516 



ANTENNAS AND RF COMPONENTS 



c Typical Ray 



Central 

 Axis " 



Antenna Feed 



Point and Focus 



of Parabola 



Antenna Aperture 

 Plane 



Fig. 10-2 Properties of a Parabolic Re- 

 flector, Showing a Typical Ray with All 

 Path Lengths abc Equal and the Path be 

 Parallel to the Central Axis. 



The Aperture. With a paraboloidal reflector, it is convenient to 

 project energy from the end of a waveguide, which is small compared with 

 the final aperture size required. The feed, being small, is effectively a point 



source of radiation which is placed 

 near the focus of the parabola (dish) 

 and with its peak radiation aimed 

 toward the center of the dish. The 

 final transmitting aperture can be 

 considered as the plane containing 

 the outer diameter of the reflector. 

 The use of such a reflector is based 

 on two independent and distinct 

 properties of the parabola. First, all 

 rays striking the dish surface from 

 the focus reflect from the dish 

 parallel to its central axis. Second, 

 all ray paths from the focus to the 

 dish to the aperture plane are of the 

 same length. These are shown in 

 Fig. 10-2, where lengths abc for all paths are equal and be is parallel to the 

 central axis. Thus, a wave front on all ray paths from a point source, at the 

 final crossing of the aperture, are automatically of equal phase. 



Properties of a Parabolic Reflector. The relative phase and power 

 density of the wave at various points in the aperture plane is called the 

 aperture illumination. The constant-phase characteristic provided by the 

 parabola is quite universally desirable. With constant phase, it can be 

 shown that maximum directivity, is achieved by constant amplitude 

 illumination. 4 The amplitude characteristic of the aperture, however, is 

 usually controlled in a prescribed manner by the beamwidth of the feed. 

 If the illumination were so adjusted that constant illumination were 

 achieved, the directivity would be maximized and the antenna physical 

 aperture would be equal to the effective receiving aperture. Unfortunately, 

 as has been mentioned, this gives rather high sidelobes; further, consider- 

 able power is lost over the edge of the dish and is not returned to the 

 aperture. Thus the actual gain is maximized and sidelobes are reduced by 

 lowering the power density from the feed about 10 db at the edge of the dish 

 relative to that at the center.^ 



The Tracking Mechanism. Now consider the tracking function 

 required of the antenna. In conical scan radar, the feed is not placed 



^Samuel Silver, ed., Microwave Antenna Theory and Design, Radiation Laboratory Series 

 (MIT), Volume 12, p. 178, McGraw-Hill Book Co., Inc., New York, 1949. 

 ^Uid., pp. 423-433. 



