Constructive interference in radiowave propagation
 

Jim Kelley wrote:
Cecil Moore wrote:

It just goes to show that people who believe they know everything
rarely know anything.

That's probably a bit of an overstatement. But they certainly can be
annoying.

Again, I post the Hecht and Born & Wolf equation for intensity-
irradiance, which is certainly an equation involving scalar values.
Please answer the question: Why do Hecht and Born & Wolf insert a
cosine term into their scalar intensity-irradiance equations? If
it is OK for them to do it, why is it not OK for me to do it?

Itot = I1 + I2 + 2*SQRT(I1*I2)*cos(A)

You seem to think the act of inserting a cosine term into a
scalar equation is an abomination. Please explain that criticism
of Hecht, Born & Wolf, and me. It's past time to put up or shut up.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Gene Fuller wrote:
Cecil Moore wrote:
Jim Kelley wrote:
You seem to be implying that there's something different about how
these electromagnetic waves change direction compared to other
electromagnetic waves. Why is that?

There is something different but not unusual. We don't
often observe wave cancellation of visible light waves
because of the problem of getting coherent beams of light
perfectly aligned. Yet, we experience RF wave cancellation
every time we adjust our antenna tuners for a Z0-match
because the perfect alignment of coherent RF waves inside
a piece of coax is an automatic given.

This is a rather curious notion. Where did you get the idea that waves
must be perfectly aligned to "cancel"?

Apologies - what I meant to say was that waves must be perfectly
aligned to totally cancel. When I say "wave cancellation", I
am usually talking about total wave cancellation, as occurs at
a perfect Z0-match. I will try not to make that same mistake
in the future.

Waves need not be perfectly aligned to partially cancel.
Waves must be perfectly aligned to totally cancel. Hope
that clears up the confusion about what I meant to say.

And of course, partial wave cancellation can extend from
almost none to almost total. However, total wave cancellation
obviously requires perfect alignment.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Cecil Moore wrote:


It is impossible for a "partially reflective surface"
to reflect 100% of the intensity.

But that's wrong. If it was right, then a partially reflective
surface couldn't be used to prevent reflections either.

Think about the transient period. You're right that after the first
bounce only half the intensity, for example, is apparent. But using
the physical reflection coefficient you can plot the intensity
increase steadily as a function of time all the way up to steady
state. That's because at every time t, the remainder of all previous
reflections are superposed. Yes, interference describes
macroscopically what happens - it's a short cut to steady state. But
nothing about the reflective surface changes - before or after steady
state. It is only your idea of 'energy in the wave' that needs to
change a little.

73, ac6xg

Constructive interference in radiowave propagation
 

Jim Kelley wrote:

Cecil Moore wrote:
It is impossible for a "partially reflective surface"
to reflect 100% of the intensity.

But that's wrong. If it was right, then a partially reflective surface
couldn't be used to prevent reflections either.

That's faulty logic born out of ignorance.
Assume s11 = 0.707 in the S-Parameter reflected
voltage equation. a1 is the normalized forward
voltage from the source. Let's assume a1 = 10.

b1 = s11(a1) + s12(a2)

the initial transient state reflection is

b1 = s11(a1) = 0.707(10) = 7.07 normalized volts

and *that term remains constant* throughout the transient
state and throughout steady-state. The impedance discontinuity
with s11=0.707 reflects 70.7% of the incident voltage, period,
no more and no less. The magnitude of a1 reflected by that
impedance discontinuity *DOES NOT CHANGE* from the very
first incidence of a1. So your statement above is obviously
false. Physical impedance discontinuities do not change their
reflection coefficients based on your whim.

So how does b1 wind up at zero? Not by changing s11(a1) as
you imply. b1 is eventually canceled by the buildup to steady-
state of s12(a2) from zero to a magnitude equal to s11(a1) and
a phase opposite of s11(a1). That's *wave cancellation* in action.
What happens to the energy in the canceled waves?

