On Jan 23, 8:35*am, Cecil Moore wrote:
Yes, signals traveling in opposite directions don't interfere.
Call this assertion A.

Consider two antennas several wavelengths apart and driven with
the same frequency. Exploring the field strength far from the
antennas we find regions with zero field strength (nulls) and
regions with increased field strength. This variation in field
strength is usually ascribed to interference and the pattern
of variation is often called an interference pattern.

Similar results can be observed with light (google "two slit
experiment").

Locate one of these nulls far from the antennas and follow it
back towards the antennas. Eventually you will be on a line
between the two antennas.

From assertion A above, is it your contention that far from
the antennas it is "interference" that causes the variation
in field strength, but that on the line drawn between the two
antennas some other mechanism is responsible?

If so, what is the other mechanism? And does it only work
exactly on the line, or does it start working when you get
close to the line? How close?

Now I suggest that interference works just as well on the
line drawn between the antennas as it does every where
else and the conditions along that line are not a special
case.

That said, when we look at the two slit experiment, it is
generally agreed that the photons are redistributed such
that there are no photons in dark regions and more photons
in the bright regions.

On the line drawn between the two antennas, there are dark
regions and bright regions (the standing wave). By analogy,
there are no photons in the dark regions and more in the
bright regions. But the photons from the two sources were
travelling towards each other. What is the mechanism that
redistributes the photons such that there are none in the
dark regions? Do the photons stop and not enter the dark
region? Or do they turn into 'dark photons' as they
transit the dark regions? What are 'dark photons'?

...Keith

Keith Dysart wrote:
From assertion A above, is it your contention that far from
the antennas it is "interference" that causes the variation
in field strength, but that on the line drawn between the two
antennas some other mechanism is responsible?

Of course not - please don't be ridiculous. If the two
antenna elements were isotropic point sources, on a
line drawn between them, there could be no interference
and there would be only standing waves in free space
along that line assuming no reflections from nearby
objects, etc.

Everywhere else there are components of waves traveling
in the same direction so interference is possible anywhere
except on that line between the point sources. When the
sources are not a point, seems to me, interference could
occur at any and all points in space.

My "assertion A above" was about transmission lines,
an essentially one-dimensional context. Two waves in
a transmission line are either traveling in opposite
directions or in the same direction.

Incidentally, I came across another interesting quote
from one of my college textbooks, "Electrical Communication",
by Albert. "Such a plot of voltage is usually referred to
as a *voltage standing wave* or as a *stationary wave*. Neither
of these terms is particularly descriptive of the phenomenon.
A *plot* of the effective values of voltage ... is *not a wave*
in the usual sense. However, the term "standing wave" is in
wide-spread use." [Emphasis is the author's]
--
73, Cecil http://www.w5dxp.com

On Jan 23, 1:12*pm, Cecil Moore wrote:
Keith Dysart wrote:
From assertion A above, is it your contention that far from
the antennas it is "interference" that causes the variation
in field strength, but that on the line drawn between the two
antennas some other mechanism is responsible?

Of course not - please don't be ridiculous. If the two
antenna elements were isotropic point sources, on a
line drawn between them, there could be no interference
and there would be only standing waves in free space
along that line assuming no reflections from nearby
objects, etc.

Everywhere else there are components of waves traveling
in the same direction so interference is possible anywhere
except on that line between the point sources. When the
sources are not a point, seems to me, interference could
occur at any and all points in space.

OK. So it is your contention that "far from the antennas
it is "interference" that causes the variation in field
strength, but that on the line drawn between the two
antennas some other mechanism is responsible".

But why do you say "Of course not" and then proceed to
paraphrase my statement?

When the mechanism abruptly changes from interference
when off the line to "standing wave" when EXACTLY (how
exact?) on the line, is there any discontinuity in
the observed field strengths?

...Keith

Keith Dysart wrote:
OK. So it is your contention that "far from the antennas
it is "interference" that causes the variation in field
strength, but that on the line drawn between the two
antennas some other mechanism is responsible".

It is unethical to bear false witness about what I said.
What I said was:

On a line drawn between two *isotropic point sources*,
when there are no reflections anywhere around, along
that line, interference is impossible. The only thing
existing along that line would be standing waves.
There is no point along that line where the power
density is not equal to the sum of the two sources,
i.e. there is superposition but no interference along
that line.

