Constructive interference in radiowave propagation
 

In my physics book, it mentions constructive and destructive wave
interference especially in reference to the the one-slit diffraction
experiment. From reading about radiowave propagation, they also
mention diffraction effects on radiowaves.

To me, it sounds like with constructive interference, the wave's
amplitude will have the chance of increasing more than what the source
actually outputted. But I wonder if this is helpful in terms of radio
communication.

In reference to a single frequency transmitted, when I think about
constructive interference and radiowave propagation, I keep picturing
a delayed signal transmitted at time_0 and another signal transmitted
at time_1 later with the same phase arriving at the receiver at the
same time.

In terms of AM, I would think this would be problematic.

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.

Thanks!

Constructive interference in radiowave propagation
 

"MRW" wrote in message
ups.com...
In my physics book, it mentions constructive and destructive wave
interference especially in reference to the the one-slit diffraction
experiment. From reading about radiowave propagation, they also
mention diffraction effects on radiowaves.

To me, it sounds like with constructive interference, the wave's
amplitude will have the chance of increasing more than what the source
actually outputted. But I wonder if this is helpful in terms of radio
communication.

the amplitude can be more in one direction than another, but the total power
can not exceed the transmitter output of course. for each constructive
interference peak there must be an area of destructive interference to make
up for it.


In reference to a single frequency transmitted, when I think about
constructive interference and radiowave propagation, I keep picturing
a delayed signal transmitted at time_0 and another signal transmitted
at time_1 later with the same phase arriving at the receiver at the
same time.

In terms of AM, I would think this would be problematic.

yep, that is what ghosts on tv signals are... if the delay is long with
respect to the modulating signal you can get effects like that. the most
common desirable uses are in antennas where there is a phase delay about
equal to the spacing of the elements of the antenna which lets you create a
stronger signal in one direction, and of course a weaker one in other
directions, allowing you to put more of the transmitter power in the
direction you want it to go. because the delays are small there is not the
problem with ghosts.



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.

yes, constructive interference is what antenna design is all about...
destructive interference has its part also to help reject interference from
undesired sources as well.

Constructive interference in radiowave propagation
 

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.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

On 6 abr, 23:37, "MRW" wrote:
In my physics book, it mentions constructive and destructive wave
interference especially in reference to the the one-slit diffraction
experiment. From reading about radiowave propagation, they also
mention diffraction effects on radiowaves.

To me, it sounds like with constructive interference, the wave's
amplitude will have the chance of increasing more than what the source
actually outputted. But I wonder if this is helpful in terms of radio
communication.

In reference to a single frequency transmitted, when I think about
constructive interference and radiowave propagation, I keep picturing
a delayed signal transmitted at time_0 and another signal transmitted
at time_1 later with the same phase arriving at the receiver at the
same time.

In terms of AM, I would think this would be problematic.

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.

Thanks!
Hello MRW,

As long as the constructive interference occurs over the full
bandwidth of your signal, it helps you without the need for
equalizing. Another way to see it is that if the delay of the
(reflected, refracted, etc) signal is far below 0.25/(RF bandwidth)
the signals will add constructively when the carriers are in phase at
the point of interference (inclusive the side bands generated by the
modulation).

This becomes more difficult (or impossible) for wide band signals. One
can see that in the frequency response of the propagation path.
Imagine when you transmit a signal with uniform power distribution
(brick wall spectrum). Receive it with an antenna and examine the
signal wit a spectrum analyzer. When the spectrum is flat (as the
original signal), then you will not have problems demodulating the
signal. However when you see many dips and peaks in the spectrum, the
information on the signal will be distorted. You will need an
equalizer (inverse FFT, deconvolution) to remove the distortion.

Another test is to transmit a very narrow pulse (amplitude modulated).
Receive the signal en show the demodulated version on an oscilloscope.
When the demodulated pulse has been stretched, you have distortion in
the modulation.

The effect of distortion in mobile systems due to multiple waves
arriving at an antenna, results in so called "frequency selective
fading".

About analog AM, the BW of the signal is about 8 kHz, As long as the
delay of reflected/refracted waves is less then 30us (that is 9 km in
distance), you will not have problems with signal distortion (valid
for surface wave propagation). With propagation via the ionosphere,
the situation is different; there the path length of several waves can
be so different, that for example waves with frequency 13.720 MHz
interfere constructively, but with frequency 13.722 MHz they interfere
destructively.

