RadioBanter - Re-Normalizing the Smith Chart (Changing the SWR into the same load)

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W5DXP wrote:

Instantaneous power is useless if an understanding is desired.

A number of texts, in presenting analyses that enhance understanding,
use both instantaneous power and the instantaneous value of the Poynting
vector as part of the explanation. Perhaps you mean that it adds
unwelcome complexity to your simplified analysis?

I agree with Hecht and all my other references. Kraus, Jasik,
Balanis, Hecht, Ramo & Whinnery all agree that dealing with
instantaneous power is a waste of time. Instantaneous power
is essentially meaningless since the definition of power
requires a length of time. There is simply no such thing as
instantaneous power.

Instantaneous power is defined in the first half dozen books I pulled
from my bookshelf. They we

Physics: Weidner & Sells
Circuit analysis: Pearson & Maler
Network analysis: Van Valkenburg ("Power, instantaneous" is even indexed)
Electromagnetics: Johnk, Holt, Kraus

You certainly have a lot to straighten us out on in your forthcoming paper!

Power is always the energy passing a
point during a slot of time. Zero energy passes a point in
dt of time as delta-t approaches zero, by definition.

By exactly the same reasoning, instantaneous current can't exist, since
it has the same relationship to charge as power does to energy. By your
argument, we should all through away that "useless" and "boring"
analysis using time-varying currents, and deal only with average
currents. The new math indeed.

. . .

Roy Lewallen, W7EL

W5DXP wrote:
. . .
This is obviously a diversionary tactic. Instantaneous power is
essentially meaningless according to all my references and I
have a bunch of them. Would you care to provide a reference that
seriously deals with instantaneous power in transmission lines?

It was just day before yesterday I posted just such a reference (Magid).
But you seemingly rejected it out of hand as being probably wrong
without making an effort to even look at it. So what's the point in
providing you with references?

Roy Lewallen, W7EL

W5DXP wrote:
Roy Lewallen wrote:

p(t) = v(t) * i(t). Period. No phase, no vectors, no cross products.

Millions of power engineers have been taught that V*I*cos(theta) is power.
Are you disagreeing with that teaching?

Yes.

p(t) = v(t) * i(t) is virtually
worthless to a power engineer since a generator is a giant heat sink.

Most of my references say that instantaneous power is virtually meaningless
for RF and optics work. Can you provide a reference that extols the virtues
of instantaneous power applied to RF? Can you list any benefit for
considering
instantaneous RF power?

I've addressed the remainder of this in another posting, except for the
last question. The last question has been the subject of many postings
I've made over the past several months. If your memory needs refreshing,
a Google search should locate them quite easily.

Roy Lewallen, W7EL

The issue of the max. value of the magnitude of the reflection
coefficient came up in our student mess. It was shown in lab that it
must approach the square root of 5, which is close to 2.41.

But what is all of the noise about reflection coefficient (or SWR),
which measures what is happening down stream, having anything at all to
do with what is happening up stream. Wow. I have never had a student
who was that illogical. However, if I were to encounter such a person
at this time of life, I would suggest a career in politics or the UN.
My hat is off to those of you who are succeeding in educating the
lurkers. I marvel at your restraint towards others.
73 Mac N8TT

--
J. Mc Laughlin - Michigan USA
Home:

Roy Lewallen wrote:
. . .
By your
argument, we should all through away that "useless" . . .

That's "throw", not "through", of course. I really do know better.

Roy Lewallen, W7EL

Ian'
I don't see from what what Reg said that this should have
been unloaded on him in such a way.
I sure hope the " establishment" or "cabal" as I call them
don't follow you and pile it on .
Seems to me that Reg is a giver not a taker as far as ham
radio is concerned, and just as much as a asset to this
group as those wish to fire off a shot at the first
oportunity to give dutch courage to all those that wil
follow.
Now you can stick the label on me also, audacity was it?
After all I did say "cabal" ? What really surprises me is
that you have not noticed over the years how many people have
been attacked so wickedly on this newsgroup by people that
infer that "all is known" to which you whimsically
referred to.
The fact is that is if this were true a thread would
never exceed five postings after a particular expert
had posted. I can only imagine that Reg has not swallowed
all you have said and thus has raised your ire such that
you have lost your cool

Art
A real XG

"Ian White, G3SEK" wrote in message ...
Reg Edwards wrote:
It should not be forgotten this newsgroup is primarily intended for
exchange of information between radio amateurs. It is fairly clear
there's a number of retired professional engineers who are also radio
amateurs who frequent this newsgroup. Fine! I'm a long-retired engineer
myself. There are also a sprinkling of engineers who are professionals
in that they are paid for their services of some sort or another. There
are some people, perhaps with some small historical justification, have
the audacity to consider themselves to form an informal Establishment.

