RADAR ANTENNAS 271 



The skies were searched for possible attackers with antennas which rotated 

 continuously in azimuth. An equally important but later use appeared 

 with the advent of great bombing attacks. Bombing radars 'painted' maps 

 of the ground which permitted navigation and bombing during night and 

 under even the worst weather conditions. In these radars also the antennas 

 were rotated in azimuth, either continuously through 360° or back and forth 

 through sectors. 



The majority of radars designed to perform these functions provided verti- 

 cal coverage by means of a special vertical pattern rather than a vertical 

 scan. It can easily be seen that such a pattern would have to be 'special.' 

 If we assume, for example, that a bombing plane is flying at an altitude of 



10.000 feet, then the radar range must be 10,000 esc 60° = 11,500 feet if a 

 target on the ground at a bomb release angle of 60° from the horizontal is to 

 be seen. Such a range would by no means be enough to pick up the target 

 at say 10° in time to prepare for bombing, for then a range of 10,000 esc 

 10° = 57,600 feet would be required. This range is far more than is neces- 

 sary for the 60° angle. It appears then that in the most efficient design the 

 radar range and therefore the radar antenna gain, must be different in dif- 

 ferent directions. 



The required variation of gain with vertical direction could be specified 

 in any one of several ways. It seems natural to specify that a given ground 

 target should produce a constant signal as the plane flies towards it at a con- 

 stant altitude. Neglecting the directivity of the target this will occur if the 

 amplitude response of the antenna is given by £ = E^cscd. This same con- 

 dition will apply by reciprocity to an early warning radar antenna on the 

 ground which is required to obtain the same response at all ranges from a 

 plane which is flying in at a constant altitude. 



This condition is not alone sufficient to specify completely the vertical pat- 

 tern of an antenna. For one thing it can obviously not be followed when 

 ^ = 0, for this would require infinite gain in this direction. Therefore a 

 lower limit to the value of 6 for which the condition is valid must be set. In 

 addition an upper limit less than 90° is specified whenever requirements per- 

 mit, since control at high angles is especially difficult. When the limits have 

 been set it still remains to specify the magnitude of the constant £o- This 

 can be done by specifying the range in one particular direction. This speci- 

 fication must of course be consistent with all the factors that determine gain, 

 including the reduction due to the required vertical spread of the pattern. 



10.1 Cosecant Antennas based on the Paraboloid 



It is evident that the standard paraboloidal antennas so far discussed will 

 not produce cosecant patterns. These patterns being unsymmetrical will 

 result only if the wave front phase and amplitude are especially controlled. 