So when you don't want distortion because of destructive and
constructive interfering wave fronts, you should have a narrow
bandwidth (that is low bitrate). This is done in multi carrier
modulation (like COFDM [TDAB, DVBT]). Many or some carriers will
suffer from destructive interference, but also many will be subjected
to constructive interference. By adding sufficient redundancy, the
data stream from the sub carriers having good signal strength can be
demodulated to the original data stream.

Relative high baud rate systems (like the GSM system) use equalizers/
echo cancellators to mitigate the effect of multi-path reflections.

Best Regards,

Wim
PA3DJS

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

Walter Maxwell wrote in
:

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

Constructive interference in radiowave propagation
 

On Sat, 07 Apr 2007 03:03:40 GMT, Walter Maxwell
wrote:

It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Hi Walt,

Well, having broached the topic, it appears time to plunge in once
again.

Several but closely related questions:
What separates "effect" from "no effect?" (They are, afterall, a
rather strict binary outcome.)

Does the binary transition from a one micro-degree longer
cycle (non-coherent) to 0 (coherence) same length cycle really
plunge us into a new physical reality of waves colliding with
rebounds and caroms where formerly there was absolutely no
interaction before?

73's
Richard Clark, KB7QHC

Constructive interference in radiowave propagation
 

On Apr 6, 11:03 pm, Walter Maxwell wrote:

It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Can one not change the location of the nulls by changing
the phase relationship of the two sources?

If so, it would seem to me that two non-coherent fields are
simply fields without a constant phase relationship and as
such, the nulls are constantly moving; still present, but
not stationary.

....Keith

Constructive interference in radiowave propagation
 

Keith Dysart wrote:
On Apr 6, 11:03 pm, Walter Maxwell wrote:

It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Can one not change the location of the nulls by changing
the phase relationship of the two sources?

If so, it would seem to me that two non-coherent fields are
simply fields without a constant phase relationship and as
such, the nulls are constantly moving; still present, but
not stationary.

...Keith

Andy writes:
Correct. One example is a television signal that is received from
two
sources : 1) a direct line to the transmitting tower and 2) a
reflection
from an airplane flying .

Even tho both received signals are generated from the same source,
the reflected signal will be changing in amplitude and phase as the
reflector,
the airplane, moves along it's flight path.

The two signals combine at the receiving antenna and the
resultant signal into the receiver will rise and fall, depending on
the resultant amplitude and phase. The maximum can be several
db above the direct signal and the null can be many many db
BELOW the direct signal.

Hence, you see the image come and go for several seconds on
your screen. After several seconds the plane will have moved to a
position
such that the reflection doesn't hit your antenna anymore, and the
problem goes away. We've all seen this. In fact, 70 years ago, this
effect (on radio signals) was what inspired the development of
radar.....

Andy W4OAH

Constructive interference in radiowave propagation
 

Richard Clark wrote:
Walter Maxwell wrote:
It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Does the binary transition from a one micro-degree longer
cycle (non-coherent) to 0 (coherence) same length cycle really
plunge us into a new physical reality of waves colliding with
rebounds and caroms where formerly there was absolutely no
interaction before?

Of course, you are being facetious but the answer is simple.
If the two signals are mutually incoherent, they don't
interfere. Permanent wave cancellation is impossible
between two waves that are not coherent. Hecht in "Optics"
devotes an entire chapter to the "Basics of Coherence Theory".
So do Born and Wolf in "Principles of Optics". Here is what
Walt was obviously saying except in Born and Wolf's words:

"If the two beams originate in the same source, the fluctuations
in the two beams are in general correlated, and the beams are
said to be completely or partially *coherent* depending on
whether the correlation is complete or partial. In beams from
different sources, the fluctuations are completely uncorrelated,
and the beams are said to be mutually *incoherent*. When such
beams from different sources are superposed, no interference is
observed under ordinary experimental conditions, the total intensity
being everywhere the sum of the intensities of the individual beams."

In case you missed it, that says *NO INTERFERENCE* between mutually
incoherent waves. Seems reasonable to say that "no interference"
means the same thing as "no effect".
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Keith Dysart wrote:
On Apr 6, 11:03 pm, Walter Maxwell wrote:

It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Can one not change the location of the nulls by changing
the phase relationship of the two sources?

