Constructive interference in radiowave propagation
 

Jim Kelley wrote:
So I'm happy to
leave it to you to explain to Cecil how waves cancel but without
anhiliating the energy "in" them.

But that's just the point, Jim. You seem to believe the
pre-existing energy in those waves has been destroyed.
They obviously possessed energy before cancellation and
you say they possess zero energy after cancellation. If
that pre-existing energy is not destroyed, where did it go?
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

On 14 Apr 2007 14:46:22 -0700, "Jim Kelley" wrote:

So I'm happy to
leave it to you to explain to Cecil how waves cancel but without
anhiliating the energy "in" them.

Hi Jim,

That would be flogging the asphalt through the stripped ribs of a dead
horse.

73's
Richard Clark, KB7QHC

Constructive interference in radiowave propagation
 

Jim Kelley wrote:
So I'm happy to
leave it to you to explain to Cecil how waves cancel but without
anhiliating the energy "in" them.

No need for that, Jim. Florida State University has done
an excellent job of explaining how wave cancellation
"redistributes" the pre-existing wave energy in "new
directions" such as the opposite direction in a
transmission line (the only other direction possible).

"... when two waves of equal amplitude and wavelength that
are 180-degrees ... out of phase with each other meet, they
are not actually annihilated, ... All of the photon energy
present in these waves must somehow be recovered or redistributed
in a new direction, according to the law of energy conservation ...
Instead, upon meeting, the photons are redistributed to regions
that permit constructive interference, so the effect should be
considered as a redistribution of light waves and photon energy
rather than the spontaneous construction or destruction of light."
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Cecil Moore wrote:
Jim Kelley wrote:
So I'm happy to
leave it to you to explain to Cecil how waves cancel but without
anhiliating the energy "in" them.

No need for that, Jim. Florida State University has done
an excellent job of explaining how wave cancellation
"redistributes" the pre-existing wave energy in "new
directions" such as the opposite direction in a
transmission line (the only other direction possible).

"... when two waves of equal amplitude and wavelength that
are 180-degrees ... out of phase with each other meet, they
are not actually annihilated, ... All of the photon energy
present in these waves must somehow be recovered or redistributed
in a new direction, according to the law of energy conservation ...
Instead, upon meeting, the photons are redistributed to regions
that permit constructive interference, so the effect should be
considered as a redistribution of light waves and photon energy
rather than the spontaneous construction or destruction of light."

The killer is that word "somehow"... "all of the photon energy must
somehow be redistributed".

Well of course it must! Nobody denies that conservation of energy will
hold, in a system with properly defined boundaries. But the weakness of
a photon model is that it cannot provide a detailed nuts-and-bolts
explanation of the mechanism by which that energy becomes redistributed
in time and space.

A wave model will provide all of that detail - and in transmission-line
problems we can use it. If we trace what happens to forward and
reflected waves of voltage (and/or current) we can predict the
magnitudes and phases of those quantities at any location, at any
instant. That gives us a complete time-dependent map of the voltage and
current across the entire system.

From that, we can also find out where the energy is - the inputs,
outputs, losses and stored energy. Sure enough, we will find that energy
is conserved within the system boundaries... but that is no big deal, we
always knew it would. In a wave model, conservation of energy is
something you should check for, but only as an overall confirmation that
you've done the sums correctly. All the useful detail came from the
analysis of the voltage and/or current waves.



--

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

Constructive interference in radiowave propagation
 

Ian White GM3SEK wrote:
Cecil Moore wrote:
"... when two waves of equal amplitude and wavelength that
are 180-degrees ... out of phase with each other meet, they
are not actually annihilated, ... All of the photon energy
present in these waves must somehow be recovered or redistributed
in a new direction, according to the law of energy conservation ...
Instead, upon meeting, the photons are redistributed to regions
that permit constructive interference, so the effect should be
considered as a redistribution of light waves and photon energy
rather than the spontaneous construction or destruction of light."

The killer is that word "somehow"... "all of the photon energy must
somehow be redistributed".

That's not a killer, Ian, that's a challenge to people
like me to figure out how. If there is indeed a "somehow",
then there has to be a "how". Please don't try to dampen
my curiosity like the church priests tried to dampen Galileo's
curiosity.

Well of course it must! Nobody denies that conservation of energy will
hold, in a system with properly defined boundaries. But the weakness of
a photon model is that it cannot provide a detailed nuts-and-bolts
explanation of the mechanism by which that energy becomes redistributed
in time and space.

I'm sure a QED explanation exists but we might have trouble
understanding it. I would like for you and others to follow
me through an energy analysis to see if you can find anything
technically wrong with it besides your revulsion to the approach.

A wave model will provide all of that detail - and in transmission-line
problems we can use it. If we trace what happens to forward and
reflected waves of voltage (and/or current) we can predict the
magnitudes and phases of those quantities at any location, at any
instant. That gives us a complete time-dependent map of the voltage and
current across the entire system.

