On Sat, 07 Apr 2007 16:10:03 GMT, Gene Fuller wrote:

Walter Maxwell wrote:
On Fri, 06 Apr 2007 23:03:42 GMT, Cecil Moore wrote:

MRW wrote:
Any comments? Really, what I'm trying to understand here is: if
constructive interference does any good in radiowave propagation. I
was thinking that with an increase in amplitude the signal would be
able to travel a little further, but the signal received may not be
accurate in terms of the information it is conveying.
Antenna gain over isotropic is an application of
constructive interference. The constructive
interference must be balanced by an equal amount
of destructive interference elsewhere to avoid
violating the conservation of energy principle.

This is what I've been trying to persuade the 'anti's' that whenthe radiation fields from two vertical dipoles
superpose at some point in space, where their magnitudes are equal and are 180° out of phase, the wave
cancellation resulting from destructive interference produces a null in a predetermined direction, and thus
prevents those fields from propagating any further in that direction. At the precise instant the null is
produced, the constructive interference following the principle of energy conservation yields an increase in
the field strength in directions away from the null direction. This explains the concept of antenna-pattern
modification, and contradicts the notion that the two fields just plow through each other with no effect on
either.

Keep in mind that the two fields are coherent because they were developed simultaneously from the same source.
It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Walt, W2D

Walt,

Your observation is "correct" only in the case that most people consider
for practical reasons. The calculation showing the null behavior is
almost invariably performed at infinite distant from the sources, i.e.,
far field condition. The path from each source to the observation point
is considered to be exactly parallel.

As you know, there are usually three or more linear dimensions that
enter into radiation calculations. In the case of two sources there are
four:

Wavelength
Size of each source
Distance between sources
Distance to the observation point

In the typical "null" presentation, such as that shown in the ARRL
publications, the distance to the observation point in always large.

Lets take another case, however. Suppose the distance between the
sources is some what larger than the wavelength. Make it large enough so
there is a region between the sources that would be considered far field
from each of the sources. Now calculate the phase differences along some
direction from the center point between the sources that eventually
points to a null region in the infinite distance. Don't pick an
obviously symmetric direction, such as broadside or end-fire, as that
would be a special case.

What you will find is that when looking at the phase difference along
the ultimate null direction is that there is no such null much closer to
the sources. The paths from the individual sources are not parallel in
this case. The null "line" is actually a curve. The waves pass right
through each other in the closer region. The "passing waves" then go on
to form nulls in the infinite distance. The nulls in the closer region
are not in the same directions as the nulls in the far field.

Again, the ground rules:

Totally coherent, monochromatic sources
Fixed phase difference
Far field conditions for each source

There are no "tricks" here; this is just a matter of simple geometry.
However, it shows that the null you believe demonstrates some permanent
interaction and annihilation of EM waves is simply a special case.

In classical, non-cosmic, non-relativistic conditions EM waves do not
interact in free space. This condition is so widely understood in the
scientific world that it becomes a prime candidate for argument on RRAA.

8-)

73,
Gene
W4SZ

Gene, at this point I can't disagree with you. However, in your next to the last paragraph in your post above,
if I interpret you correctly, you are saying that all directional arrays, such as are used in AM broadcasting,
are considered 'special' cases. Is that what you mean't to infer?

Walt, W2DU

Walter Maxwell wrote:
On Sat, 07 Apr 2007 16:10:03 GMT, Gene Fuller wrote:

[snipped]

Walt,

Your observation is "correct" only in the case that most people consider
for practical reasons. The calculation showing the null behavior is
almost invariably performed at infinite distant from the sources, i.e.,
far field condition. The path from each source to the observation point
is considered to be exactly parallel.

As you know, there are usually three or more linear dimensions that
enter into radiation calculations. In the case of two sources there are
four:

Wavelength
Size of each source
Distance between sources
Distance to the observation point

In the typical "null" presentation, such as that shown in the ARRL
publications, the distance to the observation point in always large.

Lets take another case, however. Suppose the distance between the
sources is some what larger than the wavelength. Make it large enough so
there is a region between the sources that would be considered far field
from each of the sources. Now calculate the phase differences along some
direction from the center point between the sources that eventually
points to a null region in the infinite distance. Don't pick an
obviously symmetric direction, such as broadside or end-fire, as that
would be a special case.

What you will find is that when looking at the phase difference along
the ultimate null direction is that there is no such null much closer to
the sources. The paths from the individual sources are not parallel in
this case. The null "line" is actually a curve. The waves pass right
through each other in the closer region. The "passing waves" then go on
to form nulls in the infinite distance. The nulls in the closer region
are not in the same directions as the nulls in the far field.

Again, the ground rules:

Totally coherent, monochromatic sources
Fixed phase difference
Far field conditions for each source

There are no "tricks" here; this is just a matter of simple geometry.
However, it shows that the null you believe demonstrates some permanent
interaction and annihilation of EM waves is simply a special case.

