Constructive interference in radiowave propagation
 

On Sun, 08 Apr 2007 20:26:41 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
:

Walt, I can see that you have taken my comment as personal criticism. That
was not intended, and to the extent that I may have caused that, I
apologise. In that context, it is better that I refrain from further
comment.

Regards
Owen

Hi Owen,

Please excuse the long delay in responding to your post of 4-8-07, 4:26 pm EDT. I have been away from the
computer since then, attending to personal chores that took priority over rraa.

I'm sure your comments weren't meant as a personal attack, and I accept your apology.

However, your consideration of statements appearing in Reflections as flawed on the assumption that the
concepts presented there concerning impedance matching apply only to lossless and distortionless lines, IMHO
is unfair, because it is not true.

For readers of your post who now may be questioning the reliability of statements appearing in Reflections,
I'm working on a more detailed discussion of the issue for clarification that I will enter on the rraa as a
new thread.

Walt

Constructive interference in radiowave propagation
 

Keith Dysart wrote:
So your only beef with my examples is that they do not
accurately model a "typical ham transmitter"?

Yes, I have said so about a half-dozen times now.

When you say that source impedance is a "variable", do you
mean this for a "typical ham transmitter", ...

Yes, the discussion is about typical ham
transmitters - nothing else matters to typical
hams.

Which results have been disproved on the bench?

Please research the grand argument between Warren
Bruene, w5oly, and Walter Maxwell, w2du.

Are the experiments documented in Reflections chapter 19 and
19a representative examples?

As far as I know, the Bruene/Maxwell argument first saw
light in a QST article in the early '90s and has been
raging ever since.

My read of these chapters is that they offer compelling
argument and evidence (at least for the tube style
transmitters examined) that ham transmitters are linear*,
at least over their normal region of operation.

Linear is not the requirement for your source impedance.
Constant, fixed, and linear is the requirement for your
source impedance. Nothing you have presented had the
source impedance as a linear variable.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Cecil Moore wrote:
Jim Kelley wrote:

Hint: people who write physics books know that power does not interfere.


Jim, I'll make the same deal with you that I have offered
to others with, so far, no takers. If you can prove that
I said that powers interfere, I will send you $100. If
you cannot prove that, you send me $100, and cease your
unfair straw man argument methods.

Please don't take this the wrong way, Cecil, but after seeing this
response to a fairly inert remark, I'm not feeling comfortable enough
to take you entirely at your word.

Perhaps we just misunderstand each other, but I don't know of another
way to interpret what you have written. If you don't believe that
power interferes, then why else would you continually write
interference equations in terms of power? You can't just arbitrarily
throw quantities with any units you like into equations, and then
claim authoritative references as your source, Cecil. Is it your
contention that interference is not a validating pre-requisite for use
in an interference equation?

I'm pretty sure somebody using the name Cecil Moore posted Ptot = P1 +
P2 +2*SQRT(P1*P2), and used the equation to show how two 100 watt
generators generate 400 watts. Help me out. Where did I mess up here?

73, Jim AC6XG

Constructive interference in radiowave propagation
 

Jim Kelley wrote:
Perhaps we just misunderstand each other, but I don't know of another
way to interpret what you have written.

Yes, you do, Jim. You know that I agree with you on almost
every technical point yet you incessantly try to erect
straw men so you can knock them down. You are almost
always trying to discredit someone who agrees with you.
To that unfair tactic my response is:

From: "An Energy Analysis at an Impedance Discontinuity in
an RF Transmission Line", by W5DXP, WorldRadio, Oct. 2005

"Single-source RF energy in a transmission line and laser
light are both coherent electromagnetic energy waves that
obey the laws of superposition, interference, conservation
of energy, and conservation of momentum."

"The term 'power flow' has been avoided in favor of 'energy
flow'. Power is a measure of that energy flow per unit time
through a plane. Likewise, the EM fields in the waves do the
interfering. Powers, treated as scalars, are incapable of
interference. Any sign associated with a power in this paper
is the sign of the cosine of the phase angle between two
voltage phasors."

