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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1

CHAPTER 5

Basic outline of CW radar

 If you are watching a motor-cycle race you may notice that the note of the exhaust noise appears to change as the machine passes. This is illustrated in Fig 5.

This phenomenon is known as the Doppler effect and it occurs with radio waves as well as with sound waves. As a target approaches a radar aerial the frequency of the signal reflected by the target is higher than that of the transmitted signal. Conversely if a target is moving directly away from the aerial the frequency of the reflected signal is lower than that of the transmitted signal. For stationary targets there is no change in the frequency of the reflected signal (Fig 6).

If the transmitted frequency is Ft and the new frequency to which it is changed by the Doppler effect is Fr, the difference between these two frequencies is known as the Doppler shift Fd = Ft~Fr

The magnitude of the Doppler shift is related to the velocity of a target in a straight line between the target and the aerial. A high value for the Doppler shift indicates a high target velocity.

If the target is approaching the aerial the received frequency is higher than the original transmitted frequency by the Doppler shift, i.e. Fr = Ft+Fd. If the target is moving away the received frequency is lower, i.e. Fr = Ft-Fd.

The relationship between a target's velocity and the Doppler shift, provided the target is approaching or receding in a straight line from the radar aerial, is given by the expression:

Fd = (2v/c)Ft,


 

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