Constructive interference in radiowave propagation
 

K7ITM wrote:
Well, maybe I'm mistaken, but I was under the impression that there
was someone around here who was promoting the idea that two waves
propagating in a linear medium could cancel over some non-zero finite
volume, but not cancel everywhere along their path, even though that
path was uninterrupted by any discontinuities in the medium.

Would you please name the person who said such. It certainly
was NOT me. The waves involved in the cancellation are
canceled so fast that they cannot be viewed on an o'scope.
But if they didn't exist, nothing would happen at an
impedance discontinuity.

Take the s-parameter equation, for instance.

b1 = s11(a1) + s12(a2) = 0

If s11(a1) doesn't exist, then s11 and/or a1 must not exist
either. But s11 and a1 can be measured. So if s11 and a1
exist, does s11(a1) exist only to be canceled or did it
never exist. If s11(a1) never existed, what the heck is
an s-parameter analysis good for?

Maybe
I'm mistaken, but I was under the impression that there was someone
around here who was promoting the idea that calculations based on
power rather than on voltage and current in a TEM transmission line
offered some inherent value.

An energy analysis is not supposed to replace a voltage analysis
but is supposed simply to settle the question, Where does the
energy go? If we assume that in a Z0 transmission line, that
Vfor^2/Z0 = forward joules/sec and Vref^2/Z0 = reflected joules/sec,
the energy analysis falls out from the voltage analysis.

If you don't care where the energy goes, that's cool, but some
of us, like Bruene and Maxwell, do care and have been arguing
about it for decades.

To keep an energy analysis from falling out from the voltage
analysis, we have been told that reflected waves don't exist,
and if they did exist, they would be devoid of energy content.
"I have yet to see" an EM wave that can exist devoid of
energy content.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Cecil Moore wrote:
K7ITM wrote:
No, it's about practicality. Convince me that calculations based
primarily on power (or energy) rather than on voltage and current
offer me something useful, with respect to TEM lines, and I might have
a closer look at them.

Assume you are dealing with light waves in free space
instead of RF waves in a transmission line. Would you
then find intensity (power density) calculations useful?
That's why optical physicists find them so useful.

Tom, are you familiar with an s-parameter analysis?

If so, it seems to me that b1 = s11(a1) + s12(a2) = 0
represent two wave components that immediately cancel
to zero when superposed at the impedance discontinuity.
Would you care to comment?

Cecil,

Most serious calculations by optical physicists are done through
Maxwell's Equations solvers. Intensity calculations are utterly
inadequate for exploring the details of high resolution imaging, for
example.

73,
Gene
W4SZ

Constructive interference in radiowave propagation
 

Jim Kelley wrote:
Cecil Moore wrote:
We have never discussed the point in the analysis that
I would like to discuss next. It will be brand new
territory. What have you got to lose?

Time, patience, energy......

I'm willing to furnish the bulk of the time and energy.
All you need to do is read my s-parameter analysis and
show me where I am wrong.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Gene Fuller wrote:
Most serious calculations by optical physicists are done through
Maxwell's Equations solvers. Intensity calculations are utterly
inadequate for exploring the details of high resolution imaging, for
example.

All that may be true, Gene. But don't Maxwell's
equations obey the superposition principle?
What does Maxwell say happens when we superpose
two EM waves out of phase such that destructive
interference occurs? What does Maxwell say
about the energy "lost" to destructive
interference? Where did it go?

Are intensity calculations utterly inadequate
for exploring the details of low resolution
transmission lines? :-) If the intensity
(power) calculations enumerated in the s-
parameter analysis description are utterly
inadequate, why are they used so often?
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

On 16 Apr 2007 17:50:10 -0700, "K7ITM" wrote:

On Apr 16, 3:38 pm, Richard Clark wrote:
On 16 Apr 2007 14:29:01 -0700, "K7ITM" wrote:

In my mind I was qualifying it as being waves
propagating in the same direction, since the discussion centers around
propagating EM cancelling out in a finite (non-zero) volume, and as
far as I know, there hasn't been anyone suggesting that waves on a
line in opposite directions cancel over a non-zero distance.

