Superposition
 

Gene Fuller wrote:
# Is this not written in English?

You have obviously misunderstood what I was trying
to say.

# Just what do you mean by, "superposition of two (or more)
coherent waves as the *cause* of the interference *process*"?

Superposition is necessary for interference to exist.
Superposition is not sufficient for interference to exist.
Superposition and interference are both in the cause and
effect chain of events.

# Are you playing some sort of word game by using *event* and *process*?

No, just responding to Jim Kelley's assertion that interference
is only an end result. Eugene Hecht says the "intricate color
patterns shimmering across an oil slick ... result from ...
the phenomenon of interference." The intricate color patterns
are the *result* of interference.

# Do you have a reference for the rules of that word game?

It's no word game - it's just English as plain as I can make it.
--
73, Cecil http://www.w5dxp.com

Superposition
 

Gene Fuller wrote:
Sorry. I missed it because it is not there. They don't say any such thing.

Yes they do - I distinctly remember reading it.
I will prove it to you as soon as I find my book.
--
73, Cecil http://www.w5dxp.com

Superposition
 

Cecil Moore wrote:
Gene Fuller wrote:
Sorry. I missed it because it is not there. They don't say any such
thing.

Yes they do - I distinctly remember reading it.
I will prove it to you as soon as I find my book.

I'm pretty sure it is in the section which discusses
the irradiance (power density) equation. It says the
total irradiance of two waves of the same magnitude
that are interfering can be up to four times the
irradiance of one wave.
--
73, Cecil http://www.w5dxp.com

Superposition (Antenna Arrays)
 

"Tam/WB2TT" wrote

You know that can't be right, because combining two antennas
gives 3 db gain.
_____________

The vector sum of the EM fields at every point in free space from identical
radiators fed by the same source depends in part on their relative physical
orientations, and their separation in wavelengths (see Kraus' ANTENNAS, 3rd
edition, chapters 5 and 6).

If all radiators in an array generate identical fields relative to each
other, then the peak directivity of an array of two radiators exceeds 3 dB
for radiator spacings of about 0.75 to 1.2 wavelengths (max of about 3.3
dB).

If they are spaced 1/2-wave apart then the peak directivity drops to about
1.9 dB, and at 1/4-wave separation it drops to about 0.5 dB (see Johnson &
Jasik ANTENNA ENGINEERING HANDBOOK, 2nd edition, Figure 3-4).

RF

Superposition (Antenna Arrays)
 

Richard Fry wrote:
The vector sum of the EM fields at every point in free space from
identical radiators fed by the same source depends in part on their
relative physical orientations, and their separation in wavelengths (see
Kraus' ANTENNAS, 3rd edition, chapters 5 and 6).

It's too bad that we cannot see the interference patterns
created by two radiators. Just know that all of the
interference patterns involving visible light that we can
see with our own eyes are also possible at RF frequencies.

Who hasn't been listening to a repeater that almost
completely faded out while stopped at a red light?
Letting the vehicle move a short distance brings it
back to Q5. That old familiar "picket-fencing" that
some of us have experienced is the antenna alternately
moving through zones of destructive and constructive
interference. The same thing can be caused by an
airplane flying over during local TV reception.
--
73, Cecil http://www.w5dxp.com

Superposition (Antenna Arrays)
 

"Cecil Moore" wrote
Who hasn't been listening to a repeater that almost
completely faded out while stopped at a red light?
Letting the vehicle move a short distance brings it
back to Q5. That old familiar "picket-fencing" that
some of us have experienced is the antenna alternately
moving through zones of destructive and constructive
interference. The same thing can be caused by an
airplane flying over during local TV reception.
______________

All true, but those cancellations don't originate in the transmit array.
They are the result of reflections from surfaces in the propagation
environment that arrive at the receive antenna ~180°out of phase with the
direct signal from the transmit array.

RF

Superposition
 

Cecil Moore wrote:
Cecil Moore wrote:
Gene Fuller wrote:
Sorry. I missed it because it is not there. They don't say any such
thing.

Yes they do - I distinctly remember reading it.
I will prove it to you as soon as I find my book.

I'm pretty sure it is in the section which discusses
the irradiance (power density) equation. It says the
total irradiance of two waves of the same magnitude
that are interfering can be up to four times the
irradiance of one wave.

Cecil,

The physical effect is well known and is non-controversial, even on
RRAA. What is at issue is all of the philosophical gibberish that seems
to surround the reality.

The exact words from B&W on page 289 of the 7th edition:

"the intensity varies between a maximum value Imax = 4I1, and a minimum
value Imin = 0"

In the 6th edition the same words are on page 259.

The modern convention is to use "irradiance" instead of "intensity",
since "intensity" can have multiple meanings.

What B&W *don't* say is anything about two 1 watt waves interacting,
waves exhibiting constructive and destructive interference, cause and
effects relationships, or even energy conservation.

