Constructive interference in radiowave propagation
 

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!

Constructive interference in radiowave propagation
 

"MRW" wrote in message
ups.com...
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.

the amplitude can be more in one direction than another, but the total power
can not exceed the transmitter output of course. for each constructive
interference peak there must be an area of destructive interference to make
up for it.


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.

yep, that is what ghosts on tv signals are... if the delay is long with
respect to the modulating signal you can get effects like that. the most
common desirable uses are in antennas where there is a phase delay about
equal to the spacing of the elements of the antenna which lets you create a
stronger signal in one direction, and of course a weaker one in other
directions, allowing you to put more of the transmitter power in the
direction you want it to go. because the delays are small there is not the
problem with ghosts.



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.

yes, constructive interference is what antenna design is all about...
destructive interference has its part also to help reject interference from
undesired sources as well.

Constructive interference in radiowave propagation
 

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.
--
73, Cecil http://www.w5dxp.com

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

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

Constructive interference in radiowave propagation
 

Walter Maxwell wrote in
:

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.


Walt, this seems inconsistent with the approach that I believe you seem
to use in analysing waves in transmission lines where you seem to want to
not only deal with the forward and reverse waves separately (ie to not
collapse them to a resultant V/I ratio at a point), but to deal with
multiply reflected waves travelling in the forward and reverse direction
(which is only necessary in the transient state).

Owen

Constructive interference in radiowave propagation
 

On Sat, 07 Apr 2007 03:03:40 GMT, Walter Maxwell
wrote:

It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Hi Walt,

Well, having broached the topic, it appears time to plunge in once
again.

Several but closely related questions:
What separates "effect" from "no effect?" (They are, afterall, a
rather strict binary outcome.)

Does the binary transition from a one micro-degree longer
cycle (non-coherent) to 0 (coherence) same length cycle really
plunge us into a new physical reality of waves colliding with
rebounds and caroms where formerly there was absolutely no
interaction before?

73's
Richard Clark, KB7QHC

Constructive interference in radiowave propagation
 

On Apr 6, 11:03 pm, Walter Maxwell wrote:

It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Can one not change the location of the nulls by changing
the phase relationship of the two sources?

If so, it would seem to me that two non-coherent fields are
simply fields without a constant phase relationship and as
such, the nulls are constantly moving; still present, but
not stationary.

....Keith

Constructive interference in radiowave propagation
 

Keith Dysart wrote:
On Apr 6, 11:03 pm, Walter Maxwell wrote:

It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Can one not change the location of the nulls by changing
the phase relationship of the two sources?

If so, it would seem to me that two non-coherent fields are
simply fields without a constant phase relationship and as
such, the nulls are constantly moving; still present, but
not stationary.

...Keith

Andy writes:
Correct. One example is a television signal that is received from
two
sources : 1) a direct line to the transmitting tower and 2) a
reflection
from an airplane flying .

Even tho both received signals are generated from the same source,
the reflected signal will be changing in amplitude and phase as the
reflector,
the airplane, moves along it's flight path.

The two signals combine at the receiving antenna and the
resultant signal into the receiver will rise and fall, depending on
the resultant amplitude and phase. The maximum can be several
db above the direct signal and the null can be many many db
BELOW the direct signal.

Hence, you see the image come and go for several seconds on
your screen. After several seconds the plane will have moved to a
position
such that the reflection doesn't hit your antenna anymore, and the
problem goes away. We've all seen this. In fact, 70 years ago, this
effect (on radio signals) was what inspired the development of
radar.....

Andy W4OAH

Constructive interference in radiowave propagation
 

Richard Clark wrote:
Walter Maxwell wrote:
It is true, however, that two non-coherent fields from two different sources would just plow through each
other with no effect on either.

Does the binary transition from a one micro-degree longer
cycle (non-coherent) to 0 (coherence) same length cycle really
plunge us into a new physical reality of waves colliding with
rebounds and caroms where formerly there was absolutely no
interaction before?

Of course, you are being facetious but the answer is simple.
If the two signals are mutually incoherent, they don't
interfere. Permanent wave cancellation is impossible
between two waves that are not coherent. Hecht in "Optics"
devotes an entire chapter to the "Basics of Coherence Theory".
So do Born and Wolf in "Principles of Optics". Here is what
Walt was obviously saying except in Born and Wolf's words:

"If the two beams originate in the same source, the fluctuations
in the two beams are in general correlated, and the beams are
said to be completely or partially *coherent* depending on
whether the correlation is complete or partial. In beams from
different sources, the fluctuations are completely uncorrelated,
and the beams are said to be mutually *incoherent*. When such
beams from different sources are superposed, no interference is
observed under ordinary experimental conditions, the total intensity
being everywhere the sum of the intensities of the individual beams."

In case you missed it, that says *NO INTERFERENCE* between mutually
incoherent waves. Seems reasonable to say that "no interference"
means the same thing as "no effect".
--
73, Cecil http://www.w5dxp.com