Hi Tom,

Then the challenge devolves to a self-fulfilling proposition (which
may be your point at this turn) as it requires two sources to occupy
the same point.

Well, maybe I'm mistaken, but I was under the impression that there
was someone around here who was promoting the idea that two waves
propagating in a linear medium could cancel over some non-zero finite
volume, but not cancel everywhere along their path, even though that
path was uninterrupted by any discontinuities in the medium.

Hi Tom,

'T'warn't me.

Maybe
I'm mistaken, but I was under the impression that there was someone
around here who was promoting the idea that calculations based on
power rather than on voltage and current in a TEM transmission line
offered some inherent value.

'T'warn't me.

I posted my original, "I have yet to
see...," statements as a way of saying that I'm not convinced about
the truth of either of those ideas, and it would go a long ways toward
convincing me if someone posted examples. I'm still waiting.

'T'was me.

I still
don't have a reference that a fiber optic cable is a TEM transmission
line, though I have others that say that it's not.

That example of the non-TEM fiber optic would be rare species indeed.
I've seen them, but that hardly constitutes the sole species of the
breed.

I still don't have
information on whether a soliton wave can propagate in a linear
medium, though I have references that say it is a non-linear
phenomenon that occurs in non-linear media.

Of course it can propagate in a linear medium. Solitons were first
reported in linear media - water - something like one hundred seventy
years ago. Solitons can induce non-linearity in otherwise linear
media. Solitons also interact in collision with a phase shift
afterwards. Solitons have been applied to data transmission in fiber
optics for a dozen years or more.

Your references are pretty sparse.

If you can convince me
that a wavefront coming to a Magic T doesn't see it as an impedance
discontinuity, we could perhaps post more about that--or not.

Consult Terman. He is quite compelling when it comes to describing
microwave plumbing. This hardly constitutes more than 4 pages total
reading, if you choose to move on beyond the first page of discussion.

But so far, your responses make me think you don't disagree with my
implicit suggestions:

True enough to a point.

that it's impossible to distinguish between the
condition of two cancelled waves that somehow still exist (huh?)

The elliptical huh? seems to be a curious toe in the water for many
here. Strange how a concept draws borders around energy it to make it
"disappear" simply because both contributions cancel. This is like
saying gravity disappears on a 1 square inch patch of earth when the
falling apple has come to rest on the ground. This is also akin to
the misnomer of zero-gravity environment of the astronauts in the
space shuttle.

For example (drawing away from G and towards V), if I were to place
two batteries in series opposition
- + + -
and connect a load to the two free terminals; sure, no current would
flow because there is no potential difference, but that numerical
combination doesn't make the batteries disappear. Yes, the condition
is indistinguishable from a load floating in null space, but we have a
priori knowledge of existing energy that informs us otherwise. If we
choose to be ignorant of the knowledge in that specific locality, the
map of all phase combinations around it will certainly bring it to our
attention again.

Beyond that, you're of course welcome to go off on whatever tangents
you wish. Basenote drift is the expected norm here; I engage in it
all the time myself.

The point of my going into a basenote drift is to present examples
that demonstrate what is necessary to answer your objections (like
providing two sources at one point that cancel on one side, but exist
independently on the other side of an interface). If those who
present their "theories" cannot meet these demonstrated
characteristics, then it is reasonable to reject their claims barring
their offering treatments that are equally compelling.

73's
Richard Clark, KB7QHC

Constructive interference in radiowave propagation
 

Cecil Moore wrote:
Gene Fuller wrote:
Most serious calculations by optical physicists are done through
Maxwell's Equations solvers. Intensity calculations are utterly
inadequate for exploring the details of high resolution imaging, for
example.

