Richard Clark wrote:
There is also the
amusing "mutually incoherent" redundancy.

Not my words, Richard - they are straight from Born and Wolf.
Do you really think Born and Wolf would engage in "redundancy"
if it were meaningless. Suggest that you learn the difference
between mutually inclusive and mutually exclusive.

Aside from these sophisms,
there is a conceptual, quixotic tilting at windmills in the phrase:
no effect on each other
as if waves ever affected each other (irrespective of coherence -
mutuality notwithstanding).

Coherent waves can and do affect each other. It's called
interference where the sum of the intensities is different
from the intensity of the sums. Incidentally, the intensity
of the sums is the mistake you made when you calculated
the reflection from non-reflective glass to be brighter
than the surface of the sun.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Incidentally, the intensity of the sums
^^^^^^^^^^^^^^^^^^^^^
is the mistake you made when you calculated
the reflection from non-reflective glass to be brighter
than the surface of the sun.

Sorry, that should be the "sum of the intensities".
The intensity of the sums is the way to correctly
calculate total intensity. The sum of the intensities
yields an incorrect answer as Richard earlier discovered
with his "reflections brighter than the surface of the
sun" calculation.
--
73, Cecil http://www.w5dxp.com

On Sat, 07 Apr 2007 16:10:03 GMT, Gene Fuller wrote:

Walter Maxwell wrote:
On Fri, 06 Apr 2007 23:03:42 GMT, Cecil Moore wrote:

MRW wrote:
Any comments? Really, what I'm trying to understand here is: if
constructive interference does any good in radiowave propagation. I
was thinking that with an increase in amplitude the signal would be
able to travel a little further, but the signal received may not be
accurate in terms of the information it is conveying.
Antenna gain over isotropic is an application of
constructive interference. The constructive
interference must be balanced by an equal amount
of destructive interference elsewhere to avoid
violating the conservation of energy principle.

This is what I've been trying to persuade the 'anti's' that whenthe radiation fields from two vertical dipoles
superpose at some point in space, where their magnitudes are equal and are 180° out of phase, the wave
cancellation resulting from destructive interference produces a null in a predetermined direction, and thus
prevents those fields from propagating any further in that direction. At the precise instant the null is
produced, the constructive interference following the principle of energy conservation yields an increase in
the field strength in directions away from the null direction. This explains the concept of antenna-pattern
modification, and contradicts the notion that the two fields just plow through each other with no effect on
either.

Keep in mind that the two fields are coherent because they were developed simultaneously from the same source.
It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Walt, W2D

Walt,

Your observation is "correct" only in the case that most people consider
for practical reasons. The calculation showing the null behavior is
almost invariably performed at infinite distant from the sources, i.e.,
far field condition. The path from each source to the observation point
is considered to be exactly parallel.

As you know, there are usually three or more linear dimensions that
enter into radiation calculations. In the case of two sources there are
four:

Wavelength
Size of each source
Distance between sources
Distance to the observation point

In the typical "null" presentation, such as that shown in the ARRL
publications, the distance to the observation point in always large.

Lets take another case, however. Suppose the distance between the
sources is some what larger than the wavelength. Make it large enough so
there is a region between the sources that would be considered far field
from each of the sources. Now calculate the phase differences along some
direction from the center point between the sources that eventually
points to a null region in the infinite distance. Don't pick an
obviously symmetric direction, such as broadside or end-fire, as that
would be a special case.

What you will find is that when looking at the phase difference along
the ultimate null direction is that there is no such null much closer to
the sources. The paths from the individual sources are not parallel in
this case. The null "line" is actually a curve. The waves pass right
through each other in the closer region. The "passing waves" then go on
to form nulls in the infinite distance. The nulls in the closer region
are not in the same directions as the nulls in the far field.

Again, the ground rules:

Totally coherent, monochromatic sources
Fixed phase difference
Far field conditions for each source

There are no "tricks" here; this is just a matter of simple geometry.
However, it shows that the null you believe demonstrates some permanent
interaction and annihilation of EM waves is simply a special case.

In classical, non-cosmic, non-relativistic conditions EM waves do not
interact in free space. This condition is so widely understood in the
scientific world that it becomes a prime candidate for argument on RRAA.

8-)

73,
Gene
W4SZ

Gene, at this point I can't disagree with you. However, in your next to the last paragraph in your post above,
if I interpret you correctly, you are saying that all directional arrays, such as are used in AM broadcasting,
are considered 'special' cases. Is that what you mean't to infer?

Walt, W2DU

On Sat, 07 Apr 2007 11:48:23 -0500, Cecil Moore
wrote:

There is also the
amusing "mutually incoherent" redundancy.

Not my words, Richard - they are straight from Born and Wolf.
Do you really think Born and Wolf would engage in "redundancy"
if it were meaningless.

Poor language is not excused by example. Being meaningless I leave to
your interpretations, however.

This only reveal two incidents that are amusements.

Are you sleeping with Born and Wolf now?

Richard Clark wrote:
Poor language is not excused by example. Being meaningless I leave to
your interpretations, however.

