M. K A TZI N 



CHAPTER 4 



REFLECTION AND TRANSMISSION OF 

 RADIO WAVES 



4-1 INTRODUCTION 



In the propagation of radio waves between a transmitter and receiver, 

 we are interested in the problems associated with power transfer between 

 two terminals. This involves an antenna problemi at each terminal (that is, 

 the transformation of electrical power into electromagnetic waves or vice 

 versa) and the problem of determining how the waves propagate to the 

 receiver. In the case of airborne radar, the receiving antenna is replaced by 

 the target, and interest is centered in reradiation by the target in the reverse 

 direction, back toward the transmitter. This reradiation phenomenon is 

 usually called scattering. The radar case with which we shall be primarily 

 concerned is a special case of scattering in which the angle between the 

 propagation directions of incident and scattered fields is 180°. Scattering 

 may be viewed as an antenna problem, too, for the incident field sets up in 

 the target currents whose distribution depends on the target material and 

 configuration and on the distribution of the incident field. If this current 

 distribution is known, then the field reradiated by it can be determined just 

 as though that current distribution were set up in an antenna. In propaga- 

 tion back from the target to the radar, the scattered wave is involved with 

 the same factors as in propagation from the radar to the target: the radar 

 problem involves (1) two-way propagation, and (2) back-scattering by the 

 target. Thus, in order to predict the strength of the echo received from a 

 target it is necessary to determine the characteristics of the propagation 

 mechanism and also the back-scattering properties of the target. 



The frequencies normally used for radar operation range from about 

 100 Mc/sec on up, or wavelengths of 3 meters down to less than 1 cm (see 

 Fig. 1-21). Consequently most targets are many wavelengths in dimension. 

 An antenna of corresponding size would have an extremely sharp radiation 

 pattern, so that the target, considered as an antenna, has a correspondingly 

 sharp scattering pattern. It follows that in general the field scattered 

 backward is very sensitive to target orientation. Targets which move, 

 therefore, usually give a radar echo which varies with time. Since a 



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