Consider the simple CW Radar.The transmitter generates a continuous(modulated)oscillation of frequency by the antenna A portion of the radiated energy is intercepted by the target and is scattered, some of it in the direction of the radar.where it is collected by the receiving antenna. If the target is in motion with a velocity V r relative to the radar, the received signal will be shifted in frequency from the transmitted frequency f 0 by an amount± f d as given by E q. the plus sign associated with the doppler frequency applies if the distance between target and radar is decreasing (closing target ), that is when the received signal frequency is greater than the transmitted signal frequency. The minus sign applies if the distance is increasing (receding target). The received echo signal at a frequency f 0 ± f d inters the radar via the antenna and is heterodyned in the detector (mixer) with a portion of the transmitter signal f 0 to produce a doppler beat not of frequency f d. The sign of f d is lost in this process. The purpose of the doppler amplifier is to eliminate echoes from stationary targets and amplify the doppler echo signal to a level where it can operate an indicating device. It might have a frequency-response characteristic. The low- frequency cutoff must be high to reject the d-c component caused by stationary targets, but yet it must be low enough to pass the smallest doppler frequency expected. Sometimes both conditions cannot be met simultaneously and s compromise is necessary. The upper cutoff frequency is selected to pass the highest doppler frequency expected.The indicator might be a pair of earphones of a frequency meter.If exact knowledge of the doppler frequency is not necessary, earphones are especially attractive provided the doppler frequencies lie within the audio-frequency response of the ear.
Isolation between transmitter and receiver :A signal antenna serves the purpose of transmission and reception in the simple CW radar described above. In principle, a signal antenna may be employed since the necessary isolation between the transmitted and the received signals is achieved via separation in frequency as a result of the doppler effect. In practice, it is not possible to eliminate completely the transmitter leakage. There are to practical effects which limit the amount of transmitter leakage power which can be tolerated at the receiver. These are the maximum amount of power the receiver input circuitry can withstand before it is physically damaged are its sensitivity reduced and the amount of transmitter noise due to hum, micro phonics, stray pick -up, and instability which enters the receiver from the transmitter. The amount of isolation required depends on the transmitter power and the accompanying transmitter noise as will as ruggedness and the sensitivity of the receiver. For example if the safe value of power which might be applied to s receiver were 10 m W and if the transmitter power were 1 K W, the isolation between and receiver must be at least 50 dB The amount of isolation needed in a long - range CW radar is more often determined by the noise that accompanies the transmitter leakage signal rather than by any damage caused by high power.For example, suppose the isolation between the transmitter and receiver were such that 10 m W of leakage signal appeared at the minimum detectable signal were 10-13 watt (100 dB below 1 m W), transmitter noise must be at least 110 dB (preferably 120 or 130 dB ) below the transmitted carrier The transmitter noise of concern in radar includes those noise components that lie within the same frequency range as the doppler frequencies.The greater the desired radar range, the more stringent will be the need for reducing the noise modulation accompanying the transmitter signal. If complete elimination of the direct leakage signal at the receiver could to achieved, it might not entirely solve the isolation problem since echoes from nearby fixed targets can also contain the noise components of the transmitted signal.
It will be recalled that the receiver of a pulsed radar is isolated and protected from the damaging effects of the transmitted pulse by the duplexer, which short-circuits the receiver input during the transmission period. Turning off the receiver during transmission with a duplexer is not possible in a C W radar since the transmitter is operated continuously. Isolation between transmitter and receiver might be obtained with a signal antenna by using a hybrid junction, circulator, turnstile junction, or with separate polarization.
Ferrite isolation devices such as the circulator do not suffer the 6-dB loss inherent in the hybrid junction.practical devices have isolation of the order of 20 to 50 dB. Turnstile junction achieve isolation of high as 40 to 60 dB
The largest factor are obtained with two antennas -one for transmission, the other for reception -physically separated from one another. Isolation of the order of 80 d B or more are possible with high -gain antenna. The more directive the antenna beam and the greater the spacing between antenna.
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