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!

"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.

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

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

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

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

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

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

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

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