Reg, this "Establishment" is something that you just invented. The
audacity is yours.

Yes, there are people on this newsgroup who agree on lots of technical
points. And yes, they are often quite insistent about it.

(At this point, I'll change to "we", because I am one of the people
we're talking about. And despite his chosen role as maverick,
provocateur and general gadly, so too is Reg ;-)

Nobody knows everything, but each of us has been around for long enough
to have covered most of the same territory, and to have made all the
common mistakes. But we've also been around long enough - and been
persistent enough - to have come out the other end with some correct
answers.

(Here comes Philosophy of Science in two paragraphs. Please don't give
me a hard time over the things I'm going to leave out.)

What's a correct answer? One that accurately describes external,
physical, observable, measurable reality, as nearly as we can measure it
or need to know. People who take that view are not too comfortable with
approximations and half-truths, because sooner or later we're going to
be caught out by a new situation where the "nearly correct" answers
don't work any more. In the long run, it's simply easier to work out a
correct answer that can be trusted.

Each of the people Reg is talking about (in fact, especially Reg
himself) worked out many of those answers for ourselves, way before we
met on the Internet. But when we did meet, it wasn't at all surprising
to find ourselves agreeing on so much. That's because all correct
answers are describing the same physical reality, so they *have* to
agree. The only allowable difference is to choose different viewpoints
upon the same reality; but if they are correct, all those alternative
viewpoints still *must* be consistent with one another - they must *not*
contradict in even the smallest detail (unless it's acknowledged to
involve an approximation).

Those rules are rigid, but nobody made them - reality did. The only
available choice is whether to accept them and stick by them, or to
sometimes let them slide. I choose to accept them, as much as I possibly
can, because it's actually easier in the long run. If someone else's
better logic forces me to re-evaluate, that's really not a problem
because I'm better-off than before.

Yes, people like that can be rather insistent! But it's not because we
want our personal faiths or beliefs to prevail - it's not about us at
all. It's because we want external, physical, observable, measurable
reality to prevail.

Cue for somebody to pipe up about quantum physics, Heisenberg,
uncertainty and all that. There are certain areas of science and
engineering where these apply and even dominate, and mean that certain
things are fundamentally un-knowable. Antennas, transmission lines and
electronic engineering are *not* one of those areas. It's an excellent
approximation to say that everything in these areas of physics and
engineering is fixed and "real", and - if we work hard enough at it -
knowable.

The exceptions, such as noise and the internal physics of
semiconductors, can all be treated by engineers as macro-scale
observables. A good example is the tunnel diode, which relies on a
probabilistic quantum phenomenon that just can't happen according to
classical science... but at the engineering level you can still rely on
the device data. You don't see "Probably" stamped on each page.

Cue now for someone else to pipe up "But we don't know everything yet."
No, we don't... but for every genuine new discovery, there are billions
of mistakes, experimental and logical errors. When straightened out,
these simply confirm that what we already knew is correct. You'd better
learn to trust that body of existing knowledge much more that you trust
yourself! I certainly do, and again life is easier as a result.

The good thing about the Internet is that we can all learn much faster
than before. I certainly have. The bad thing is that we no longer make
our mistakes in private. Stuff that used to end up as crumpled balls of
paper, to be thought about more clearly tomorrow, is now being published
every night to be read by thousands of people and archived forevermore.
It's becoming harder and harder to identify the good information among
all the rest.

To anyone who has already made those same mistakes, it seems such a
waste to see people wandering off and doing it all over again. It's very
hard to stand by and let it happen...

"J. McLaughlin" wrote
The issue of the max. value of the magnitude of the reflection
coefficient came up in our student mess. It was shown in lab that it
must approach the square root of 5, which is close to 2.41.

================================

You don't need a lab. All you need is a pencil and the back of a cigarette
packet.

Theoretical Max possible value = 1+Sqrt(2) exactly = 2.4142136 . . . . .

Can't imagine where you get 5 from.

It occurs when line Zo = Ro - jXo has an angle of -45 degrees, ie., when Xo
= -Ro, and when the line is terminated in an inductive reactance of +jXo.

----
Reg.

On Tue, 19 Aug 2003 03:06:15 -0700, Roy Lewallen
wrote:

Thanks for the analogy.

