click image to enlarge
I refer to:
http://herrero-radio-astronomy.blogspot.com/2011/05/localization-of-type-iii-event-on.html
Please click on the illustration to see an enlargement.
Figure 1 (from Thejappa and McDowall 2010) shows the variation of the fundamental and second harmonic radiation frequencies with heliocentric radial distance.
For a given radial velocity of the source, the drift rate will be proportional to the slope of the curve.
Assuming the heliocentric radial velocity is approximately constant, we see that the drift rate will generally decrease with time, producing the usual shape seen in wide bandwidth spectra.
The present illustration shows how figure 1 was computed.