Constructive interference in radiowave propagation
 

Roy Lewallen wrote:
Next, do the same for a transmission line. Show how two coherent
traveling waves can be produced which will propagate together in the
same direction but out of phase with each other, resulting in a net zero
field at all points beyond some summing point.

Roy, it is done all the time represented by the
S-Parameter equation for the reflected wave
toward the source. Assuming an impedance discontinuity
in a transmission line:

b1 = s11(a1) + s12(a2) = 0

If b1=0, then s11(a1) and s12(a2) are indeed two coherent
traveling waves propagating together in the same direction
but out of phase with each other. The net reflected wave
field is zero.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Cecil Moore wrote:


One more example of an ignorant person making fun of something
he doesn't understand.

Cecil,

Sorry, I simply cannot keep up with you. We seem to be switching from RF
to optics in order to explain something or other, and now we are
switching from optics to S-parameters to explain the explanation.

You accuse me of saying that waves never "cancel", right after I told
you that "cancel" is not a good description in detailed technical
analysis. Do you actually read anything here?

Going back to your oft-stated line about Galileo, you seem to be on the
other side now. You keep insisting on a Cecil-centric universe, with the
consequent requirement to add more and more crystalline spheres to
explain all of your "creative" ideas.

Stick to the basic EM understanding that has stood for more than 100
years. No need to keep inventing new principles. The old ones work just
fine.

(You must be getting desperate; the ad hominem attacks have started.) 8-)

73,
Gene
W4SZ

Constructive interference in radiowave propagation
 

Richard Fry wrote:
The total average power available at the hybrid output for both of these
conditions is twice that of a single tx without the hybrid.

Does the quote from Born and Wolf support this?

Yes, there is obviously no interference between the
two transmitters if the powers simply add together.
You have doubled the current capability without
doubling the voltage capability - that's not
interference.

For interference to occur, both the E-fields and
the H-fields must be superposed at the same time
such that both fields increase or decrease by the
same percentage. So how do we double the voltage
and double the current in the 50 ohm transmission
line to the antenna?

Put a transformer on the output of each transmitter.
Wire the secondaries in series. We have doubled the
voltage output. Now drive the 50 ohm load with that
doubled voltage and see what happens. This is equivalent
to constructive interference. Hope you are running
the transmitters at half power. :-)
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Gene Fuller wrote:
Sorry, I simply cannot keep up with you. We seem to be switching from RF
to optics in order to explain something or other, and now we are
switching from optics to S-parameters to explain the explanation.

So are you saying the physics concepts from the field
of optics are wrong? Are you saying the S-Parameter
equations are invalid? If not, seems you are having
a hard time defending your concepts against those
valid concepts. That should tell you something about
your (simplified short cut) concepts.

I'm going to keep it up until you give up on the notion
that you already know everything and therefore reality
obeys your every whim. You cannot dismiss wave cancellation
simply because you find that part of reality distasteful.

In analyzing an impedance
discontinuity in a transmission line, the S-Parameter
equations are accepted as valid. b1 is the normalized
reflected voltage toward the source.

b1 = s11(a1) + s12(a2) = 0

When b1 is zero, waves s11(a1) and s12(a2) have been
canceled because they are of equal magnitude and opposite
phase. Simple cause and effect - not rocket science.

If you can prove those concepts from the field of optics
are invalid and that an S-Parameter analysis is invalid,
now would be the time.

You accuse me of saying that waves never "cancel", right after I told
you that "cancel" is not a good description in detailed technical
analysis. Do you actually read anything here?

Wave cancellation occurs all the time, Gene. Every time
a ham tunes his antenna tuner for zero reflected power,
he has caused two reflected waves to cancel. I am amazed
at how many otherwise intelligent posters to this newsgroup
attempt to engage in the copout of sweeping under the rug
anything that they do not understand and/or don't want to
deal with.

Stick to the basic EM understanding that has stood for more than 100
years.

That's exactly what I am doing. You seem to be relatively
ignorant of those century old concepts. I don't know when
interference was first explained but it was long before
you and I were born. I am repeating principles that have
been around for a century including the wave reflection
model.

What has actually happened is that seductive short cuts
have left many ignorant of the basic principles, e.g.
Standing waves can exist without reverse traveling waves.
Reverse traveling waves exist without a source of energy.
Waves never interact. etc. etc. etc.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Gene Fuller wrote:
We seem to be switching from RF
to optics in order to explain something or other, ...

Gene, I forgot to ask. At exactly what EM frequency
do the RF waves stop obeying the century old laws of
physics for visible light?
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

On Apr 8, 10:20 pm, Cecil Moore wrote:
Keith Dysart wrote:
This is good; a continuum with high coherence at one end, low
coherence at the other and medium in the middle, and, of course,
since the ends are infinitely small, no such thing as perfect
coherence or "NO" coherence (at least in the real world).

But remember that definition is for fiber optics
sources, not amateur radio sources. Coherency
in amateur radio systems can get as close as
a zero reading on a reflected power meter.

Still there are those nagging assertions of Born
and Wolf that for two equal magnitude signals,
the total intensity possible for incoherent
signals is double the intensity of one signal.
The total intensity possible for coherent
signals is four times the intensity of one
signal.

I take this to mean that with largely incoherent sources
the intensity is doubled everywhere.

With largely coherent sources, the average intensity is
doubled everywhere, but there is also a spatial distribution
where the peak intensity is 4 times, but the minimum is zero
(thus the same average of two).