One can mathematically and conceptually conceive two opposite-traveling
waves that add up to the observed standing wave, and that's fine. The
problem comes with assigning power or energy to the waves. Then you run
into the problem of how one wave got the energy over the barrier into
the pocket and the other wave took the same amount back out, without
transfering any air molecules across the barrier in the process.

The average power analysis looks to me something like this. Suppose you
have two batteries each with exactly 2 volts potential and zero internal
resistance, with a 2 ohm resistor connected between their positive
terminals. The negative terminals are connected together. You replace
the battery on the left with a short (turning it off), and observe that
the current through the resistor is one amp to the left. Then you hook
the left hand battery back up and turn the right hand battery "off" by
replacing it with a short. You observe that there's one amp now flowing
through the resistor to the right. Finally, turn both batteries "on" by
putting them both in place. You can use superposition to conclude,
correctly, that there's zero current flowing in the resistor.

But it's silly to insist that there's a forward two watt "power wave"
flowing to the right, and another two watt wave flowing to the left. You
subtract one from the other and, sure enough, get zero. But are the
"power waves" real? Studying and analyzing these imaginary waves is
surely a lot more interesting than simply looking at the circuit and
noting that the "boring" (as Cecil calls it) net power is zero. But
aren't you studying ghosts?

Interesting question, isn't it? It seems to me that this is a class
of problem that is succeptable to solution by several approaches,
classical network theory, or transmission line theory and perhaps
others. Identifying the solution that best matches physically
observable phenominia isn't always obvious. Applying Ockham's Razor
(spell it any of the alternative ways you wish) one might prefer the
simpilist theory that produces correct results.

But, to some extent the artifical conditions necessary in such a
statement are responsible for some of difficulty. I = E/R but as as
you make R zero, you have introduced a singularity into Ohm's law, so
in some sense I isn't determinable. There is also a finite propagation
time for current flow across the resistor, (and through the battery,
for that matter) and that might argue for a transmission line
solution.

It also reminds me a bit of the question posed in electromagnetic
theory class. You've probably seen it before as well. You have two
wires going through the room you are in. With a clamp-on ammeter, you
measure 1 amp in each wire, in opposite directions. With a volt meter,
you measure 1,000 volts between the two wires. When asked to calculate
the power flow, you answer 1,000 watts, being 1KV * 1A.

However, the actual circuit is 4 wires, and you only see two in the
room. Two loops of 1 volt batteries with a 1 ohm series resistance,
with the two loops bridged via a 1KV source. No current flow from the
1 KV battery and the total power is 2 watts, being 1 watt in each
loop. As I recall, the only way to get the correct answer is to
properly determine the Poynting vector, but it's damn near 40 years
since I looked at it.

1 ohm A B 1 ohm
+---WWW--------+ +---WWW--------+
| | | |
| | | |
+ + + = 1 volt battery
| = | +
| | | |
+--------------+--+||---+--------------+
A B
1KV battery

You can only see the wires at A-A and B-B.

Jack K8ZOA

Even more risky is adding the things. This time hook two one volt
batteries in series with the 2 ohm resistor and energize one at a time.
With the upper one on and the lower one "off" (replaced with a short)
you get 1/2 amp. You've got a "power wave" of I^2 * R = 1/2 watt. Turn
the lower one on and the upper one "off", and you get another "power
wave" of 1/2 watt, in the same direction. Turn them both on, and you
have a power flow of, um, 2 watts. Welcome to the new math.

Roy Lewallen, W7EL

Sounds like a variation on the old two identical caps with equal
charge are shorted together. Where does the excess energy go?

Jack Smith wrote:

Roy:

Interesting point and I don't recall reading or hearing it elsewhere.

The following is dashed off without fully thinking it through, so no
warranty on its accuracy.

If you think of a sound wave (longitudinal transmission, of course) in
a lossless acoustic transmission line terminated with a short, the
individual air molecules within each 1/4 wave section are likewise
trapped since at the 1/4 wave points there is zero sound pressure.
This may be a useful analogy for the electromagnetic transverse
propagating T-line.

Jack K8ZOA

W5DXP wrote:

wrote:
I see your confusion and apologize for not being completely clear.
When I say P = V * I, P, V and I are instantaneous values, the only
ones worth exploring if an understanding is desired.

Instantaneous power is useless if an understanding is desired.
I agree with Hecht and all my other references. Kraus, Jasik,
Balanis, Hecht, Ramo & Whinnery all agree that dealing with
instantaneous power is a waste of time. Instantaneous power
is essentially meaningless since the definition of power
requires a length of time. There is simply no such thing as
instantaneous power.

May I recommend differentiation: In the limit, as t approaches 0....
Very useful results can be obtained with this technique.

