I really do wish you'd learn to read. I did NOT say I don't care
where the power goes.

Forward and Reverse Power
 

K7ITM wrote:
Cecil Moore wrote:
If you, like Roy and Tom, don't care where the power
goes,

I really do wish you'd learn to read. I did NOT say I don't care
where the power goes.

I apologize, Tom. To me, the following quote sounded like
you don't care.

The stuff about constructive/destructive interference with respect to
figuring out what happens inside a source is, to me, just so much
dancing on the head of pins.

If you had to chose the correct implication which would it be?

A. I care about what happens inside a source.

B. I don't care what happens inside a source.
--
73, Cecil http://www.w5dxp.com