THE RADAR RECEIVER 707 



2.11 Input Signal Characteristics 



The low amplitude of the signal at the input terminals of the radar 

 receiver requires that this signal be efficiently utilized. The power of the 

 received signal at this point under somewhat idealized free space assump- 

 tions is given by the following: 



Pr = 



16Tr'~D^ 



Where G = Power gain of common transmitting and receiving antenna 

 P = Transmitted power of radar system 



Ae = Equivalent flat plate area of target (This represents an equiva- 

 lent flat plate normal to the incident beam which reradiates 

 all impinging energy) 

 D = Range to the target. 

 The two following sample computations are illustrative of the military 

 radar system conditions: 



1. Naval Vessel Search Radar System 

 Frequency = 3000 mc 



Target range = 25 nautical miles 



Target-Destroyer (Effective flat plate area = 0.03 sq meter at 3000 mc) 



(This value has been determined from a study of target response with 



military radar systems) 

 Power gain of Antenna = 30 db 

 Transmitter Peak Power = 100 kw 

 Received Peak Power = 13 X 10"" watts 



2. Airborne Search Radar System 

 Frequency = 10,000 mc 

 Target Range = 70 nautical miles 



Target-Destroyer (Effective flat plate area = 0.2 sq meters 



at 10,000 mc) 

 Power Gain of Antenna = 30 db 

 Transmitter Peak Power = 100 kw 

 Received Peak Power = 1 1 X 10^" watts 

 A reduction of the available received signal power, as computed above, 

 is to be expected in practice due to multiple path effects and absorption and 

 refraction effects over the propagation path. 



2.12 Input Circuit Noise Considerations 



While it is possible to conceive of providing sufficient gain within a radar 

 receiver to meet any desired sensitivity requirement, this sensitivity caimot 

 usefully be employed beyond certain limits as determined by the amplitude 