In classical, non-cosmic, non-relativistic conditions EM waves do not
interact in free space. This condition is so widely understood in the
scientific world that it becomes a prime candidate for argument on RRAA.

8-)

73,
Gene
W4SZ

Gene, at this point I can't disagree with you. However, in your next to the last paragraph in your post above,
if I interpret you correctly, you are saying that all directional arrays, such as are used in AM broadcasting,
are considered 'special' cases. Is that what you mean't to infer?

Walt, W2DU

Hi Walt,

Yes, those are special cases, but those special cases are the only ones
that most people care about.

What I was trying to say might be better illustrated by the following:

Two coherent laser beams from the same source can be arranged by
suitable mirrors to intersect at some angle. There will most definitely
be interference in the region of intersection, but the beams will
continue through unchanged. If one measured a beam somewhere downstream
from the intersection region it would not be possible to determine that
it had crossed another beam earlier.

The beams "interfere" but they do not "interact". I know this sounds
goofy, and it is critical to keep the definitions straight. When I say
the beams do not interact I mean that they do not cause any changes in
the other beam. The fact that the beams interfere means that the sum of
the fields shows the characteristic constructive and destructive
behavior. It does not mean that the waves are henceforth changed.

OK, so how does this square with the observation that there are nulls in
patterns from two or more RF sources? It is actually very
straightforward. In the far field the waves from the separate sources
are virtually parallel. Just like Timex, they interfere and they keep on
interfering. They never really pass beyond the intersection region.

I know it seems like a subtle, or even meaningless, distinction. Do the
waves interfere forever or do they actually annihilate each other? For
many purposes it does not matter. However, the non-interaction of waves
in free space is pretty basic to all of EM analysis.

73,
Gene
W4SZ

Cecil, W5DXP wrote:
"Antenna gain over isotropic is an application of constructive
interference."

Yes. An often offered annalog is an inflated spherical balloon. It
contains the same amount of air no matter how it is squeezed. Sqeeze it
one place and it bulges elsewhere.

An isotropic antenna, could one be constructed, would radiate equally
well in all directions. As a radiation pattern becomes lopsided, the
bulge is filled with the energy squeezed from elsewhere.

Directive gain of an antenna is a power ratio. It`s the power that you
would have to put into an isotropic wersus the power you have to put
into the gain antenna to lay the same signal on a point in the preffered
direction.

Other things equal, if a gain antenna radiates twice the power in the
preferred direction as an isotropic, it has a gain of 3 dBi.

Best regards, Richard Harrison, KB5WZI

On 6 abr, 23:37, "MRW" wrote:
In my physics book, it mentions constructive and destructive wave
interference especially in reference to the the one-slit diffraction
experiment. From reading about radiowave propagation, they also
mention diffraction effects on radiowaves.

To me, it sounds like with constructive interference, the wave's
amplitude will have the chance of increasing more than what the source
actually outputted. But I wonder if this is helpful in terms of radio
communication.

In reference to a single frequency transmitted, when I think about
constructive interference and radiowave propagation, I keep picturing
a delayed signal transmitted at time_0 and another signal transmitted
at time_1 later with the same phase arriving at the receiver at the
same time.

In terms of AM, I would think this would be problematic.

Any comments? Really, what I'm trying to understand here is: if
constructive interference does any good in radiowave propagation. I
was thinking that with an increase in amplitude the signal would be
able to travel a little further, but the signal received may not be
accurate in terms of the information it is conveying.

Thanks!
Hello MRW,

As long as the constructive interference occurs over the full
bandwidth of your signal, it helps you without the need for
equalizing. Another way to see it is that if the delay of the
(reflected, refracted, etc) signal is far below 0.25/(RF bandwidth)
the signals will add constructively when the carriers are in phase at
the point of interference (inclusive the side bands generated by the
modulation).

This becomes more difficult (or impossible) for wide band signals. One
can see that in the frequency response of the propagation path.
Imagine when you transmit a signal with uniform power distribution
(brick wall spectrum). Receive it with an antenna and examine the
signal wit a spectrum analyzer. When the spectrum is flat (as the
original signal), then you will not have problems demodulating the
signal. However when you see many dips and peaks in the spectrum, the
information on the signal will be distorted. You will need an
equalizer (inverse FFT, deconvolution) to remove the distortion.

Another test is to transmit a very narrow pulse (amplitude modulated).
Receive the signal en show the demodulated version on an oscilloscope.
When the demodulated pulse has been stretched, you have distortion in
the modulation.

The effect of distortion in mobile systems due to multiple waves
arriving at an antenna, results in so called "frequency selective
fading".