You seem to have missed (Born and Wolf)'s point.
Between coherent and mutually incoherent is
a span of signals which they call partially
incoherent. There are degrees of incoherency
as can be seen from your postings. :-)
--
73, Cecil http://www.w5dxp.com

Owen Duffy wrote:
Walt, this seems inconsistent with the approach that I believe you seem
to use in analysing waves in transmission lines where you seem to want to
not only deal with the forward and reverse waves separately (ie to not
collapse them to a resultant V/I ratio at a point), but to deal with
multiply reflected waves travelling in the forward and reverse direction
(which is only necessary in the transient state).

I think what Walt is trying to do is explain that there is
no interference at power up. As the reflections build up,
the interference builds up, until there is total
destructive interference toward the source during steady-
state and total constructive interference toward the load.
Without interference, a Z0-match would not be possible.

The principle of superposition gives us permission to
analyze the forward and reverse separately and collapse
them to a resultant V/I ratio later. If one wants
to use the simplified mashed-potatoes approach, that is
OK since the results are the same in either case.
--
73, Cecil http://www.w5dxp.com

Walter Maxwell wrote in
:

On Sat, 07 Apr 2007 05:03:51 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
m:

On Fri, 06 Apr 2007 23:03:42 GMT, Cecil Moore
wrote:

MRW wrote:
Any comments? Really, what I'm trying to understand here is: if
constructive interference does any good in radiowave propagation.
I was thinking that with an increase in amplitude the signal would
be able to travel a little further, but the signal received may
not be accurate in terms of the information it is conveying.

Antenna gain over isotropic is an application of
constructive interference. The constructive
interference must be balanced by an equal amount
of destructive interference elsewhere to avoid
violating the conservation of energy principle.

This is what I've been trying to persuade the 'anti's' that whenthe
radiation fields from two vertical dipoles superpose at some point
in space, where their magnitudes are equal and are 180° out of
phase, the wave cancellation resulting from destructive interference
produces a null in a predetermined direction, and thus prevents
those fields from propagating any further in that direction. At the
precise instant the null is produced, the constructive interference
following the principle of energy conservation yields an increase in
the field strength in directions away from the null direction. This
explains the concept of antenna-pattern modification, and
contradicts the notion that the two fields just plow through each
other with no effect on either.


Walt, this seems inconsistent with the approach that I believe you
seem to use in analysing waves in transmission lines where you seem to
want to not only deal with the forward and reverse waves separately
(ie to not collapse them to a resultant V/I ratio at a point), but to
deal with multiply reflected waves travelling in the forward and
reverse direction (which is only necessary in the transient state).

Owen

Owen, it appears that you've misinterpreted my approach. In developing
a condition for impedance matching, such as adding a series or shunt
stub at the proper place on a transmission line, the object has always
been to generate a new reflection at the stub point of the opposite
phase to that appearing on the line at the stub point. Thus when the
stub reflection and the load reflection superpose at the stub point,
the resulting reflection coefficients of voltage and current form
either a virtual open circuit or a virtual short circuit. These
conditions are produced because when the load impedance is greater
than Zo, the resultant reflection coefficient angles at the stub point
are 0° for voltage and 180° for current, establishing a virtual open
circuit at the stub point to rearward traveling waves. When the load
impedance is less than Zo, the resultant reflection coefficient angles
are 180° for voltage and 0° for current, establishing a virtual short
circuit at the stub point for rearward traveling waves.

Hi Walt,

I read the above, and I think I can see what you are getting at, however
I think it is flawed.

If you were to try to extend this method to explain the common two stub
tuner (where the length of the stubs is adjustable and the distance
between them is fixed), you will have to deal with a situation where the
load end stub junction does not present the "virtual o/c or s/c" you
describe, your "total re-reflector concept" and you come to need to
calculate the situation on the source side of the load end stub (possibly
by conventional methods?).

Walk your explanation around a Smith chart, and explain why, if the
principles on which your explanation are based are correct, why energy
fills a 3/4 wave hi Q coaxial resonator rather than being blocked by the
virtual s/c or o/c at the first voltage minimum or current minimum.

Someone else persuing the theme that reflected waves always travel all
the way back to the source, seems to come to a position that some kinds
of matching produce a complementary reflected wave, and that really there
are two (or more) reflected waves, its just that they have zero net
energy. Some of us would accept that if the resultant is zero, there is
no wave. Otherwise, you would see a multitude of net-zero waves all
around us to complicate every analysis.

These "new" and alternative explanations are questionable and don't seem
better than the conventional explanations of a transmission line that are
set out in just about any reputable transmission lines text. What
advantages do these explanation have, who are they targeted at? Is the
"total re-reflector" concept to appeal to a dumbed down audience who can
get their mind around a bunch of words that describe specific situations
in a simple and appealing way, but an incorrect explanation nonetheless?

I think it is a real challenge to teach people a simple explanation of
what happens without telling them convenient lies that have to be
unlearned to develop further. The "reflected wave is (always) dissipated
in the PA as heat" is an example of one of those convenient lies.