So your premise is completely flawed. s11(a1) doesn't change.
s12 doesn't change. s21 doesn't change. s22 doesn't change.
What changes is the s12(a2) term which is the reflections
from the load. b1 decreases increment by increment due to
the wave cancellation between the fixed value of s11(a1)
and the ever increasing value of s12(a2) until steady-state
is reached and b1 has become zero. At steady-state, s11(a1)
is still equal to 7.07 normalized volts. It has not changed.
Contrary to your assertion, it will not change as long as
a1 is applied.

s11(a1) = 7.07 unchanging throughout the initial transient
build-up and through steady-state. Anything else would
require magic.

b1 is initially equal to 7.07 because a2 is zero.

s12(a2) will eventually build up from 0 to 7.07 at which
point the *net reflections are eliminated by wave cancellation*.

As b1 is decreasing to zero at steady-state, b2 is increasing
to its steady-state value in the other direction. b1 is
undergoing increasing destructive interference and b2 is
undergoing increasing constructive interference until the
time when b1 = 0 and therefore |b1|^2 = 0, i.e. net reflections
are eliminated.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

Owen Duffy wrote:
"I am not quite sure about the concept of energy at a point that you
discuss, isn`t it zero."

Not when radio waves are passing by. These waves were likely produced by
electrical energy in a wire somewhere that spread into space around the
wire. Radio waves alternate around a zero value. If symmetrical about an
axis, the waveforms may have zero average values. But that is not how we
value the intensity of a rafio wave. We give it an rms or effective
value which is 0.707 times its maximum voltage profuced during the
cycle.

When speaking of power in an alternating energy value, it is not correct
to say rms power. The effective a-c power value is its average.

I`m not a teacher, never have been, and never intend to be. I think I
got into this discussion by declaring that 50% of the power in a wave
resided in each of its two constituents.

I shall argue no more nor try to explain any more on the topic of radio
waves in this thread. Fred Terman is the master of all masters in my
books and I suggest beginning on page one of his 1955 version of
"Electronic and Radio Engineering" to learn all about "Radio Waves".

From page 1:
"One-half of the electrical energy contained in the wave exists in the
form of electrostatic energy, while the remaining half is in the form of
magnetic energy."

Best regards, Richard Harrison, KB5WZI

Constructive interference in radiowave propagation
 

On Apr 13, 6:13 pm, Cecil Moore wrote:
Jim Kelley wrote:

Cecil Moore wrote:
It is impossible for a "partially reflective surface"
to reflect 100% of the intensity.

But that's wrong. If it was right, then a partially reflective surface
couldn't be used to prevent reflections either.

That's faulty logic born out of ignorance.

Partially reflective surfaces can (and are) in fact used to prevent
reflections, just as they are used to 100% re-reflect partial
reflections from a load.

The magnitude of a1 reflected by that
impedance discontinuity *DOES NOT CHANGE* from the very
first incidence of a1.

That was the main point of my post, Cecil. The reflective coefficient
DOES NOT CHANGE. You're the one who claims that it does.

What happens to the energy in the canceled waves?

There is no energy "in" cancelled waves. Your ideas in that regard are
faulty. Energy only exists where fields aren't cancelled. That
should be obvious even to someone with propensities such as yours.

ac6xg

Constructive interference in radiowave propagation
 

On 14 Apr 2007 09:53:11 -0700, "Jim Kelley" wrote:

What happens to the energy in the canceled waves?

There is no energy "in" cancelled waves.

Hi Jim,

How has this inversion arrived? Cecileo offering emphatic testimony
to the Cardinals "It certainly doesn't move!" and no energy "in"
cancelled waves?

Both waves exist as the absence of either would easily reveal. There
may be no power to extract due to their offsetting contributions, but
that doesn't prove they have vanished (which, in the context of sight,
interference, and light diminishing in regions necessarily demands a
load to demonstrate).

