10-2] SOME FUNDAMENTAL CONCEPTS USEFUL IN DEVELOPMENT 513 



degree of accuracy on one or more specific targets. This function is referred 

 to as tracking. Often, to save weight in a fire-control radar, these two 

 functions are required from the same antenna. If this is the case the search 

 and track functions are usually not required simultaneously. The AI radar 

 discussed in Chapter 2 is an example of such a system. In other cases, such 

 as the AEW system of- Chapter 2, the search and tracking functions are 

 carried on simultaneously in what is called a track-while-scan 7node of 

 operation. 



In view of these things, the most salient feature of the radar antenna 

 with which we are concerned is directivity. It is related to the radar 

 angular resolution and range. Explicitly, directivity is 47r times the ratio 

 of the maximum radiation power angular density to the total power 

 radiated by the antenna.^ That is to say, directivity is a measure of the 

 antenna's ability to concentrate power in a particular direction. This 

 antenna characteristic and others such as beamwidth and gain are related 

 through the antenna effective aperture.^ 



10-2 SOME FUNDAMENTAL CONCEPTS USEFUL IN THE 

 DEVELOPMENT OF RADAR ANTENNA 

 REQUIREMENTS 



The effective antenna area or aperture Ag may be defined for a lossless, 

 matched antenna as 



Ae = 4^^^. (10-1) 



where Pioad is the power delivered to its load and ^density is the power 

 density in an incident plane wave. It should be said at this point that the 

 antennas of which we speak are assumed to be linear and reciprocal. 

 Antenna characteristics such as aperture, directivity, etc. are unchanged 

 whether the antenna is used in transmission or reception. Sometimes it is 

 more convenient to give a physical picture of the phenomena in terms of a 

 receiving antenna. At other times the physical picture as a transmitter is 

 more easily visualized as in the previous definition of directivity. This 

 selection is only a matter of convenience and in no way affects antenna 

 performance, whether the antenna is being used in reception or in trans- 

 mission. 



In either case the aperture concept is especially useful for many radar 

 antennas because (as in the definition) losses are relatively low, the antenna 

 is usually well matched to the load, and the effective aperture is within 



"^Standards on Antennas: Definitions of Terms, The Institute of Radio Engineers, 1948. 

 2John D. Kraus, Antennas, Sees. 3-2 and 3-3, McGraw-Hill Book Co.., Inc., New York, 1950. 



