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

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

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

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

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

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

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

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

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

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