8-6] STATIC AND DYNAMIC ACCURACY REQUIREMENTS 405 



Although the Rieke diagram specifies the frequency and power output for 

 any load, the pulling figure of the magnetron is defined by the total fre- 

 quency variation resulting from a load which produces a VSWR of 1.5 when 

 it is changed through a phase of 360°. The load corresponding to this 

 condition is shown as a circle in Fig. 8-7. The total frequency variation 

 caused by pulling might therefore be 13 Mc. A typical conical scanning 

 antenna produces relatively small phase variation. The measured phase 

 variation of a typical load is plotted as sectors A, B, and C in Fig. 8-7. The 

 most unfavorable position for the phase is at Sector A, for which the 

 frequency may be pulled a total of approximately 4 Mc. Pulling can also 

 result from a wide-angle scanning antenna looking through discontinuities 

 in the radome as well as the discontinuities in phase of a rotating feedhorn 

 in a conical scanning radar. Sector C is most favorable for elimination of 

 transient frequency pulling caused by phase changes; however, power 

 output variations are relatively large. The transmitted signal will be 

 amplitude-modulated by this effect, and the resultant amplitude mod- 

 ulation on the received signals introduces errors in antenna pointing. 

 Accordingly, Sector B represents the most favorable alternative from the 

 standpoint of low-frequency pulling and minimum amplitude modulation 

 of the transmitter. It will be observed, however, that these advantages 

 are purchased at the price of lower-than-rated power output. 



In a well-designed antenna-radome combination, rapid phase changes 

 with the position of the antenna are not usually severe. In a conical 

 scanning radar the greatest pulling effect results from the rotation of the 

 feedhorn. The phase may change quite rapidly with feedhorn position, 

 and the frequency of pulling is therefore high. A typical system has been 

 observed to generate two phase rotations in one revolution of the feedhorn 

 with some abrupt changes. The frequency variation is thus predominantly 

 at frequencies greater than twice the lobing frequency of the antenna. 



Table 8-2 gives some typical pulling characteristics of a conical scanning 

 system. 



Table 8-2 PULLING CHARACTERISTICS OF A 

 TYPICAL CONICAL SCANNING SYSTEM 



Frequency of FM Peak Deviation {Mc) 



f\ (lobina frequency) 0.5 



2/i ^ 1.7 



3/i 0.25 



4/i 0.25 



8-6 STATIC AND DYNAMIC ACCURACY REQUIREMENTS 



Tuning errors in the radar receiver degrade the output signal-to-noise 

 ratio of the radar. Maximum range performance of the radar is thus a 