If so, it would seem to me that two non-coherent fields are
simply fields without a constant phase relationship and as
such, the nulls are constantly moving; still present, but
not stationary.

If the waves are mutually incoherent, there is
NO interference which means no effect on each other.
Constructive and destructive interference is
impossible between two mutually incoherent waves
(under ordinary experimental conditions).
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

AndyS wrote:
Hence, you see the image come and go for several seconds on
your screen.

This seems to fall under the concept of partial
coherence. In "Principles of Optics", Born and Wolf
devote an entire chapter to it.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Richard Clark wrote:
Several but closely related questions:
What separates "effect" from "no effect?" (They are, afterall, a
rather strict binary outcome.)

It is a rather strict binary outcome when we are
discussing coherent vs mutually incoherent waves
as Walt obviously was. The gray area in between
to which you are alluding is called "partial
coherence". It is the region between "coherent"
and "mutually incoherent" which makes it three-state,
not binary, much like a logic 0 vs a logic 1 with
an in between region.

We generally would not have to worry about "partial
coherence" in a transmission line but if you want to
nit-pick that subject on rraa, be our guest.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

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

Walter Maxwell wrote in
:

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.

If you want more details on how the resultant reflection coefficients are developed I'll be glad to furnish
it.

Walt, W2DU

Constructive interference in radiowave propagation
 

Cecil Moore wrote:
AndyS wrote:
Hence, you see the image come and go for several seconds on
your screen.

This seems to fall under the concept of partial
coherence. In "Principles of Optics", Born and Wolf
devote an entire chapter to it.


Cecil,

In my line of work I get to deal with partial coherence every day. The
fading of TV signals due to multipath reflections from airplanes is not
at all what B&W are describing.

73,
Gene
W4SZ

Constructive interference in radiowave propagation
 

On Apr 7, 9:17 am, Cecil Moore wrote:
Keith Dysart wrote:
On Apr 6, 11:03 pm, Walter Maxwell wrote:

It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Can one not change the location of the nulls by changing
the phase relationship of the two sources?

If so, it would seem to me that two non-coherent fields are
simply fields without a constant phase relationship and as
such, the nulls are constantly moving; still present, but
not stationary.

If the waves are mutually incoherent, there is
NO interference which means no effect on each other.
Constructive and destructive interference is
impossible between two mutually incoherent waves
(under ordinary experimental conditions).

By "NO interference" did you mean "sufficiently close to zero
that it can be ignored for engineering purposes", or "exactly
zero"?

If the former, a bit more precision in your writing would be
valuable. The use of CAPITALs certainly suggests the latter.

If the latter, how incoherent do the waves have to be before
the interference suddenly drops to ZERO.

....Keith

Constructive interference in radiowave propagation
 

Keith Dysart wrote:
By "NO interference" did you mean "sufficiently close to zero
that it can be ignored for engineering purposes", or "exactly
zero"?

If *mutually incoherent*, then exactly zero, according to
Born and Wolf. "Mutually incoherent" excludes any possibility
of coherency.

If the former, a bit more precision in your writing would be
valuable. The use of CAPITALs certainly suggests the latter.

Note that I didn't say anything about partially coherent
waves or partially incoherent waves. Whether two waves are
coherent or mutually incoherent is indeed a binary situation.
Any middle ground is thus excluded from my statements.

If the latter, how incoherent do the waves have to be before
the interference suddenly drops to ZERO.

I believe mutually incoherent means the same thing as
perfectly incoherent.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

On 7 Apr 2007 08:55:58 -0700, "Keith Dysart" wrote:

If the waves are mutually incoherent, there is
NO interference which means no effect on each other.
Constructive and destructive interference is
impossible between two mutually incoherent waves
(under ordinary experimental conditions).

By "NO interference" did you mean "sufficiently close to zero
that it can be ignored for engineering purposes", or "exactly
zero"?

Hi Keith,

Your question of parsing "NO" reveals one of those binary shifts in an
otherwise analog word that has me puzzled too. There is also the
amusing "mutually incoherent" redundancy. 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).

If the past is an indicator of future activity, this topic is about to
split into other discussion with a desperate attempt to appear to be
answering for these strange theses.

73's
Richard Clark, KB7QHC

Constructive interference in radiowave propagation
 

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?
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

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?

