512 ANTENNAS AND RF COMPONENTS 



oscillator is likely to be a klystron. The mixer is usually a pair of semi- 

 conductor diodes. If the radar frequency is low enough for a traveling 

 wave tube to be available with a competitively low noise figure, such a tube 

 may be used for an RF amplifier. 



Radar frequency techniques are in a transition region between the 

 methods of ray optics and lumped circuits. Here a host of specialized 

 antennas and components have been developed which have in common 

 an outer metallic covering to prevent loss of energy by radiation. Here, 

 also, special techniques of measurement and calculation are used which 

 have in common their dependence on field theory. In some ways, this is 

 a difficult part of the frequency spectrum in which to work. The compo- 

 nents are expensive, large, and closely toleranced. The measurements are 

 restricted, and analysis by field methods is cumbersome. On the other 

 hand, there are certain functions that can be performed here more easily 

 than at other frequencies, especially in the field of nonreciprocal devices 

 and of the so-called hybrid couplers. 



This chapter will inspect a few of the antennas and components in this 

 frequency band and will show the engineering approach to problems related 

 to these devices. A method of looking at antennas is developed which is 

 particularly useful for microwaves, and several antenna types are described, 

 such as the parabolic reflector and the linear array. These are typical of 

 important classes of radar antennas. Quite a number of waveguide compo- 

 nents are presented in enough detail to give the reader a familiarity with 

 their problems and applications, but not enough detail for design without 

 further reference. For this reason, references are included to which the 

 reader can refer for design. 



10-1 ANTENNAS: INTRODUCTION TO RADAR ANTENNAS 



Radar information concerning a specific target consists of range and 

 direction quantities and their rates of change. The range information is 

 concerned with time measurement and as such has little to do with the 

 antenna design. Direction information, on the other hand, comes directly 

 from the antenna through its ability to concentrate radiated power in one 

 direction relative to another. It is to this end that the radar antenna is 

 planned. 



In general, there are two distinctly different types of direction infor- 

 mation required by the radar of the antenna. First, information may be 

 necessary to a comparatively rough degree of accuracy concerning the 

 many targets that may exist in a sector of space of tactical interest around 

 the antenna. This function is referred to as search in fire-control radar or 

 as ground surveillance in mapping radar. Second, information involving 

 both direction and rates of change of direction may be required to a high 