....Keith

Constructive interference in radiowave propagation
 

On Apr 9, 9:04 am, Cecil Moore wrote:
Richard Fry wrote:
The total average power available at the hybrid output for both of these
conditions is twice that of a single tx without the hybrid.

Does the quote from Born and Wolf support this?

Yes, there is obviously no interference between the
two transmitters if the powers simply add together.
You have doubled the current capability without
doubling the voltage capability - that's not
interference.

For interference to occur, both the E-fields and
the H-fields must be superposed at the same time
such that both fields increase or decrease by the
same percentage. So how do we double the voltage
and double the current in the 50 ohm transmission
line to the antenna?

Except that Richard's description sure seems to meet the
requirements of coherency. Can you offer a way for use
to know whether two signals are coherent?

Secondly, I am at a complete loss to understand how you
can be arguing that when two signals of a particular
power interfere, the result is 4 times the power. This
sure seems like you're getting something from nothing.

What happened to the staunch acceptance of 'conservation of
energy'?

....Keith

Constructive interference in radiowave propagation
 

On Apr 9, 3:35 am, Roy Lewallen wrote:
I'd vowed that I wouldn't hit this tarbaby yet again. But here I go.

Among the junk science being bandied about here is the following
supposition:

Suppose you have beams from two identical coherent lasers which, by a
system of (presumably partially reflective and partially transmissive)
mirrors, are made to shine in exactly the same direction from the same
point (which I'll call the "summing point"). Further, suppose that the
paths from the two lasers to this summing point differ by an odd number
of half wavelengths. So beyond the summing point, where the laser beams
exactly overlie each other, there is no beam because the two exactly
cancel. Or, in other words, the sum of the two superposed fields is
zero. The recurring argument is that because each laser is producing
energy and yet there is no net field and therefore no energy in the
summed beams, something strange has happened at the summing point (or
"virtual short circuit"), and creative explanations are necessary to
account for the "missing energy". One such proposed explanation is that
the mere meeting of the two beams is the cause of some kind of a
reflection of energy, and that each wave somehow detects and interacts
with the other.

Well, here's what I think. I think that no one will be able to draw a
diagram of such a summing system which doesn't also produce, due solely
to the reflection and transmission of the mirrors, a beam or beams
containing exactly the amount of energy "missing" from the summed beam.
No interaction(*) of the two beams at or beyond the summing point is
necessary to account for the "missing" energy -- you'll find it all at
other places in the system. Just as you do in a phased antenna array,
where the regions of cancelled field are always accompanied by
complementary regions of reinforced field. Somewhere, in some bounce
from a mirror or pass through it, the beams will end up reinforcing each
other is some other direction. My challenge is this: Sketch a system
which will produce this summation of out-of-phase beams, showing the
reflectivity and transmissivity of each mirror, and showing the beams
and their phases going in all directions from the interactions from each
mirror. Then show that simple interaction of the beams with the mirrors
is insufficient to account for the final distribution of energy.

Next, do the same for a transmission line. Show how two coherent
traveling waves can be produced which will propagate together in the
same direction but out of phase with each other, resulting in a net zero
field at all points beyond some summing point. But also calculate the
field from waves reflected at the summing point and elsewhere in the
system due to simple impedance changes. Show that this simple analysis,
assuming no interaction between the traveling waves, is insufficient to
account for all the energy. A single case will do.

Until someone is able to do this, I'll stand firm with the unanimous
findings of countless mathematical and practical analyses which show
superposition of and no interaction between waves or fields in a linear
medium.

(*) By "interaction" I mean that one beam or wave has an effect on the
other, altering it in some way -- for example, causing it to change
amplitude, phase, orientation, or direction. I'm not including
superposition, that is the fact that the net field of the two waves is
the sum of the two, in the meaning of "interaction".

Roy Lewallen, W7EL

Constructive interference in radiowave propagation
 

Keith Dysart wrote:
I take this to mean that with largely incoherent sources
the intensity is doubled everywhere.

Being a little more precise:
With mutually incoherent equal-magnitude sources,
the maximum possible peak intensity is double the
intensity of a single wave. There's no interference.

For "largely incoherent sources", the peak intensity
would be slightly more than double.

With largely coherent sources, the average intensity is
doubled everywhere, but there is also a spatial distribution
where the peak intensity is 4 times, but the minimum is zero
(thus the same average of two).

With mutually coherent equal-magnitude sources,
the maximum possible peak intensity is four times
the intensity of a single wave, i.e. there is total
constructive interference. (This can happen
at a Z0-match in an RF transmission line.)

For "largely coherent sources" the peak intensity
would be slightly less than four times.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

On Apr 9, 3:35 am, Roy Lewallen wrote:
I'd vowed that I wouldn't hit this tarbaby yet again. But here I go.

Thanks for doing so.

The junk science is often presented with very rational sounding
arguments and it can be difficult to detect the flaws. This example
was a case for me and you expose the flaw nicely.

What I have difficulty with is deciding on the value of the junk
science. On the one hand it misleads many; on the other, debunking
provides opportunity to develop deeper understanding.

When I first started lurking on this group many, many years ago,
I didn't even know that there was a question about "where does the
reflected power go". Following the debates and iterating to the
correct answers has been extremely educational, much more so than
just accepting the correct explanations without question.

The promoters of junk science fulfill an important role in this
process and I can't decide if their net effect is good or bad.
The bad effects are, of course, when they successfully lead
others astray.

On the whole, good or bad? I haven't decided.

....Keith