If you don't believe that power
interferes, then why else would you continually write interference
equations in terms of power?

Because that's what Dr. Best, Hecht, and Born and Wolf do.
Intensity, irradiance, and Poynting vectors are *power*
densities. Multiply the following intensity-irradiance
equation by unit-area and what do you get? Why, you get
WATTS of power!

Itot = I1 + I2 + 2*SQRT(I1*I2)cos(A) in watts/unit-area

Multiplying both sides of the equation by unit-area

Ptot = P1 + P2 + 2*SQRT(P1*P2)cos(A) in watts

I assume that's how Dr. Best came up with the equation.

Or don't even multiply by unit area - just use Poynting
vectors instead where each P above is a Poynting vector.
The dimensions of a Poynting vector are identical to
the dimensions of irradiance and intensity.

You can't just arbitrarily throw
quantities with any units you like into equations, and then claim
authoritative references as your source, Cecil.

I have not done that, Jim. The Poynting vector has exactly
the same units as intensity and irradiance. Multiply both
sides of the equation by unit-area and the result is WATTS,
the unit of power and that's fully consistent with the
rules of mathematics.

Is it your contention
that interference is not a validating pre-requisite for use in an
interference equation?

The question appears to have a trivial answer so it must
be just another straw man. Yes, interference exists if
interference exists and interference doesn't exist if
interference doesn't exist. Satisfied?

I'm pretty sure somebody using the name Cecil Moore posted Ptot = P1 +
P2 +2*SQRT(P1*P2), and used the equation to show how two 100 watt
generators generate 400 watts. Help me out. Where did I mess up here?

False so just another straw man. I said two 100 watt generators
do *NOT* generate 400 watts because there is no interference
between the generators. Maybe you need your glasses changed?
I did follow that posting up showing how two 100 watt *waves*
can engage in total constructive interference and obtain a total
of 400 watts.

Even you should be able to figure out how two 100 watt generators
can be made to produce a forward power of 400 watts in a
transmission line with an SWR of 5.83:1 and that is perfectly
consistent with Hecht, and Born & Wolf's intensity equations.

************************************************** ****************
HECHT AND BORN & WOLF'S TOTAL CONSTRUCTIVE INTERFERENCE EQUATIONS
ASSUME THERE IS AN EQUAL MAGNITUDE OF TOTAL DESTRUCTIVE INTERFERENCE
OCCURRING SOMEWHERE ELSE TO SATISFY THE REQUIREMENTS OF THE
CONSERVATION OF ENERGY PRINCIPLE. For every total constructive
interference equation I1 + I2 + 2*SQRT(I1*I2), there exists a
total destructive interference equation I1 + I2 - 2*SQRT(I1*I2)
The destructive interference and constructive interference
always sum to a net power density of ZERO such that the AVERAGE
power always remains the same.
************************************************** ****************

Let's say we have two light waves of 100 watts/cm^2 intensity
and we cause them to interfere constructively. Here's the
intensity equation for total constructive interference.

I1 + I2 + 2*SQRT(I1*I2) = 100 + 100 + 2*SQRT(100*100) = 400 watts/cm^2

Multiply both sides of the equation by cm^2 and you get
400 watts of power and indeed that square cm would be
getting very warm. Somewhere else exists a square cm
with zero intensity.

That is Born and Wolf's equation (16a) on page 259 of the 4th
edition of "Principles of Optics). The equation is correct
and the dimensions are correct. It is also Hecht's equation (9.15)
on page 388 of the 4th edition of "Optics". Those authors all label
the 2*SQRT(I1*I2) term as the *interference term*.

Let's say we have two RF waves or 100 watts/cm^2 intensity and
we cause them to interfere constructively in a coax transmission
line with a cross sectional area of one square cm.