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

On Sat, 07 Apr 2007 17:59:03 GMT, Cecil Moore
wrote:

Between coherent and mutually incoherent is
a span of signals which they call partially
incoherent.

As Reggie would say

"BAFFLEGAB!"

Constructive interference in radiowave propagation
 

On Sat, 07 Apr 2007 21:16:06 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
:

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

Walter Maxwell wrote in
:

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

Owen,

Are you saying that those who support my explanation of stub matching are telling lies!?
I can't believe you said that.

I explained this exact matching function in great detail in October 1974 QST. It was repeated in Reflections
edition 1 in 1990, and again in edition 2 in 2001. It was also published in QEX in the Mar/Apr 1998 issue.

I had to submit my manuscripts to the engineering departments of four different divisions of RCA for approval
before being allowed to even submit them to QST to determine if they were interested in publishing them. One
of the RCA engineers who reviewed my text was Jack Young, then the chief engineer of the transmitter section
of the RCA Broadcast Engineering Division. He complimented me on my explanation of stub matching using the
reflection coefficients as the parameters. On accepting my papers for publication in QST, the former Technical
Director of the League, George Grammer, W1DF, also complimented me on the unique approach explaining stub
matching, and also for my explanation of conjugate matching. Grammer was probably the most highly educated
engineer ever gracing the ARRL.

It's been 33 years since I published my presentation using reflection coefficients, and to date no one except
some of the posters on this NG have disputed it. Are you saying that all who reviewed my writings that
appeared in QST and in Reflection were being lied to?

You suggested walking through the Smith Chart with the problem. Owen, the Smith Chart is Fig. 1 in my
presentation that proves my position is correct. Please review the QEX article I referenced above to see it.
With all due respect, Owen, I believe you have misunderstood, or perhaps misconstrued the procedure I
presented. Would you please review it again to see where you might have gone wrong?

Walt

Constructive interference in radiowave propagation
 

Walter Maxwell wrote in
:

is correct. Please review the QEX article I referenced above to see
it. With all due respect, Owen, I believe you have misunderstood, or
perhaps misconstrued the procedure I presented. Would you please
review it again to see where you might have gone wrong?

Hi Walt,

I have reviewed Chapter 3 of Reflections II which you kindly sent me. I
think it contains the deveopment to which you refer.

It seems to me that Chapter 3 depends entirely on an assumption that the
phase relationship of the current and voltage of a travelling wave is 0 deg
or 180 deg, depending on the direction. This only holds true for lossless
lines and distortionless lines, and so the "proofs" developed in the
chapter are not general proofs. For example, the proof that reflected power
is purely real and of magnitude |E-|^2/Zc is not developed for the general
case, and happens to not be correct for the general case.

Owen

Constructive interference in radiowave propagation
 

On Apr 7, 7:31 pm, Richard Clark wrote:
On Sat, 07 Apr 2007 17:59:03 GMT, Cecil Moore
wrote:

Between coherent and mutually incoherent is
a span of signals which they call partially
incoherent.

That should have been "partially coherent".

As Reggie would say "BAFFLEGAB!"

Don't blame me - blame Born and Wolf - whom a lot of people respect.
--
73, Cecil, w5dxp.com

Constructive interference in radiowave propagation
 

On Apr 7, 11:45 pm, "Cecil Moore" wrote:
On Apr 7, 7:31 pm, Richard Clark wrote:

On Sat, 07 Apr 2007 17:59:03 GMT, Cecil Moore
wrote:

Between coherent and mutually incoherent is
a span of signals which they call partially
incoherent.

That should have been "partially coherent".

As Reggie would say "BAFFLEGAB!"

Don't blame me - blame Born and Wolf - whom a lot of people respect.

Do Born and Wolf offer crisp definitions of the boundaries
between coherent, partially coherent, and mutually incoherent?

Or is it a continuum arbitrarily divided into 3 regions for
the purposes of discussion?

....Keith

Constructive interference in radiowave propagation
 

On Sun, 08 Apr 2007 03:32:48 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
:

is correct. Please review the QEX article I referenced above to see
it. With all due respect, Owen, I believe you have misunderstood, or
perhaps misconstrued the procedure I presented. Would you please
review it again to see where you might have gone wrong?

Hi Walt,

I have reviewed Chapter 3 of Reflections II which you kindly sent me. I
think it contains the deveopment to which you refer.

