10-6] SEVERAL ANOMALOUS EFFECTS IN ANTENNAS 523 



These data are typical of measurements on conical scan systems out to 

 one beamwidth; in this region there is little difference between one antenna 

 and another. Beyond this point, however, the characteristics depend upon 

 the detail of the sidelobe structure. Fig. 10-4 is carried to this extreme to 

 show that angular pull-in (as would be expected intuitively) is maintained 

 to approximately the point where the two extreme lobe positions cross for 

 the second time. Compare Figs. 10-4(a) and 10-4(b). From this kind of 

 information, the optimum antenna crossover may be selected for a given 

 application. Note that a l|-db crossover gives optimum linearity. As the 

 crossover is deepened, angular sensitivity increases and the angular pull-in 

 increases slightly. Adversely, however, per cent harmonic distortion 

 increases. Typical second-harmonic distortion produced at various cross- 

 over values are shown in Fig. 10-4(c). Higher harmonics also are generated, 

 but to a smaller degree. The presence of these harmonic distortions 

 requires special consideration in the design of the receiver and angle 

 tracking system. Another factor to consider is tracking range, which may 

 decrease somewhat at deeper crossover because of the lower power incident 

 on the target. A 1^-db crossover is often considered optimum. 



Procedures very similar and analogous to this may be repeated for other 

 problems such as monopulse radar, beam-riding missiles, etc. 



10-6 SEVERAL ANOMALOUS EFFECTS IN ANTENNAS FOR 

 TRACKING SYSTEMS 



Radar return from targets or ground return may dictate the wave 

 polarization to be used for a particular design. At other times, the choice 

 between vertical, horizontal, and circular polarization is less important; 

 or possibly, it is a weighted consideration between conflicting requirements. 

 The effects of polarization upon signal return are discussed in another 

 chapter of this book. One other factor in the polarization choice needs 

 mentioning, however. 



In addition to the ground return normally returned from the sidelobes, 

 there is inevitable scattering from the antenna and aircraft in a vertical 

 direction, giving a strong return from the ground directly beneath the 

 aircraft. This has been termed altitude line because of its appearance on a 

 B scope as a line at a range equal to the airplane's altitude. Sometimes 

 this return is so strong that tracking of a target at a range equal to the 

 altitude line becomes difficult. As the ground return directly beneath the 

 antenna is polarization insensitive, and as a parabolic antenna often has 

 one direction of this right angle scattering which is substantially less than 

 the other, the antenna may be polarized to favor the direction of low ground 

 return. This factor should be taken into account when the radar polari- 

 zation is chosen. 