All that may be true, Gene. But don't Maxwell's
equations obey the superposition principle?
What does Maxwell say happens when we superpose
two EM waves out of phase such that destructive
interference occurs? What does Maxwell say
about the energy "lost" to destructive
interference? Where did it go?

Are intensity calculations utterly inadequate
for exploring the details of low resolution
transmission lines? :-) If the intensity
(power) calculations enumerated in the s-
parameter analysis description are utterly
inadequate, why are they used so often?

Cecil,

Changing the topic again? So soon?

You made a claim about optical physicists. I pointed out that your claim
is simply not correct. You then start babbling about low resolution
transmission lines. What a surprise!

You seem to be going back and forth about the utility of bringing optics
into the discussion on antennas and transmission lines. I doubt that
many here would expect different physical principles to apply to the two
wavelength regimes. I wonder if there might be a practical reason why
the preferred computational techniques are somewhat different?

The physics does not change, but the mathematical convenience does
change. Yes, that seems to be a recurring theme from me.

8-)

73,
Gene
W4SZ

Constructive interference in radiowave propagation
 

Gene Fuller wrote:
Changing the topic again? So soon?

No, just asking questions, Gene, like any grasshopper
worshiping at the feet of a guru is supposed to.
Please stop avoiding the questions with non-technical
diversions.

Do Maxwell's laws abide by the superposition principle?
It is a question with a simple yes/no answer. If they
do abide by the superposition principle, the forward
wave and reflected wave can be analyzed separately
and then superposed. Every individual wave component,
e.g. s11(a1), s12(a2), s21(a2), and s22(a2) can be
analyzed separately and then superposed. What do you get
when you apply Maxwell's equations to s11(a1)? Hopefully,
the same voltage, current, and energy as any other valid
analysis. If not, there's a distinct problem that needs
to be solved.

You made a claim about optical physicists. I pointed out that your claim
is simply not correct.

And I asked you to explain why it is not correct and
you very carefully avoided answering. One wonders why.

I doubt that
many here would expect different physical principles to apply to the two
wavelength regimes.

My point exactly, Gene. The two fields should agree in
every way (except lingo). If you switch from voltage and
current to EM fields, nothing should change. But when you
admit that, you are forced to admit that voltages and
currents associated with EM waves are bound by a set of
restrictions, one of them being that they must at all
times, travel at c(VF) and cannot, by definition, stand
still as long as they exist as EM waves.

Intensity, irradiance, and Poynting vectors are just
different names for the same physical phenomenon. To
assert that power density in a transmission line doesn't
obey the same rules as light intensity is just nonsense.
The energy content of component waves has been known for
decades in the field of optics and it applies just as
well to RF waves as it does to light waves.

The physics does not change, but the mathematical convenience does
change.

My point exactly! No matter what the mathematical convenience,
(except for the lingo) the two fields should agree in every way.
When they appear to disagree, there is a contradiction somewhere.
Seems to me, in the quest to fit EM waves into the voltage and
current mold, some have forgotten that EM waves are not DC.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

On Apr 17, 12:33 am, Richard Clark wrote:
On 16 Apr 2007 17:50:10 -0700, "K7ITM" wrote:
....
I still
don't have a reference that a fiber optic cable is a TEM transmission
line, though I have others that say that it's not.

That example of the non-TEM fiber optic would be rare species indeed.
I've seen them, but that hardly constitutes the sole species of the
breed.

So give me a reference already. I find lots of references, including
ones that explain the propagation, that talk about TM, TE, hybrid, and
even quasi-TEM mode propagation in a fiber. What boundary conditions
are there in an optical fiber that give TEM mode?


I still don't have
information on whether a soliton wave can propagate in a linear
medium, though I have references that say it is a non-linear
phenomenon that occurs in non-linear media.

Of course it can propagate in a linear medium. Solitons were first
reported in linear media - water - something like one hundred seventy
years ago.

Solitons can induce non-linearity in otherwise linear
media. Solitons also interact in collision with a phase shift
afterwards. Solitons have been applied to data transmission in fiber
optics for a dozen years or more.

