i?^ DARAN TENNA S 307 



14.12 .1 3 CM Anti-Aircraft Radar Antenna. 



To obtain greater discrimination between a given target and other targets, 

 or between a target and its surroundings, the wavelength was reduced to 

 the 3 cm region. An antenna for this wavelength was designed to employ 

 the conical scan principle. In this case the parabolic reflector was 30 inches 

 in diameter and transmitted a beam approximately 3° wide at the 3db point 

 with a beam shift of 1.5° making a total of 4.5° with the antenna scanning. 

 The minor lobes are down more than 22 db (one way) from the maximum; 

 and the gain of this antenna is ?)5 db. 



In the 3 cm system in which a Cutler feed was used, the axis of the beam 

 was rotated in an orbit by "nutation" about the mechanical axis of the 

 antenna. This was accomplished by passing circular waveguide through 

 the hollow shaft of the driving motor. The rear end of the feed (choke 

 coupling end) was fixed in a ball pivot while the center (near the reflector) 

 was off set the proper amount to develop the required beam shift. This 

 off set was produced by a rotating eccentric driven by the motor. The 

 latter was a 440 volt, 60 cycle, 3 phase motor rotating at approximately 1800 

 rpm which resulted in a scanning rate of 30 cycles per second. The two- 

 phase reference voltage generator was integral with the driving motor. 



It was found necessary at these radio frequencies to use a cast aluminum 

 reflector and to machine the reflecting surface to close tolerances in order to 

 attain the consistency in beam width and beam direction required for 

 accurate pointing. An antenna assembly for the 3 cm anti-aircraft radar 

 is shown in Fig. 57. 



15. Land Based Radar Antennas 

 15.1 The SCR-545 Radar ''Search'' and "Track" Antennas''' 



The SCR-545 Radar Set was developed at the Army's request to meet 

 the urgent need for a radar set to detect aircraft and provide accurate tar- 

 get tracking data for the direction of anti-aircraft guns. 



This use required that a narrow beam tracking antenna be employed to 

 achieve the necessary tracking accuracy, furthermore, a narrow^ beam 

 antenna suitable for accurate tracking has a very limited field of view and 

 requires additional facilities for target acquisition. This was provided by 

 the search antenna which has a relatively large field of view and is provided 

 with facilities for centering the target in its field of view. These two an- 

 tennas are integrated into a single mechanical structure and both radar axes 

 coincide. 



The "Search" antenna operates in the 200 mc band and is com- 



" Section 15.1 was written by A. L. Robinson. 



