It is possible to detect moving targets by radiating unmodulated continuous wave (CW) energy instead of radiating in the form of pulses. Continuous wave (CW) radar makes use of the Doppler effect for target speed measurements.

In CW system the carrier frequency f is transmitted and the received signal is at frequency $f ± f_d$. The + or - ve sign is depending upon movement toward the radar or away from it. The frequency modulated CW radar is used for obtaining range as well as the velocity of the target.

A small portion of transmitted frequency and output of local oscillator are mixed in transmitter mixer and the sum $(f_t + f_i)$ is fed to receiver mixer. The receiver mixer also receives the Doppler shifted signal from the receiver antenna, this produces an output difference frequency i.e. 30 MHz ± Doppler frequency.

The output of this receiver is amplified and demodulated again and the signal from the second detector is simply Doppler frequency without a ±

sign preceding it so it is not possible to conclude that whether the target is approaching or moving away. For a common transmitting and receiving antenna, the circulator which provides the isolation up to 30-40 dB is used to provide necessary isolation between transmitter and receiver.

Advantages of CW Doppler

1. CW Doppler radar has no blind speed.
2. CW Doppler radar is capable of giving accurate measurements of relative velocities.
3. CW Doppler radars are always on, they need low power and arc compact in size.
4. They can be used for small to large range with high degree of efficiency and accuracy.
5. The performance of radar is not affected by stationary object.

Disadvantages of CW Doppler Radar

1. The maximum range of CW Doppler radar is limited by the power that radar can radiate.
2. The target range cannot be calculated by CW Doppler radar.
3. There is possibility of ambiguous results when number of targets are more.

Applications of CW Doppler Radar

1. CW Doppler radars are used where only velocity information is of interest and actual range is not needed e.g. in police radar for catching cars travelling above the speed limit.
2. Measuring motion of waves on water level.
3. Traffic counters.
4. Intrusion alarm.
5. Runway monitors.
6. Cricket ball speed measurement.