Your references are pretty sparse.

Yours seem non-existent. Mine at least did a good job explaining the
phenomena.

From Wikipedia, for example, about solitons:
"The stability of solitons stems from the delicate balance of
"nonlinearity" and "dispersion" in the model equations. Nonlinearity
drives a solitary wave to concentrate further; dispersion is the
effect to spread such a localized wave. If one of these two competing
effects is lost, solitons become unstable and, eventually, cease to
exist. In this respect, solitons are completely different from "linear
waves" like sinusoidal waves. In fact, sinusoidal waves are rather
unstable in some model equations of soliton phenomena. Computer
simulations show that they soon break into a train of solitons."

There is specific mention of the Kerr effect--a nonlinearity in
optical media that support soliton transmission. One of the
references I saw specifically said that solitons are solutions to non-
linear differential equations. Since the equations governing the
behaviour of waves derive from the properties of the propagation
medium, I expect that any medium that can propagate a soliton is
nonlinear. Another reference specifically addressed the nonlinearity
of water as a transmission medium, as a necessary part of its being
able to propagate solitons.



If you can convince me
that a wavefront coming to a Magic T doesn't see it as an impedance
discontinuity, we could perhaps post more about that--or not.

Consult Terman. He is quite compelling when it comes to describing
microwave plumbing. This hardly constitutes more than 4 pages total
reading, if you choose to move on beyond the first page of discussion.

I find nothing in the index of my "Radio Engineers' Handbook" by
Terman under either "Magic" or "Hybrid". Sorry. The three different
coaxial "Magic T" hybrid designs I DID find all do show an impedance
discontinuity: the junction of more than two lines of equal impedance
and/or impedance steps in through-lines. Sorry.

Time to move on.

Cheers,
Tom

Constructive interference in radiowave propagation
 

On 17 Apr 2007 08:30:41 -0700, K7ITM wrote:

On Apr 17, 12:33 am, Richard Clark wrote:
On 16 Apr 2007 17:50:10 -0700, "K7ITM" wrote:
...
I still
don't have a reference that a fiber optic cable is a TEM transmission
line, though I have others that say that it's not.

That example of the non-TEM fiber optic would be rare species indeed.
I've seen them, but that hardly constitutes the sole species of the
breed.

So give me a reference already. I find lots of references, including
ones that explain the propagation, that talk about TM, TE, hybrid, and
even quasi-TEM mode propagation in a fiber. What boundary conditions
are there in an optical fiber that give TEM mode?

Hi Tom,

This is curious request indeed. Can you name any example of light
that is not TEM? Let's see, wikipedia's entry for TEM includes Fiber
Optics as example (along with the sources and illustrations for many
modes). TEM00 is the principle mode of the ubiquitous "single mode"
fiber optic that is laid in the millions of miles every year.

One vendor of Fiber modeling software
http://www.zemax.com
specifically at
http://www.zemax.com/kb/articles/154...MAX/Page1.html
offers:
"ZDC thanks Steve Dods of OptiWave Corporation for supplying the
SMF-28 fiber simulation data used in this article.

"In the article How to Model Coupling Between Single-Mode Fibers
SMF-28 single mode fiber is modeled using data from the
manufacturer's datasheet. The only data provided on the optical
radiation produced at 1.31 is the mode field diameter, which is
stated to be 9.2 ± 0.4 µm.

"As a result, the fiber mode of both launch and receiver fibers
was entered as a Gaussian (TEM0,0) mode of waist 4.6µ. The
resulting fiber coupling calculation agrees well with experimental
measurement."

Corning SMF-28 has been in production for nearly 20 years.

I still don't have
information on whether a soliton wave can propagate in a linear
medium, though I have references that say it is a non-linear
phenomenon that occurs in non-linear media.

Of course it can propagate in a linear medium. Solitons were first
reported in linear media - water - something like one hundred seventy
years ago.

