RADAR ANTENNAS 



299 



switcher, which is a motor driven capacitor that has a single rotor plate and 

 four stator plates, one for each group of dipoles. The phase shift intro- 

 duced into the four feed lines by the lobe switching mechanism causes the 

 antenna beam to be "lobed" or successively shifted to the right, up, left 

 and down as the rotor of the capacitor turns through 360 degrees. 



Mounted centrally on the front of the antenna at the junction of the two 

 parabolic antennas is a smaller auxiliary antenna consisting of two dipole 

 elements and a parabolic reflector, the purpose of which is to reduce the 

 minor lobes that are present in the main antenna beam. The auxiliary 



Fig. 49 — Mark 3 Radar Antenna on Battleship New Jersey. 



antenna beam is not lobe switched and is sufficiently broad in both the 

 horizontal and vertical planes to overlap both the main antenna beam and 

 the first minor lobes. The auxiliary antenna feed is so designed that its 

 field is in phase with the field of the main beam of the main antenna. This 

 causes the feed of the auxiliary antenna to "add" to the field of the main 

 antenna in the region of its main beam, but to subtract from the field in the 

 region of its first minor lobes. This occurs because the phase of the first 

 minor lobes differs by 180 degrees from that of the main beam. As a result, 

 the field of the main beam is increased and the first minor lobes are greatly 



