Consider the case of an EM source moving away from you at constant
velocity, with nothing but you with your measurement system, the EM
source, and freespace. What gives rise to dispersion?

K7ITM wrote:

Consider the case of an EM source moving away from you at constant
velocity, with nothing but you with your measurement system, the EM
source, and freespace. What gives rise to dispersion?


There is a small dispersion caused by the modulation scheme. The
modulating frequencies are slightly different from the carrier frequency
and therefore have slightly different doppler shift.

Restated: the Doppler shift on a 144.500 MHz carrier is different from
the Doppler shift on a 15 KHZ FM signal. How much? That's an exercise
for the student ... so sez my Prof!!

The doppler shift of 144.500 MHz is different from the doppler shift of
a 144.515 MHz sub carrier. If the FM moves the sub carrier doppler moves.

OK, Amos, I see what you wrote as a set of changes of frequency, all in
the same proportion, exactly as I'd expect from the Doppler formulas
I've seen, including relativistic ones. But how does that relate to
dispersion? Perhaps kb7qhc will offer us a definition of dispersion...

On 8 Mar 2006 12:24:13 -0800, "K7ITM" wrote:

Consider the case of an EM source moving away from you at constant
velocity, with nothing but you with your measurement system, the EM
source, and freespace. What gives rise to dispersion?

Doppler does. However, you should follow the nature of the thread in
that it is discussing a spectrum of emission, not just one frequency.
When you have more than one associated frequency, and the source(s)
are in a moving reference plane, these frequencies are all shifted by
the proportion of their frequency in relation to their speed. This
gives rise to corruption of waveshape in modulations - dispersion.

OK, so all wavelengths shift by the same ratio. In what way does that
change the "waveshape in modulations," other than to simply change the
time scale by that same ratio?

Or--does Doppler shift (alone) give rise to different propagation
velocities at different frequencies? Does Doppler shift change an
impulse to a chirp (or the right chirp to an impulse)?

On 8 Mar 2006 14:15:20 -0800, "K7ITM" wrote:

OK, so all wavelengths shift by the same ratio.

Yes, in fact they do. I mistakenly ascribed a disproportionality to
the relation of the doppler sidebands and the doppler carrier.

Net answer, no dispersion.

Thanks, Richard. I'd have been happy to learn that indeed Doppler
shift does lead to dispersion -- "any day I learn something new is a
good day." But coming to agreement was also good. Now, for the next
part of the story, Doppler shift _in_combination_with_ multipath indeed
does lead to dispersion, I believe. That's why I was careful in my
original question to say, "Doppler alone?" See for example
http://www.eleceng.adelaide.edu.au/s...grad/Yu05.html. I
get the impression that Ms. Yu is a pretty bright young woman.

Cheers,
Tom

On 8 Mar 2006 15:16:05 -0800, "K7ITM" wrote:

I'd have been happy to learn that indeed Doppler shift does lead to dispersion

Hi Tom,

I'd gotten it in my mind that the higher sideband would have increased
more in relation to the carrier than the lower sideband would have (or
versa vice) hence dispersion. Scribbling out the math in response to
your questions resolved that error.

73's
Richard Clark, KB7QHC