It seems to me that Chapter 3 depends entirely on an assumption that the
phase relationship of the current and voltage of a travelling wave is 0 deg
or 180 deg, depending on the direction. This only holds true for lossless
lines and distortionless lines, and so the "proofs" developed in the
chapter are not general proofs. For example, the proof that reflected power
is purely real and of magnitude |E-|^2/Zc is not developed for the general
case, and happens to not be correct for the general case.

Owen

Owen, your statement that my writings in Reflections are flawed is shocking. The feeling I get from it is like
getting sucker punched in the stomach.

Are you so narrowly oriented academically that the difference between lossless and low-loss conditions is so
great that general principles cannot be applied to situations where real-world low-loss elements are involved?

It is generally accepted that voltage and current travel forward on a low-loss line with 0° phase difference.
In low-loss lines the effect of the small negative-reactance component in the Z0 due to the loss is routinely
disregarded as insignificant. Likewise, when voltage and current travel rearward on a low-loss line, resulting
from reflection at a mismatched termination, it is generally accepted that they travel with a 180° phase
difference, disregarding the small error caused by the insignificantly-small reactance in the Z0.

Calculations performed when disregarding the small error still yield practical results in hands-on operations.
On the other hand, if everyday practical operations required calculating with the academically-perfect
conditions of the Z0, time would be lost due to the unnecessary complications involved in the calculations.

Your stated position is that applying general principles that are academically correct only with lossless
elements to operations involving low-loss elements is flawed.

C'mon, Owen, let's get practical and rescind your impeachment of Reflections.

Walt

Constructive interference in radiowave propagation
 

On Apr 7, 5:51 pm, Gene Fuller wrote:
The beams "interfere" but they do not "interact".

Of course, you can give examples where the waves survive the
superposition. But what we are talking about is when the waves do
NOT survive the superposition.

How about wave cancellation, Gene? When two coherent waves traveling
in the same direction in the same path with equal magnitudes and
opposite
phases interact, they cease to exist in the direction of the original
travel.
Ir's senseless to argue that waves that cease to exist during the
process
of superposition have not interacted with each other, don't you think?
--
73, Cecil, w5dxp.com

Constructive interference in radiowave propagation
 

On Apr 8, 6:57 am, "Keith Dysart" wrote:
Do Born and Wolf offer crisp definitions of the boundaries
between coherent, partially coherent, and mutually incoherent?
Or is it a continuum arbitrarily divided into 3 regions for
the purposes of discussion?

I don't have Born and Wolf with me at the moment but I seem
to recall that they said that any signal that was not single
frequency could not be 100% coherent. Presumably a signal
with modulation would be partially coherent.
--
73, Cecil, w5dxp.com

Constructive interference in radiowave propagation
 

Cecil Moore wrote:
On Apr 7, 5:51 pm, Gene Fuller wrote:
The beams "interfere" but they do not "interact".

Of course, you can give examples where the waves survive the
superposition. But what we are talking about is when the waves do
NOT survive the superposition.

How about wave cancellation, Gene? When two coherent waves traveling
in the same direction in the same path with equal magnitudes and
opposite
phases interact, they cease to exist in the direction of the original
travel.
Ir's senseless to argue that waves that cease to exist during the
process
of superposition have not interacted with each other, don't you think?
--
73, Cecil, w5dxp.com


Cecil,

It is easy to give examples where the waves survive the superposition,
because they always do. It is rather strange that you are making this
argument after all the back and forth about traveling waves and standing
waves. Do we now have multiple flavors of EM waves? Some that obey
superposition and some that don't?

I must have missed class the day they went over the theory of
"cancellation". Is this another one of those convenient descriptions of
results that you keep trying to remold into fundamental physical laws?

I stand 100% behind my two messages to Walt. If you actually read them
you would note that I said for most cases it makes no difference whether
the waves interfere forever or whether they interact and "cancel". As
Owen pointed out a little while ago, we generally don't want to carry
around lots of zero components in an analysis.

The bottom line is that EM waves do not interact in free space.
Linearity and superposition could not hold if that were the case.
Maxwell's equations would need to be recast.

There are exactly enough physical laws and principles now. There is no
need to invent more on RRAA.

73,
Gene
W4SZ

Constructive interference in radiowave propagation
 

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