524 ANTENNAS AND RF COMPONENTS 



Another antenna problem affecting system performance has been termed 

 Jahe lock-on. In this event, a radar tracking lock is achieved with an 

 exceptionally strong target which is at some angle other than zero with 

 the paraboloid centerline. This is seen in all types of radar systems, but 

 practice has found conical scanning systems less susceptible to the problem 

 than others. With beams having cylindrical symmetry in their patterns, 

 including the sidelobes, a false lock-on would not be possible. The beam 

 asymmetry, however, caused by offset feed which is necessary to produce 

 the required error signal gives sidelobes which are not cylindrically sym- 

 metric. A three-dimensional picture of this phenomenon would show the 

 major lobe as a large symmetrical central peak, but the sidelobes as a rather 

 irregular range of low mountains arranged around the foot of the major 

 lobe. In this case, a sidelobe peak may follow a conical motion similar to 

 the main lobe, and some form of lock-on can be achieved at this crossover 

 with a sufficiently strong target. 



It is difficult to exclude this possibility from antenna measurements alone, 

 and as a result, it is usually best to test the completed antenna for false 

 lock-on with the actual system with which it is to be used. 



A final effect to be mentioned with narrow-beam antennas is harmonic 

 generation by large targets at close ranges. When the target subtends a 

 large portion of or exceeds a beamwidth, the target structure modulates 

 the return as the beam sweeps past a wing or tail surface. This modulation 

 gives a false error signal causing angular steering errors at short ranges. 

 The phenomenon is not related to target glint noise, although they both 

 become effective at short range. The only relief for this problem as far as 

 the antenna is concerned is to widen the antenna beam. 



10-7 THE LINEAR ARRAY AS A FAN BEAM ANTENNA 

 FOR SURVEILLANCE 



An antenna array is a combination of two or more elementary radiators 

 so connected to give pattern — and thus gain — reinforcement in certain 

 directions and pattern cancellation in other directions. One class of 

 antennas used often in radar is a long linear array of elementary radiators 

 so driven as to produce field reinforcement broadside to the array. These 

 antennas find application in ground surveillance and in AEW radar. 



The Slotted Waveguide as a Fan Beam Antenna for Ground 

 Surveillance. System requirements for these search antennas have 

 little in common with the fire-control antennas previously discussed. The 

 pattern is broadbeam in elevation of a modified cosecant squared (csc^ B) 

 form'" to give uniform illumination of a considerable segment of the earth's 



'"Silver, op. at.., p. 471. 