Owen

On Apr 7, 4:16 pm, Owen Duffy wrote:
The "reflected wave is (always) dissipated
in the PA as heat" is an example of one of those convenient lies.

Are you sure that's not a straw man? Who, exactly, has voiced that lie
(besides Keith and his ten cent resistor?) :-)
--
73, Cecil, w5dxp.com

Cecil Moore wrote:
Gene Fuller wrote:
However, it shows that the null you believe demonstrates some
permanent interaction and annihilation of EM waves is simply a special
case.

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


"... 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."

In classical, non-cosmic, non-relativistic conditions EM waves do not
interact in free space. This condition is so widely understood in the
scientific world that it becomes a prime candidate for argument on RRAA.

Florida State University seems to disagree. "Upon meeting"
in free space, the interfering photons are "redistributed".
RF waves are EM waves. Just because we cannot see them is
no reason to assert that they act differently from EM waves
that we can see.

Hecht, in "Optics", says about interference:

"At various points in space, the resultant irradiance can
be greater, less than, or equal to I1 + I2 depending on
the value of I12 ..." I12 is previously defined as the
interference term. Hecht's "various points in space"
seem to contradict your assertion that waves "do not
interact in free space".

From Born and Wolf: "Thus if light from a source is divided
by suitable apparatus into two beams which are then superposed,
the intensity in the region of superposition is found to vary
from point to point between maxima which exceed the sum of the
intensities in the beams, and minima which may be zero."

If "region of superposition" is not referring to the free
space point of interference, to what is it referring?

When one can see with one's own eyes the interaction of
two light beams in free space, how can you possibly deny
the existence of that interaction?


Cecil,

Wake up!

You are not even on topic. None of those quotes are related to the
discussion at hand.

73,
Gene
W4SZ

Walter Maxwell wrote:
On Sat, 07 Apr 2007 16:10:03 GMT, Gene Fuller wrote:

[snipped]

Walt,

Your observation is "correct" only in the case that most people consider
for practical reasons. The calculation showing the null behavior is
almost invariably performed at infinite distant from the sources, i.e.,
far field condition. The path from each source to the observation point
is considered to be exactly parallel.

As you know, there are usually three or more linear dimensions that
enter into radiation calculations. In the case of two sources there are
four:

Wavelength
Size of each source
Distance between sources
Distance to the observation point

In the typical "null" presentation, such as that shown in the ARRL
publications, the distance to the observation point in always large.

Lets take another case, however. Suppose the distance between the
sources is some what larger than the wavelength. Make it large enough so
there is a region between the sources that would be considered far field
from each of the sources. Now calculate the phase differences along some
direction from the center point between the sources that eventually
points to a null region in the infinite distance. Don't pick an
obviously symmetric direction, such as broadside or end-fire, as that
would be a special case.

What you will find is that when looking at the phase difference along
the ultimate null direction is that there is no such null much closer to
the sources. The paths from the individual sources are not parallel in
this case. The null "line" is actually a curve. The waves pass right
through each other in the closer region. The "passing waves" then go on
to form nulls in the infinite distance. The nulls in the closer region
are not in the same directions as the nulls in the far field.

Again, the ground rules:

Totally coherent, monochromatic sources
Fixed phase difference
Far field conditions for each source

There are no "tricks" here; this is just a matter of simple geometry.
However, it shows that the null you believe demonstrates some permanent
interaction and annihilation of EM waves is simply a special case.

In classical, non-cosmic, non-relativistic conditions EM waves do not
interact in free space. This condition is so widely understood in the
scientific world that it becomes a prime candidate for argument on RRAA.

8-)

73,
Gene
W4SZ

Gene, at this point I can't disagree with you. However, in your next to the last paragraph in your post above,
if I interpret you correctly, you are saying that all directional arrays, such as are used in AM broadcasting,
are considered 'special' cases. Is that what you mean't to infer?

Walt, W2DU

Hi Walt,

Yes, those are special cases, but those special cases are the only ones
that most people care about.

What I was trying to say might be better illustrated by the following:

Two coherent laser beams from the same source can be arranged by
suitable mirrors to intersect at some angle. There will most definitely
be interference in the region of intersection, but the beams will
continue through unchanged. If one measured a beam somewhere downstream
from the intersection region it would not be possible to determine that
it had crossed another beam earlier.

The beams "interfere" but they do not "interact". I know this sounds
goofy, and it is critical to keep the definitions straight. When I say
the beams do not interact I mean that they do not cause any changes in
the other beam. The fact that the beams interfere means that the sum of
the fields shows the characteristic constructive and destructive
behavior. It does not mean that the waves are henceforth changed.

OK, so how does this square with the observation that there are nulls in
patterns from two or more RF sources? It is actually very
straightforward. In the far field the waves from the separate sources
are virtually parallel. Just like Timex, they interfere and they keep on
interfering. They never really pass beyond the intersection region.

I know it seems like a subtle, or even meaningless, distinction. Do the
waves interfere forever or do they actually annihilate each other? For
many purposes it does not matter. However, the non-interaction of waves
in free space is pretty basic to all of EM analysis.

73,
Gene
W4SZ