From that, we can also find out where the energy is - the inputs,
outputs, losses and stored energy. Sure enough, we will find that energy
is conserved within the system boundaries... but that is no big deal, we
always knew it would. In a wave model, conservation of energy is
something you should check for, but only as an overall confirmation that
you've done the sums correctly. All the useful detail came from the
analysis of the voltage and/or current waves.

I agree with everything except your last sentence.
There is lots of useful information to be had from
tracking the energy through the system including
how and why the energy in the reflected wave changes
direction and momentum. If you think that information
doesn't matter or is not useful, then that's your
opinion. But please don't condemn the individuals who
find that information useful and go for an explanation.
And please don't say that explanation is wrong if you
cannot prove it to be invalid.

In the process of tracing forward and reflected waves,
we must remember that they obey the laws of physics
including their energy contents. The average forward energy
per unit time in a forward voltage of Vf RMS volts is
Vf^2/Z0 joules/sec, an assumption upon which the S-Parameter
analysis system is based. The average reflected energy per
unit time in a reflected RMS voltage is Vr^2/Z0 joules/sec.

In an S-Parameter analysis, if you square any of the
normalized voltage terms, you get joules/sec. Someone
said that at microwave frequencies, the powers are
often easier to measure than the voltages and currents.
The powers can be measured and the voltages and currents
calculated from the power measurements.

In optics, physicists don't have the luxury of dealing
with voltages and currents. They must necessarily deal
with energy and power. That field of physics is older
(and wiser) than RF engineering and they deal with power
reflection coefficients, not voltage reflection coefficients.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Jim Kelley wrote:
Partially reflective surfaces can (and are) in fact used to prevent
reflections, just as they are used to 100% re-reflect partial
reflections from a load.

Let's look at one of those reflective surfaces from
the standpoint of the forward wave in an S-Parameter
analysis.

a1----|
|----s21(a1)
s11(a1)----|

a1 is the normalized forward voltage, e.g. 10
s11 is the voltage reflection coefficient, e.g. 0.707
s21 is the voltage transmission coefficient

|a1|^2 is the forward power called Pfor1 in my energy
analysis article.

|s11(a1)|^2 is the reflected power called P3 in my
energy analysis article.

|s21(a1)|^2 is the transmitted power called P1 in
my energy analysis article.

The point is that s11(a1) is a steady-state value for
normalized reflected voltage that never makes it through
the impedance discontinuity.

|s11(a1)|^2 is the steady-state reflected joules/sec
that never makes it through the impedance discontinuity.

Here is a fill in the blank question for you and anyone
else who wants to respond.

If a Z0-match exists, the above values of normalized voltage
and joules/sec do not reach the source during steady-state
because __________________________________________________ _.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Cecil Moore wrote:
The point is that s11(a1) is a steady-state value for
normalized reflected voltage that never makes it through
the impedance discontinuity.

I ended the sentence too soon. It never makes it through
the impedance discontinuity without help from somewhere.

|s11(a1)|^2 is the steady-state reflected joules/sec
that never makes it through the impedance discontinuity.

Same he It never makes it through the impedance discontinuity
without help from somewhere.

If a Z0-match exists, the above values of normalized voltage
and joules/sec do not reach the source during steady-state
because __________________________________________________ _.

What is the nature of that "help from somewhere"?
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Cecil Moore wrote:
Jim Kelley wrote:
Partially reflective surfaces can (and are) in fact used to prevent
reflections, just as they are used to 100% re-reflect partial
reflections from a load.

Partially reflective surfaces cannot, by themselves,
reflect 100% of the incident energy. If it's partial,
it's not 100%, by definition. Any partially reflective
surface needs help from interference in order to
achieve 100% reflection. You know, that interference
that you deny exists.

That was the main point of my post, Cecil. The reflective coefficient
DOES NOT CHANGE. You're the one who claims that it does.

You continue to lie about what I said. I have said any
number of times that the physical reflection coefficient,
s11, is fixed and does NOT change. Why does someone who
is technically correct need to stoop to lying?

There is no energy "in" cancelled waves.

The waves existed along with their energy components before
they were canceled. What happens to those energy components
after the waves are canceled. If one sets one phase equal
zero and the other phase equal 180 degrees, what happens to
the energy in the two waves at:

http://micro.magnet.fsu.edu/primer/j...ons/index.html


There are two waves on the left existing with their respective
voltage and joules/sec. The result of total destructive interference
is zero voltage and zero joules/sec. What happened to the original
joule/sec components?


Cecil,

I pointed out a few days ago that the FSU Java applet you lean on so
heavily these days is a simple tutorial device designed by a grad
student and a programmer. As shown, it is physically impossible, since
there is no mechanism in place to cause the waves to suddenly jump
together and interfere.

It is a useful picture showing how sine waves with differing phases add
together; no more and no less. It is a simple matter of mathematics. It
is not a new discovery in the world of RF or optics.

73,
Gene
W4SZ

Constructive interference in radiowave propagation
 

Cecil Moore wrote:
Jim Kelley wrote:
So I'm happy to
leave it to you to explain to Cecil how waves cancel but without
anhiliating the energy "in" them.

