RADAR ANTENNAS 



249 



from an initial source or 'primary feed'. This distribution can occur in 

 both dimensions at once, as from a point source. Alternatively the energy 

 can be radiated from a primary source but be constrained to lie between 

 parallel conducting plates so that it is at first distributed only over a long 

 narrow aperture or 'line source'. Distribution over the other dimension 

 occurs only after radiation from the line source. 



In order to control the amplitude across the aperture of an array antenna 

 we must design the branching junctions so that the desired power division 

 occurs in each one. When the energy is distributed by radiation from a 

 primary source we must control the amplitude by selecting the proper pri- 

 mary feed directivity. 



We can control the phase of an array antenna by choosing properly the 

 lengths of the branching lines. Alternatively we can insert appropriate 

 phase changers in the lines. 



When the energy is distributed by primary feeds, methods resembling 

 those of optics can be used to control phase. The radiation from a point 

 source is spherical in character. It can be 'focussed' into a plane wave by 

 means of a paraboloidal reflector or by a spherical lens. The radiation from 

 a point source between parallel plates or from a uniphase line source is 

 cylindrical in character. It can be focussed by a parabolic cylinder or a 

 cylindrical lens. 



In Table A we have indicated a possible classification of methods of radar 

 antenna construction. This classification is based on the differences dis- 

 cussed in the foregoing paragraphs. 



Table A 

 Classification of Methods of Radar Antenna Construction 

 - Dipoles 



r Arrays of 



Methods 

 of Radar 

 Antenna 

 Construe 

 tion 



Optical 

 Methods 



Polyrods 

 Optical Elements 



r Point 

 sources 



Spherical < and 

 Optics 



Dipole Arrays 



Wave Guide 

 Apertures 



I Spherical r^"^"^^"''^ 

 -- Elements-! Lenses 



- Arrays 



r Line Reflectors] 



Cylindrical ^0"^^^^ [ j^^^^^^ 



Optics < and 



Cylindrical J deflectors 



- Elements "1 t 



Lenses 



