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.