Chapter 7 

 PROPAGATION ASPECTS OF EQUIPMENT OPERATION 



7.1. 



GENERAL PROBLEM 

 Introduction 



FOR A STANDARD atmosphere and with the basic 

 assumptions set forth in Section 5.1.4, the 

 relation between the factors affecting the power of a 

 set and the gain factor A is given in equations (3) 

 and (5) in Chapter 5. The problem of computing A 

 depends on the set in the sense that some sets are 

 designed to operate in free space, others with the aid 

 of reflection from the sea, as in low -angle and surface 

 coverage. 



The characteristics of a set as given in the manu- 

 facturer's description or in Tables 3, 4, and 5 at the 

 end of this chapter may not represent the tme values 

 for a set in field use. Expected set performance, 

 such as maximum range and coverage, can be calcu- 

 lated on the basis of the set's rated characteristics. 

 Such perfomaance can be termed "normal." If a 

 set is behaving abnormally, it may be that it is not 

 functioning most efficiently. Unfortunately, the 

 problem is complicated by the possible presence of 

 atmospheric ducts and by the variability and diffi- 

 culty of finding accuratelj^ the radar cross sections 

 of aircraft and ships. 



Ducts are especially important for low antennas 

 in surface search. In the case of communication 

 sets, the most important item of information from a 

 propagation standpoint is the maximum range. In 

 the case of radar, not only is knowledge of maximum 

 range wanted but also the ability to estimate the 

 size and type of the target. 



7.1.2 



The Performance Figure 

 and Efficiency 



The maxinium range of a set depends on the peak 

 power output Pp of the transmitter, the minimum 

 ■detectable power Pmm (see Sections 2.3.1 to 2.3.5) 

 of the receiver, and the antenna gains Gi and G^. 

 These can be grouped to give a performance figure. 

 For communication, this figure is {Pp/Pmin)GiG2- 

 For radar, the gains G'l and G2 are generally equal. 

 'The performance figure is then (Pp/Pmin)^''. The 

 160 



ratio of the actual performance figure to the max- 

 imum pos.?ible value, or the difference in decibels, 

 gives the efficiency of the set. In field use, it is 

 generally impossible to measure the working per- 

 formance figure ■with any precision and methods for 

 obtaining a rough measure must be employed. 



'■'■' Effect of Reflection 



It has been pointed out in Chapters 5 and 6 that 

 reflection maj' increase the maximum range of a 

 radar up to twice the free-space value. This aid 



10 20 30 40 



DISTANCE -KILOMETERS 

 D Experifnentoi Po'nt^ 



Figure 1. Effect of beam tilt on coverage for a radar. 



the early detection of aircraft at low angles. How- 

 ever, the minima which occur in the resulting inter- 

 ference pattern prevent the continuous tracking of 

 an airship coming in. 