P1 + P2 + 2*SQRT(P1*P2) = 100 + 100 + 2*SQRT(100*100) = 400 watts/cm^2.

With those dimensions, the intensity equation and the Poynting
vector equation are EXACTLY the same. This is equation 12 from
Dr. Best's QEX article, "Wave Mechanics of Transmission Lines,
Part 3: ..." in the Nov/Dec 2001 edition. The fixed cross sectional
area of the coax is redundant and we can choose to deal entirely
with watts.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Jim Kelley wrote:
I'm pretty sure somebody using the name Cecil Moore posted Ptot = P1 +
P2 +2*SQRT(P1*P2), and used the equation to show how two 100 watt
generators generate 400 watts. Help me out. Where did I mess up here?

Jim, you asked me not to tell you what you said but instead
to furnish the quote showing what you said. Above you are
violating the rules you asked me to abide by. Produce that
quote and I will explain it.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

On Apr 12, 3:23 pm, Cecil Moore wrote:
K7ITM wrote:
If you don't believe there's a solution to the example Keith posted,
you have no right to believe in the results of a measurement with a
vector network analyzer, and you should certainly not trust the
indicated output level of any signal generator.

Methinks you have missed the context of the discussion.
If the model doesn't work for an IC-706 it is not much
use to amateur radio operators. I have already said that
a valid model can be had for a signal generator equipped
with a circulator load.

The only problem with this statement is the assumption that
the result can only be achieved with a circulator. It only
takes a 10 cent resistor. You really should put down your
optics books for a few hours and crack open a basic circuit
theory or transmission line text. Or google, "'lattice
diagrams' reflection". For matching at the source,
only 10 cent resistors are used.

....Keith

Constructive interference in radiowave propagation
 

On Apr 12, 3:37 pm, Cecil Moore wrote:
Keith Dysart wrote:
So your only beef with my examples is that they do not
accurately model a "typical ham transmitter"?

Yes, I have said so about a half-dozen times now.

When you say that source impedance is a "variable", do you
mean this for a "typical ham transmitter", ...

Yes, the discussion is about typical ham
transmitters - nothing else matters to typical
hams.

So when a poster presents a problem in a context other "typical
ham transmitters", why do you dispute the answers. If you can't
discuss the problem in the context presented by the poster, why
not have the courtesy to stay out. Others may be interested in
learning about how things relate in contexts other than "typical
ham transmitters". Why sabotage the discussions by arguing and
arguing and then saying "Oh, I only meant my comments in the
context of "typical ham transmitters" WHICH WAS NOT THE CONTEXT
OF THE PROBLEM STATEMENT. And kindly stop using the Texas A&M
example of TVSG and 1000 feet of line. It is clearly out of
your context of "typical ham transmitters".

Perhaps, in your dissertations on optics, it would be valuable
to state that they apply only in the context of "typical ham
transmitters". This might make it clear to the reader that
your suggestions are not generally applicable and could reduce
the wasted bits.

Which results have been disproved on the bench?

Please research the grand argument between Warren
Bruene, w5oly, and Walter Maxwell, w2du.

Are the experiments documented in Reflections chapter 19 and
19a representative examples?

As far as I know, the Bruene/Maxwell argument first saw
light in a QST article in the early '90s and has been
raging ever since.

My read of these chapters is that they offer compelling
argument and evidence (at least for the tube style
transmitters examined) that ham transmitters are linear*,
at least over their normal region of operation.

Linear is not the requirement for your source impedance.
Constant, fixed, and linear is the requirement for your
source impedance. Nothing you have presented had the
source impedance as a linear variable.

No indeed, the source impedance was a constant and resistive
in all my examples. That meets the needs for linear analysis.
Remember f(a+b) = f(a) + f(b)?

You are claiming that for "typical ham transmitters" the
source impedance is undefinable. This is quite at odds with
the exposition in Reflections chapters 19 and 19a. Or maybe not,
after all, Reflections is quite precise and claims only for a
specific class of ham transmitters, which, I suppose, may not
be typical. Though they look so to me.

