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.