The language of photon shuffling and energy re-distribution lends the
logic of divine intervention to scientific theory. These verbs are
active and require an actor. If we were to travel down that path, the
patterns of intelligent design interference would be explained in
epicycles and crystalline spheres of angels' guiding results.

73's
Richard Clark, KB7QHC

Constructive interference in radiowave propagation
 

On Apr 14, 11:27 am, Richard Clark wrote:
On 14 Apr 2007 09:53:11 -0700, "Jim Kelley" wrote:

What happens to the energy in the canceled waves?

There is no energy "in" cancelled waves.

Hi Jim,

How has this inversion arrived? Cecileo offering emphatic testimony
to the Cardinals "It certainly doesn't move!" and no energy "in"
cancelled waves?

Both waves exist as the absence of either would easily reveal. There
may be no power to extract due to their offsetting contributions, but
that doesn't prove they have vanished (which, in the context of sight,
interference, and light diminishing in regions necessarily demands a
load to demonstrate).

The language of photon shuffling and energy re-distribution lends the
logic of divine intervention to scientific theory. These verbs are
active and require an actor. If we were to travel down that path, the
patterns of intelligent design interference would be explained in
epicycles and crystalline spheres of angels' guiding results.

73's
Richard Clark, KB7QHC

Hi Richard,

I have to admit that I do have difficulty arguing with nonsense, and
you've caught me at it. I've tried explaining this to Cecil in the
context of energy transfer, but without success. So I'm happy to
leave it to you to explain to Cecil how waves cancel but without
anhiliating the energy "in" them.

73, ac6xg

Constructive interference in radiowave propagation
 

On Sat, 14 Apr 2007 11:27:11 -0700, Richard Clark wrote:

On 14 Apr 2007 09:53:11 -0700, "Jim Kelley" wrote:

What happens to the energy in the canceled waves?

There is no energy "in" cancelled waves.

Hi Jim,

How has this inversion arrived? Cecileo offering emphatic testimony
to the Cardinals "It certainly doesn't move!" and no energy "in"
cancelled waves?

Both waves exist as the absence of either would easily reveal. There
may be no power to extract due to their offsetting contributions, but
that doesn't prove they have vanished (which, in the context of sight,
interference, and light diminishing in regions necessarily demands a
load to demonstrate).

The language of photon shuffling and energy re-distribution lends the
logic of divine intervention to scientific theory. These verbs are
active and require an actor. If we were to travel down that path, the
patterns of intelligent design interference would be explained in
epicycles and crystalline spheres of angels' guiding results.

73's
Richard Clark, KB7QHC

Richard, I love the way you talk about 'epicycles and crystalline spheres of angels'.

Walt

Constructive interference in radiowave propagation
 

Jim Kelley wrote:
Partially reflective surfaces can (and are) in fact used to prevent
reflections, just as they are used to 100% re-reflect partial
reflections from a load.

Partially reflective surfaces cannot, by themselves,
reflect 100% of the incident energy. If it's partial,
it's not 100%, by definition. Any partially reflective
surface needs help from interference in order to
achieve 100% reflection. You know, that interference
that you deny exists.

That was the main point of my post, Cecil. The reflective coefficient
DOES NOT CHANGE. You're the one who claims that it does.

You continue to lie about what I said. I have said any
number of times that the physical reflection coefficient,
s11, is fixed and does NOT change. Why does someone who
is technically correct need to stoop to lying?

There is no energy "in" cancelled waves.

The waves existed along with their energy components before
they were canceled. What happens to those energy components
after the waves are canceled. If one sets one phase equal
zero and the other phase equal 180 degrees, what happens to
the energy in the two waves at:

http://micro.magnet.fsu.edu/primer/j...ons/index.html

There are two waves on the left existing with their respective
voltage and joules/sec. The result of total destructive interference
is zero voltage and zero joules/sec. What happened to the original
joule/sec components?
--
73, Cecil http://www.w5dxp.com