All of those are things written by more casual writers, such as Hecht,
Melles-Griot, and the FSU Java dudes. There is nothing wrong with that
type of explanation for simple illustration, but it runs out of gas when
trying to support detailed analysis. One quickly ends up with silliness
such as waves that are launched and then cancel destructively within a
short (but undefined) distance. None of that nonsense occurs if one
simply applies the standard analysis techniques such as used by B&W.

73,
Gene
W4SZ

Superposition
 

Gene Fuller wrote:
"the intensity varies between a maximum value Imax = 4I1, and a minimum
value Imin = 0"

Yes, that's essentially what I have been saying. The peak
intensity (irradiance) can be double the intensity of
the combined intensity of both superposed waves.

What B&W *don't* say is anything about two 1 watt waves interacting,
waves exhibiting constructive and destructive interference, cause and
effects relationships, or even energy conservation.

Eugene Hecht calls the last term in the irradiance equation
the "interference term". He talks about "total destructive
interference" and "total constructive interference". The sign
of the interference term indicates whether the interference
is destructive (-) or constructive (+).

All of those are things written by more casual writers, such as Hecht,
Melles-Griot, and the FSU Java dudes. There is nothing wrong with that
type of explanation for simple illustration, but it runs out of gas when
trying to support detailed analysis. One quickly ends up with silliness
such as waves that are launched and then cancel destructively within a
short (but undefined) distance. None of that nonsense occurs if one
simply applies the standard analysis techniques such as used by B&W.

Exactly what nonsense are you referring to? Please be specific. It
is difficult to defend myself from assertions of "nonsense" with no
specific allegations.

I gather from the above that wave cancellation due to superposition
is against your religion. Since all impedance discontinuities cause
reflections, exactly how and why do those reflected waves cease to
exist? Please be specific.
--
73, Cecil http://www.w5dxp.com

Superposition
 

Cecil Moore wrote:

If waves are not 'doing things' to other waves, how
are reflected waves eliminated by thin-films on glass?

A reasonable question, for a third grader. It's like asking: if '-2'
is not doing something to '2', then how can the result be zero when
they combine? For the case A + B = 0, B does not change A, and A does
not change B even though their sum happens to be zero. (Certain other
problems arise when you try to algebraically add commodities which
cannot be negative - power for example.)

As I've told you many times, you could keep from becoming confused on
these points if you would work them through from the standpoint of
fields, rather than power. Not doing so is leading to problems in your
understanding of what actually goes on.

Imagine you're floating above the ground between two closely spaced
football field sized capacitor plates. There is one on either side of
you, and they are parallel to each other. Consider now that one of
them becomes highly charged with respect to the other, and to Earth.
(Some source of energy would be required in order for this to happen
and to keep it charged in air.) You would then experience a strong
electric field.

Now imagine the other plate becomes highly charged in magnitude and
polarity equal to the opposite plate. It now produces a field equal
in magnitude and opposite in direction to the other plate. The net
field from your perspective is now zero, but one can imagine that from
a different perspective the total field is much greater with the two
plates being charged. But you'll note that nothing actually happened
to the field from either the first plate or the second plate, yet
between the two plates their effect was canceled. Their ability to do
work on a charged particle is negated. But in the way that many of
your references points out, that ability has moved to different points
in space. Please note that nothing moved that 'ability' there other
than the charge which was applied to the second capacitor plate.

An interference pattern doesn't 'cause' energy to move around, fields
don't move other fields just as waves don't move other waves and
photons don't move other photons.

73, Jim AC6XG

Superposition
 

Jim Kelley wrote:
As I've told you many times, you could keep from becoming confused on
these points if you would work them through from the standpoint of
fields, rather than power.

As you know, I did exactly that in a private email to
you, Jim, and it didn't change anything.

Imagine you're floating above the ground ...

Just last night in a dream, I imagined that I was floating
above ground. Since I can also imagine that I went to the
moon, do you really consider imagination to be a tool of
knowledge?

An interference pattern doesn't 'cause' energy to move around, fields
don't move other fields just as waves don't move other waves and photons
don't move other photons.

Then exactly what "redistributes the photons to regions that
permit constructive interference", as the FSU web page says?
Is it really imagination that accomplishes that magic feat?
If not, exactly how and why and what redistributes (moves)
those photons?

I will be happy to engage you in a step by step mathematical
explanation/discussion of what happens during superposition but
all you have done so far is hand-waving and ad hominem attacks.

Given the power-density equation:

Ptotal = P1 + P2 + 2*SQRT(P1*P2)cos(A)

May I assume that from what you have said so far, that P1
and P2 never existed in the first place???? If they never
existed, wouldn't their magnitudes be zero in violation
of every rule of physics concerning reflections????

Jim, you have *NEVER* said what you think causes total
re-reflection of reflected waves (aside from your magical
imagination). Please enlighten us with some math and details
that don't violate the laws of physics.
--
73, Cecil http://www.w5dxp.com