Solitons can induce non-linearity in otherwise linear
media. Solitons also interact in collision with a phase shift
afterwards. Solitons have been applied to data transmission in fiber
optics for a dozen years or more.

Your references are pretty sparse.

Yours seem non-existent. Mine at least did a good job explaining the
phenomena.

To which there is scant difference as nearly every point you recite
has already been anticipated in my earlier post (shown above). Your
rebuttal that water is non-linear is already answered in this same
quote. If this is basenote drift, we are now into the treble clef.

If you can convince me
that a wavefront coming to a Magic T doesn't see it as an impedance
discontinuity, we could perhaps post more about that--or not.

Consult Terman. He is quite compelling when it comes to describing
microwave plumbing. This hardly constitutes more than 4 pages total
reading, if you choose to move on beyond the first page of discussion.

I find nothing in the index of my "Radio Engineers' Handbook" by
Terman under either "Magic" or "Hybrid". Sorry. The three different
coaxial "Magic T" hybrid designs I DID find all do show an impedance
discontinuity: the junction of more than two lines of equal impedance
and/or impedance steps in through-lines. Sorry.

Time to move on.

For others that are not moving on, but interested in the use and
issues of reflection to the source driving a Magic T, I quote work
from Q MEASUREMENTS FOR HIGH-Q CAVITIES
R. A. RAPUANO and J. HALPERN, MIT (1946):

"The heart of this equipment is the "magic T". This is an
eight-terminal network (Fig. 3) in waveguide or coax having
symmetry properties analogous to those of a "hybrid coil".
In the case of an ideal T, power entering the E aria is divided
equally between S1 and S2, both parts being out of phase; none
goes directly to H. Power entering the H arm is divided equally
between S1 and S2, with both parts now in phase; no power goes
directly to E. Power reflected from the loads on S1 and S2,
however, can be coupled from H to E, depending upon the magnitude
and phase of the terminal impedances on S1 and S In the case of
two short circuits the power going from H to E can be caused to
vary from zero to the full amount depending on their position
along the line. If a short circuit is placed on S1 and a resonant
cavity is placed on S2, then the power going from H to E is a
function of frequency. The power reflected back from H is the
difference between the input and the loss due to transmission
through E and absorption in the resonator."

Figure 3 (use fixed font):
S1
||
||
||
H ======== ======== E
||
||
||
S2

where the interior blank space represents the plumbing too difficult
to render here.

I would further offer that Walt is working on a fairly similar
treatment employing the "Rat Race" (alluded to as a Hybrid Coil in the
monograph extract above). The discussion above is germane in that
sense and would be beneficial to those who eventually see his
rebuttals to arguments pressed against him.

73's
Richard Clark, KB7QHC

Constructive interference in radiowave propagation
 

On Apr 17, 2:57 pm, Richard Clark wrote:
....

Richard, it really doesn't much matter to me what modes fiber optic
cable supports. If there are types that support true TEM mode, I'd be
happy to hear about it. So far, though, I've followed links from over
a dozen searches and found NO reference that claims that true TEM mode
is supported by a fiber, be it single-mode or multi-mode. I've gone
to the Wikipedia pages you suggested and other pages there, and found
quite a bit of info about fiber optic cables and their modes. In all
that, I have found no claim that true TEM mode is supported. I
followed the link you provided to the simulation software provider,
and found only that they modeled a particular cable as having TEM 0,0
mode; nowhere could I see a claim that the cable modeled actually
propagates by true TEM mode. The way the article was worded sounded
to me like the TEM entry was an approximation. In my research, the
closest to a claim of true TEM mode I've found has been in one recent
article that says TEM would be the ideal, but the best anyone's been
able to do is quasi-TEM or TEM-like.

You're welcome to think it's true TEM if you wish, of course, but your
saying it, over and over if you wish, isn't going to be nearly as
convincing as if we can find one, even one, ligitimate reference that
claims true TEM.

Cheers,
Tom