On 8 Mar 2006 12:04:22 -0800, "K7ITM" wrote:

kb7qhc wrote, "Doppler is going to give rise to dispersion..."

Doppler alone? Care to elaborate on that?

What do you mean by doppler alone?

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.

You can probably get lots of comments from users of Doppler DF antenna
systems about the performance to be expected. (There's a "fox hunting"
email list that could get you into a group with lots of
direction-finding experience quickly.) Of course, since indicated
direction depends on the detected phase of the received Doppler-derived
FM modulation, relative to the antenna "rotation," it is important to
use a detection path that maintains a very stable phase shift. It's
generally useful in practice to have a very narrow filter on the
Doppler-derived FM, so that other modulation on the signal does not
interfere with your detection. You can do that with switched-capacitor
"analog" filters, or with digital filters, locked to the
antenna-switching subsystem. Expect multipath to mess up your
indication just as it can with any other direction-finding technique.
Many practical factors affect how well your antenna system will work.
Beware of the mutual impedance among all the antenna elements: you
will probably want to insure that elements which are de-selected are
non-resonant. If you don't need very fast indication of bearing, you
may do better with a rotating directional antenna, ESPECIALLY in terms
of minimum detectable signal. If you want to know just how well this
Doppler DF scheme is going to work with a particular signal (with
perhaps broadband complex modulation), I'd suggest that it's a pretty
easy experiment to set up, either with the antenna and receiver you
plan to use, or even by generating a version of the source signal with
FM added. I have this vision of you trying to detect something that
looks like 5MHz wide random noise modulated by a 50kHz-deviation FM,
using a receiver with a 50kHz or 100kHz bandwidth, and perhaps being
very disappointed in performance.

Finally, why didn't you indicate in your original posting that it was a
particular direction-finding application that you were interested in?
That probably would have gotten to the point a bit quicker.

Cheers,
Tom

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