If the elements are not point sources, interference
is obviously possible at each and every point. I assume
your example elements are not point sources so what you
claimed was my contention was a false statement. If
you can't win the arguments without making false
statements about what I said, you lose anyway.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote
Keith Dysart wrote:
From assertion A above, is it your contention that far from
the antennas it is "interference" that causes the variation
in field strength, but that on the line drawn between the two
antennas some other mechanism is responsible?

Of course not - please don't be ridiculous. If the two
antenna elements were isotropic point sources, on a
line drawn between them, there could be no interference
and there would be only standing waves in free space
along that line assuming no reflections from nearby
objects, etc.
______________

Cecil, hopefully you understand that even isotropic radiators near each
other and excited on the same frequency with the same amount of power will
generate far-field pattern nulls.

Maybe I'm misunderstanding you.

RF

Richard Fry wrote:
Cecil, hopefully you understand that even isotropic radiators near each
other and excited on the same frequency with the same amount of power will
generate far-field pattern nulls.

Maybe I'm misunderstanding you.

I'm trying to understand how a line drawn between two
"isotropic radiators near each other" could ever be
in the far field.
--
73, Cecil http://www.w5dxp.com

Roy Lewallen wrote:
Sine waves are another problem -- there, we can easily have overlapping
waves traveling in the same direction, so we'll run into trouble if
we're not careful. I haven't worked the problem yet, but when I do, the
energy will all be accounted for. Either the energy ends up spread out
beyond the overlap region, or the energy lost during reflections will
account for the apparent energy difference between the sum of the
energies and the energy of the sum. You can count on it!

An example from optics will make the situation clear.

http://www.w5dxp.com/thinfilm.GIF

At t3, when the 0.009801 watt internal reflection arrives
to interfere with the 0.01 watt external reflection, what
is the resulting reflected power toward the source?

Anyone who can answer that simple question from the field
of optics will understand what happens to the energy in
a transmission line.

Hint: the reflected power is *not* 0.01w - 0.009801w.
--
73, Cecil http://www.w5dxp.com

Roy Lewallen wrote:
[... very nice explanation]

Sine waves are another problem -- there, we can easily have
overlapping waves traveling in the same direction, so we'll run into
trouble if we're not careful. I haven't worked the problem yet, but
when I do, the energy will all be accounted for. Either the energy
ends up spread out beyond the overlap region, or the energy lost
during reflections will account for the apparent energy difference
between the sum of the energies and the energy of the sum. You can
count on it!

As always, I appreciate any corrections to either the methodology or
the calculations.

Roy Lewallen, W7EL

How about analyzing a vibrating string? If you play guitar, there's a very
nice note you can make by plucking a high string, then putting your finger
at exactly the correct spot and removing it quickly. The note will jump to
a much higher frequency and give a much purer sound. Clearly, the
mechanical energy has split into two waves that cancel at the node.

In principle, you could show the node is stationary, thus contains no
energy. But there is energy travelling on both sides of the null point -
you can hear it.

You can also create other notes by touching different spots on the
vibrating string. These create standing waves with energy travelling in
both directions, but cancelling at the null points. Very similar to
transmission lines.

Regards,

Mike Monett

Mike Monett wrote:

How about analyzing a vibrating string? If you play guitar, there's a very
nice note you can make by plucking a high string, then putting your finger
at exactly the correct spot and removing it quickly. The note will jump to
a much higher frequency and give a much purer sound. Clearly, the
mechanical energy has split into two waves that cancel at the node.

In principle, you could show the node is stationary, thus contains no
energy. But there is energy travelling on both sides of the null point -
you can hear it.

You can also create other notes by touching different spots on the
vibrating string. These create standing waves with energy travelling in
both directions, but cancelling at the null points. Very similar to
transmission lines.

Regards,

Mike Monett

Sounds like a great idea. I'll look forward to seeing your analysis.

Roy Lewallen, W7EL

Roy Lewallen wrote:

Mike Monett wrote:

[...]

Sounds like a great idea. I'll look forward to seeing your analysis.

Roy Lewallen, W7EL

LOL! I stopped playing guitar years ago!

Regards,

Mike Monett