Gene Fuller wrote:
Why do you attribute such magic to the word "interference"? Do you think
that Hecht's "interaction" is any different than superposition?

It is not magic. "Interference" and "superposition" simply
have different definitions.

Interference is a subset of superposition, i.e. interference
cannot occur without superposition but superposition can occur
without interference. This subject is covered in every optics
text that I have ever seen, including Born and Wolf. Given two
waves of equal power densities (irradiances) if the resultant
irradiance is not equal to the sum of the two irradiances, then
interference has occurred.

What if the waves are not quite anti-parallel, say at an angle of 179
degrees? Is interference now possible?

Impossible in a transmission line which is the context.
In free space, I would guess that interference is possible
in their common direction of travel.

Suppose the waves are only 1 degree from parallel. Does that negate the
interference?

For coherent waves in free space, that would ensure interference
until the beams diverged. It should result in the usual light
and dark interference rings.

Repeating: This is a distinction with no technical value.

Maybe it would help if you published a video of you waving
your hands as you scream that assertion at the top of your
lungs? :-)
--
73, Cecil http://www.w5dxp.com

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

"Cecil Moore wrote
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.
_______________

Everywhere it exceeds 2*D^2/lambda in length, where D is the greatest
dimension of the array.

RF

On Jan 23, 1:46*pm, Cecil Moore wrote:
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.

And you also wrote:
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.

I am having great difficulty finding any difference
between my writing:

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".

and your paraphrase.

...Keith

Richard Fry wrote:
"Cecil Moore wrote
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.

Everywhere it exceeds 2*D^2/lambda in length, where D is the greatest
dimension of the array.

Are you saying that the sources that are "near each other"
are far enough apart to be in each other's far field?
How could that be if the two sources are D apart?
--
73, Cecil http://www.w5dxp.com

Keith Dysart wrote:
I am having great difficulty finding any difference
between my writing:

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".

and your paraphrase.

The difference is that your example contained elements
that are not zero dimensions. My assertions covered only
antenna elements of zero dimensions. I repeat:

On a line drawn between two coherent isotropic radiators,
in the absence of any reflections, interference along
that line is impossible because the average total
power density all along that line is constant.
There is no interference in standing waves given
"interference" as defined by Eugene Hecht in "Optics".
--
73, Cecil http://www.w5dxp.com

"Cecil Moore"
Everywhere it exceeds 2*D^2/lambda in length, where D is the greatest
dimension of the array.

Are you saying that the sources that are "near each other"
are far enough apart to be in each other's far field?
How could that be if the two sources are D apart?
_________

It isn't necessary for the radiators to be in each other's far field.

Here is a link to a plot of the far-field elevation pattern of a linear
array of three isotropic sources at one lambda vertical spacing.

http://i62.photobucket.com/albums/h8...picSources.gif

RF

On Jan 23, 2:21*pm, Cecil Moore wrote:
Keith Dysart wrote:
I am having great difficulty finding any difference
between my writing:

* 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".

and your paraphrase.

The difference is that your example contained elements
that are not zero dimensions. My assertions covered only
antenna elements of zero dimensions. I repeat:

On a line drawn between two coherent isotropic radiators,
in the absence of any reflections, interference along
that line is impossible because the average total
power density all along that line is constant.
There is no interference in standing waves given
"interference" as defined by Eugene Hecht in "Optics".

So then, for "two coherent isotropic radiator",
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".

...Keith