532 ANTENNAS AND RF COMPONENTS 



radome may result in a loss of accuracy. The range loss is simply calculated 

 from the percent loss in power that occurs when a radome is inserted 

 between an antenna and a detector. ^^ Losses are typically between 10 and 

 30 per cent and are more or less independent of the angle of look through 

 the radome. Let us glance first at the system problem. 



The System Approach to Radome Design. The aforementioned 

 radome losses degrade the range capability of the radar by 2 to 8 per cent. 

 The loss in accuracy, on the other hand, arises primarily from refractions 

 of the radar beam as it transits the radome at other than normal incidence. 

 This causes the apparent line-of-sight to change, thereby introducing a 

 static measurement error. Further, the amount and direction of refraction 

 is not constant over the entire radome; thus when the radar is tracking a 

 target, spurious angular rate signals will be produced by relative movement 

 between the radome and the 'line of sight. 



Other contributions to radar inaccuracy come from radome reflections 

 affecting the transmitter frequency and from general deterioration of the 

 antenna sidelobes. Paragraph 8-5 considered the effect of the transmitter 

 frequency pulling problem on the design of the AFC. The deterioration of 

 the sidelobes can similarly have a serious effect upon tracking and the 

 altitude line. The system designer must remember this fact when he 

 evaluates antenna patterns measured without the radome. 



While it is possible in principle to compensate for known radome errors 

 by suitable system design, this line has not been followed. Rather, the 

 effort has been to reduce the errors below some tolerable figure. Depending 

 upon the use to which the system is put, the tolerable errors may vary for 

 diflferent portions of the radome. For example, let us consider one facet of 

 the radome design problem for the AI radar discussed in Chapter 2. During 

 the tracking phase, the AI radar measures range, azimuth, and elevation 

 lead angles, and the azimuth and elevation space angular rates of the radar 

 line-of-sight to the target. The magnitude of the lead angle during the final 

 phases of tracking may vary from near zero for head-on or tail-on attacks 

 to 40-50° for attacks upon the target's beam. During steering, the pilot 

 may roll the aircraft to bank angles exceeding ±60° at rolling rates up to 

 60° /sec. This rolling action causes the position of the radome relative to 

 the antenna beam to change with a speed that is a function of rolling rate 

 and lead angle. If the boresight error caused by the radome is variable, 

 this variation, coupled with the rolling action, will cause spurious angular 

 rate signals which degrade system accuracy and can also contribute to 

 instability of the steering loop. 



From this discussion, it can be seen that boresight error changes are 

 likely to be more significant for large lead angles. Thus, to provide the 



iSRadar Design Criteria for Precision Guidance Radar, p. 77, Contract AF33(038)-12283. 