But that's just the point, Jim. You seem to believe the
pre-existing energy in those waves has been destroyed.
They obviously possessed energy before cancellation and
you say they possess zero energy after cancellation. If
that pre-existing energy is not destroyed, where did it go?


Cecil,

Now that you have access to a copy of Born and Wolf, you might dig
inside to see if you can improve your understanding of conservation of
energy. It is not quite as simple as you seem to believe.

B&W discuss the Poynting vector and its use in an overview in the first
chapter. I don't have the 4th edition. I have a couple of later editions
that contain identical language, so perhaps the same thing is in the 4th
edition.

In any case, here is the relevant quote. My explanations are enclosed in
[...]. Otherwise the paragraph is completely intact.

"It should be noted that the interpretation of S [Poynting vector] as
energy flow (more precisely as the density of energy flow) is an
abstraction which introduces a certain degree of arbitrariness. For the
quantity which is physically significant is, according to (41), not S
itself, but the integral of S [dot] n taken over a closed surface.
Clearly, from the value of the integral, no unambiguous conclusion can
be drawn about the detailed distribution of S, and alternative
definitions of the energy flux density are therefore possible. One can
always add to S the curl of an arbitrary vector, since such a term will
not contribute to the surface integral as can be seen from Gauss'
theorem and the identity div curl = 0. However, when the definition has
been applied cautiously, in particular for averages of small but finite
regions of space or time, no contradictions with experiments have been
found. We shall therefore accept the above definition in terms of the
Poynting vector of the density of the energy flow."

[ S and n are vectors, shown in bold type in the original. ]

Now for my comments.

Two important concepts are contained in the B&W quote. First, the math
involved with Poynting vectors is not quite as simple as many amateur
radio operators seem to believe. It does not make any sense to simply
add and subtract Poynting vectors in elementary fashion and expect to
get correct results. This is true even for your favorite case of a
one-dimensional problem such as a transmission line.

Second, the Poynting vector by itself means little. It is only the
integral over a closed surface that has physical reality. In your
favorite case of reflections and re-reflections the only useful
non-trivial application of the Poynting vector would be the integration
of the Poynting vector over a small region that includes the line
discontinuity inside. And even then, only the total energy balance can
be determined. Put in direct terms, there is no available information,
and no need for any information about what happens to the energy
contained in the various component waves you like to consider. It simply
does not matter. The only energy balance that counts is the net energy
flowing through the surface of the integration volume. Anything else is
merely in your imagination. B&W allow you to add anything you like, as
long as it is the curl of a vector. But there is no physical reality in
doing so.

It has been pointed out numerous times that modern physical theory is
correct by design. Ian again pointed out that fact earlier today. If the
wave equations, the field equations, force equations, or whatever are
analyzed correctly the energy balance will automatically work out
correctly as well. A check of energy balance is sometimes useful to
highlight any errors that might have been made in the math, but no new
physical information should be expected.

Finally, it is well known by all physicists, and I believe most
engineers, that energy considerations by themselves can be very useful
for analyzing physical problems. Much of higher level classical
mechanics and essentially all of quantum mechanics techniques are energy
based. The so-called Hamiltonian formulation is well-known and widely
used. It is no more or less correct than techniques based on forces and
other fields, but the Hamiltonian technique is often much more
computationally convenient.

73,
Gene
W4SZ

Constructive interference in radiowave propagation
 

Gene Fuller wrote:
I pointed out a few days ago that the FSU Java applet you lean on so
heavily these days is a simple tutorial device designed by a grad
student and a programmer. As shown, it is physically impossible, since
there is no mechanism in place to cause the waves to suddenly jump
together and interfere.

Good Grief, Gene! You are arguing that because you cannot
view them in the present that they never existed in the
past. Such is nonsense.The left hand side is a historical
plot of the points of the waves before they interfere.
Of course, those points only exist back in history and
no longer exist in the present because everything in the
present is happening at a point. Do you also deny the
existence of the historical yearly temperature plot points
because they don't still exist today? Please get real.
Here's a temperature chart to which you can apply your
"impossible" logic concepts.

http://en.wikipedia.org/wiki/Global_warming

Paraphrasing your idea: "As shown, it is physically
impossible, since there is no mechanism in place to
cause more than one temperature to exist at the
present time."

That java example is an example of implementing the
S-Parameter equation b1 = s11(a1) + s12(a2) which
is CERTAINLY NOT IMPOSSIBLE. By adjusting the magnitudes
and phase angles of a1 and a2, any degree of interference
can be obtained. One wave is s11(a1) and the other wave
is s12(a2). Of course, the interference happens at a point
(or plane) so fast that it is impossible to view in real
time. But by using deductive reasoning and the known
laws of physics, we are able to come up with valid java
scripts like the above. Your confusion is in assuming
all those points have to exist simultaneously in the
present, a really, really ridiculous notion.

They do not and cannot exist simultaneously in the present
just as temperatures on a temperature plot of past years
do not and cannot exist in the present anymore. Those points
on the java script existed back in time and are plotted in
a similar manner to plotting temperatures that no longer
exist in the present.
--
73, Cecil http://www.w5dxp.com