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