About analog AM, the BW of the signal is about 8 kHz, As long as the
delay of reflected/refracted waves is less then 30us (that is 9 km in
distance), you will not have problems with signal distortion (valid
for surface wave propagation). With propagation via the ionosphere,
the situation is different; there the path length of several waves can
be so different, that for example waves with frequency 13.720 MHz
interfere constructively, but with frequency 13.722 MHz they interfere
destructively.

So when you don't want distortion because of destructive and
constructive interfering wave fronts, you should have a narrow
bandwidth (that is low bitrate). This is done in multi carrier
modulation (like COFDM [TDAB, DVBT]). Many or some carriers will
suffer from destructive interference, but also many will be subjected
to constructive interference. By adding sufficient redundancy, the
data stream from the sub carriers having good signal strength can be
demodulated to the original data stream.

Relative high baud rate systems (like the GSM system) use equalizers/
echo cancellators to mitigate the effect of multi-path reflections.

Best Regards,

Wim
PA3DJS

On Sun, 08 Apr 2007 20:26:41 GMT, Owen Duffy wrote:
Owen, the following is a copy of your post of 4-8-07, and my response 0n 4-12-07 to which you haven't
responded. Perhaps you haven't seen my response, or perhaps you chose not to respond, which is ok either way.


Walter Maxwell wrote in
:

Walt, I can see that you have taken my comment as personal criticism. That
was not intended, and to the extent that I may have caused that, I
apologise. In that context, it is better that I refrain from further
comment.

Regards
Owen

Hi Owen,

Please excuse the long delay in responding to your post of 4-8-07, 4:26 pm EDT. I have been away from the
computer since then, attending to personal chores that took priority over rraa.

I'm sure your comments weren't meant as a personal attack, and I accept your apology.

However, your consideration of statements appearing in Reflections as flawed on the assumption that the
concepts presented there concerning impedance matching apply only to lossless and distortionless lines, IMHO
is unfair, because it is not true.

For readers of your post who now may be questioning the reliability of statements appearing in Reflections,
I'm working on a more detailed discussion of the issue for clarification that I will enter on the rraa as a
new thread.

Walt

Walter Maxwell wrote:
On Sun, 08 Apr 2007 20:26:41 GMT, Owen Duffy wrote:
Owen, the following is a copy of your post of 4-8-07, and my response 0n 4-12-07 to which you haven't
responded. Perhaps you haven't seen my response, or perhaps you chose not to respond, which is ok either way.

Perhaps instead of asking Owen to point out what is wrong with
your writings, he would be more comfortable discussing his
theory, given the Vr and Ir terms that he uses, of how the
energy associated with that Vr and Ir wave gets its direction
and momentum changed at a Z0-match when Vr and Ir are
canceled/re-reflected/redistributed.
--
73, Cecil, w5dxp.com

Cecil Moore wrote:

Perhaps instead of asking Owen to point out what is wrong with
your writings, he would be more comfortable discussing his
theory, given the Vr and Ir terms that he uses, of how the
energy associated with that Vr and Ir wave gets its direction
and momentum changed at a Z0-match when Vr and Ir are
canceled/re-reflected/redistributed.
--
73, Cecil, w5dxp.com

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?

73, ac6xg

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.

Here's a very simple example. The measured forward
and reflected powers are given. The source and load
impedances are irrelevant and the length of the Z01
and Z02 lines are irrelevant. Any one of these
measured values could be unknown and solved for by
calculations based on the conservation of energy
principle.

------Z01------+------Z02------
Pfor1=100w-- Pfor2=200w--
--Pref1=0w --Pref2=100w

We have 100 joules/sec incident upon the Z0-match
point from the direction of the source. We have 100
joules/sec incident upon the Z0-match point from
the direction of the load. Those waves combine
to obtain 200 joules/sec toward the load. It is
obvious that Pref2 has to change direction and
momentum for that condition to exist.

The power reflection coefficient, rho^2, is obviously
0.5 so the voltage reflection coefficient, rho, is
just as obviously +/- 0.707, depending upon whether
[Z02 Z01] or [Z02 Z01}.

The direction and momentum of the Pref2 reflected wave
obviously reverses at the Z0-match point '+'. Exactly
how does the direction and momentum of the Pref2 wave
get reversed? Where are the physics equations for that
process that we hams label "re-reflection"? You and others
have been strangely silent on that subject preferring to
kibitz rather than provide any technical insight.

An exact duplicate of the above conditions would exist
with a 100w laser beam traveling through 1/2WL of thin
film with an index of refraction of 5.83.

A B
i=1.0 | i=5.83 | i=1.0
100w laser---air---|--1/2WL thin-film--|---air---...
--Pref1=0w | --Pref2=100w | --Pref3=0w
Pfor1=100w | Pfor2=200w-- | Pfor3=100w--

What happens to reverse the direction and momentum of
the internal reflection in the thin film? Hint: Both
Hecht and Born & Wolf give the equations for what happens
at plane A. And yes, the S-Parameter equations agree 100%
with them.
--
73, Cecil http://www.w5dxp.com