Power is always the energy passing a
point during a slot of time. Zero energy passes a point in
dt of time as delta-t approaches zero, by definition.

Well, if you do a bit of fudging you can always make it work. But
I do not observe these fudge terms in the expressions related to
forward and reverse power in transmission lines.

Then you haven't read Dr. Best's QEX article. The interference is
there and takes the form of 2*Sqrt(P1)*Sqrt(P2) for 100% constructive
interference.

Of circuit theory I have a reasonable grasp, optics I leave to others.

Too bad. The field of optics has already solved the problems with which
you are wrestling. Take a look at this web page to figure out how Z0-
matching works.

http://www.mellesgriot.com/products/optics/oc_2_1.htm

As mentioned previously: stored in the line and moving back and
forth between quarter wave boundaries.

Mentioned, but not explained. How does the energy's momentum change
direction in a constant Z0 environment? What causes the back and
forth movement?

Back in basic circuit theory, some years ago, it was permitted to
short points of equal voltage and open conductors with no current.
Has this changed in the intervening years?

Ahhhh, I see your confusion. Circuit theory and transmission line
theory are not the same thing. Many have tried to mix the two
models and fallen on their faces. Transmission line theory is
simply more complicated than basic circuit theory.

Well, slightly more. The capacitances and inductances are spatially
distributed. Should make the math a bit more complicated but hasn't
introduced anything new.

If you don't
believe it, replace a transmission line with ghosting with an
equivalent circuit - the ghosting disappears. Are they really
equivalent?

Well if the ghosting disappears, it was not the 'equivalent circuit'.
No surprises there.

Because the observed voltages and currents are the same. The circuit
has been replaced by one which is indistinguishable from the first.

BS! I can certainly distinguish between a short and a non-shorted
transmission line. Please try again.

Just to recap, the experiment was: a matched source connected to an open
transmission line. After the source is turned on (and a brief wait for
settling), measure any voltage or current you desire at any point on
the line. I short the points of zero voltage and cut the points with
zero current. You can now remeasure any current or voltage at any
points you choose to tell me whether the shorts or cuts are in or out.

Since you can not tell the difference, the circuit with cuts and opens
is identical to the one without.

You have gone from 100%
transmission of waves to 100% reflection of waves.

An intriguing assertion, but one which can not be demonstrated through
any observations made on the circuit.

Of course it can by observing ghosting in a TV signal. With the line
shorted, there is no ghosting. With the line not shorted, there is
ghosting. Why is that so difficult to understand?

We were, I thought, discussing an open transmission line, not one
carrying a TV signal. You have changed the experiment.

I am not sure why you are looking so hard for reflections. I am not
attempting to claim that there are any. I simply claim that no energy
can cross the boundaries because the current or voltage is always 0.

In the real world, if no RF energy crosses a boundary, then that energy
is reflected since it cannot stand still. Of course, the supernatural
world is an entirely different matter.

Basic electriciy. This says nothing about the presence or absence of
reflections or whether there is a mechanism to prevent the energy
crossing the boundary or it just happens through the dynamics of the
distributed capacitance and inductance.

Unfortunately, this is beyond the ability of basic electricity to
handle.

It takes the wave reflection model (or quantum physics)

Oh, I don't think we need quantum physics to understand energy
distributions in ideal transmission lines. Let's not scare off
the neophytes.

to
handle it. Einstein said a model should be as simple as possible,
but not too simple. Once again, you seem to be trying to force
reality to obey your model instead of vice versa.

Oh, I fully understand what a 'directional watt' meter will indicate.

Then why are you so confused? :-)

May I suggest that you perform the same experiment with a real
instantaneous watt meter:

This is obviously a diversionary tactic.

Not at all. A much better understanding will be had if both the
indications of real wattmeters and 'directional watt' meters are
understood. Limiting yourself to only one instrument limits the
opportunity for full understanding.

Instantaneous power is
essentially meaningless according to all my references and I
have a bunch of them. Would you care to provide a reference that
seriously deals with instantaneous power in transmission lines?

It's not so hard that you need a text book. V, I, cos(theta) and
a little thinking will suffice. Draw a sine wave for the voltage
and one for the current shifted by 90 degrees. Compute the
power (which will be a new sine wave at double the frequency).
Observe that the power goes positive and negative representing
forward and reverse energy flow. At any instance (at a single point)
the power is either positive or negative. Energy is either flowing
in one direction or the other at any given instance.
And remember that at any point where the voltage or current is
always 0, no energy is ever flowing.

....Keith