Electronics Forum / Basics / December 2005

Doppler shift is dependent on the relative velocity between the target
and the receiver, and the frequency shift is proportional to the
relative velocity.

Doppler shift is given as (for a generating source moving relative to
an independent receiver):

f' = f( 1 / [ 1+ (v / Co)] ) where f' is the received frequency, f the
transmitted frequency, v the relative velocity between source and
receiver and Co the medium velocity. For free space and electromagnetic
radiation, that's 3 X 10^8 m/s to a close approximation.

For a target of a radar gun, the solution is f' = 2{f( 1 / [ 1+ (v /
Co)] )} because the doppler effect occurs both on the outbound and
return trips.

These equations are for targets moving much less than the speed of
light (beyond that relativistic effects become significant).

Note that the relative velocity is |v| x cos theta where |v| is the
absolute velocity of the target + receiver and theta the angle between
the target motion and the direct line between target and receiver. For
a stationary radar gun, it is the velocity of the target times the
cosine of the angle involved.

A rate change of frequency will occur is the target accelerates
relative to the receiver. (Acceleration may be positive or negative)

Cheers

PeteS

the frequecy shift of the reflected signal will be greater for greater speed
of aproach.

the magnitude of the reflected signal depends on the surface of the object
and it's distance etc.

Bye.
  Jasen