So I take it that you no longer agree with the analysis presented
in Reflections 19 and 19a. I am pretty sure that you have stated
agreement in the past.

It would be valuable if you were to expand on the reasons for your
change of thought. What convinced you that "typical ham transmitters"
could not have their source impedance measured?

....Keith

Constructive interference in radiowave propagation
 

Owen Duffy wrote:
Cecil, you have conveniently clipped the context (as you do), the
relevant context being the line-load interface and source-line interface.

Following published usenet rules, I trim the part to which
I am not replying.

Statements in some explanations (by others) like "This clearly proves
that reflected power and forward power in a transmission line are both
real power, and that no fictitious power, or reactive volt-amperes,
exists in either one." seem incompatible with the basic AC circuit theory
explanation of a reactive load which must exchange reactive energy with
the transmission line over a complete cycle (and the same effect at the
source end).

Those statements are generally about lossless lines where the
Z0 is purely resistive. In the lossless wave reflection model,
there is no reactive energy in the transmission line. The forward
voltage is in phase with the forward current and the reflected
voltage is 180 degrees out of phase with the reflected current.
Both V*I*cos(theta) terms are in watts with zero vars. Of course,
real world transmission lines have (hopefully negligible) vars.

BTW, I am not surprised at your dissertation apparently dismissing the
distributed impedance model of a line, because after all it is the
solution of that model that gives us the classic transmission line
equations that you seem to not want to use.

It is NOT the distributed impedance model to which I object. It is
the lumped circuit model which assumes the speed of light is infinite
and 75m loading coils don't occupy any space. Here's a quote from an
IEEE white paper at: http://www.ttr.com/TELSIKS2001-MASTER-1.pdf

"Consequently, lumped element circuit theory does not (and cannot)
accurately embody a world of second order partial differential
equations in space and time."
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Keith Dysart wrote:
The only problem with this statement is the assumption that
the result can only be achieved with a circulator.

Please stop putting words into my mouth. I said it could
be achieved with a circulator. I did *NOT* say it could
*only* be achieved with a circulator.

It only takes a 10 cent resistor.

That is naive in the extreme and makes you sound about
ten years old. If a 10 cent resistor would accomplish
that in the real world, nobody would ever buy a
circulator.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

On Apr 12, 8:06 pm, Cecil Moore wrote:
Owen Duffy wrote:
Cecil, you have conveniently clipped the context (as you do), the
relevant context being the line-load interface and source-line interface.

Following published usenet rules, I trim the part to which
I am not replying.

Statements in some explanations (by others) like "This clearly proves
that reflected power and forward power in a transmission line are both
real power, and that no fictitious power, or reactive volt-amperes,
exists in either one." seem incompatible with the basic AC circuit theory
explanation of a reactive load which must exchange reactive energy with
the transmission line over a complete cycle (and the same effect at the
source end).

Those statements are generally about lossless lines where the
Z0 is purely resistive. In the lossless wave reflection model,
there is no reactive energy in the transmission line. The forward
voltage is in phase with the forward current and the reflected
voltage is 180 degrees out of phase with the reflected current.
Both V*I*cos(theta) terms are in watts with zero vars. Of course,
real world transmission lines have (hopefully negligible) vars.

There are two models that can accurately describe the same
phenomenon: the power folk like VAs, Watts and VARs; the RF folk
like forward and reverse travelling waves. But the phenomenon
is the same. One can indeed describe what happens on a transmission
line in terms of VAs, Watts, and VARs and any line that has a
reverse wave will be found to have VARs. The different terms apply
to different models, not to different situations.

Just another reason why you have to be careful when you think
that the forward and reverse waves necessarily represent real
power. Trust the power folk on this one; they know what represents
real power, they are billing for it. And its Watts. Net. They
only power that counts is the power you can bill for.

....Keith