294 BELL S YSTEM TECH NIC A L JOURNA L 



14.4 The %rodified S J/ Mark 27 Radar Antenna 



The SJ antenna described above performed a remarkable and timely fire 

 control job as a lobing antenna but was found to be unsatisfactory when 

 rotated continuously to produce a Plan Position Indicator (PPI) presenta- 

 tion. In the PPI method of presentation range and angle are presented as 

 radius and angle on the oscilloscope screen. Consequently a realistic map 

 of the strategic situation is produced. This map is easily spoiled by false 

 signals due to large minor lobes of the antenna. 



Since it had been established that the PPI picture was valuable for 

 navigation and warning as well as for target selection it was decided to 

 modify the antenna in a way that would reduce these undesirably high minor 

 lobes. These were evidently due principally to the shadowing effect of the 

 massively built double primary feed. Accordingly a new reflector was de- 

 signed which in combination with a slightly modified feed provided a much 

 improved pattern. 



The new reflector was different in configuration principally in that it was 

 a partially offset section of a paraboloid. The reflector surface was also 

 markedly different in character since it was built as a grating rather than a 

 solid surface. This reduced water drag on the antenna. In addition 

 the grating was less visible at a distance, an advantage that is obviously 

 appreciable when the antenna is the only object above the water. 



This modified antenna was used not only on submarines as part of the 



SJ-1 radar but also on surface vessels as the Mark 27 Radar Antenna. 



Figure 46 shows one of these antennas. Its electrical characteristics are 



as follows: 



Gain > 20 db 



Horizontal Half Power Beamwidth = 8° 



Vertical Half Power Beamwidth = 17° 



Vertical Beam Character — Some upward radiation 



Lobe Switching Beam Separation — approximately 5° 



Gain reduction at beam cross-over < 1 db 



Polarization — -Horizontal 



14.5 The SH and Mark 16 Autenna^^ 



The antennas designed for the SH and Mark 16 Radar Equipments are 

 practically identical. The SH system was a shipborne combined fire con- 

 trol and search system, and the Mark 16 its land based counterpart was used 

 by the Marine Corps for directing shore batteries. 



These systems operated at 9.8 cm. The requirement that the system, 

 operate as a fire control as well as a search system imposed some rather 

 stringent mechanical requirements on the antenna. For search purposes, 

 the antenna was rotated at 180 rpm, and indications were presented on a 

 plan position indicator. For fire control data, slow, accurately controlled 

 motion was recjuired. Bearing accuracy is attained by lobe switching in 



'^Written by R. J. Philipps. 



