FIRE-CONTROL RADARS FOR NAVAL VESSELS 17 



CXAS having regularly out-performed much higher powered equipment 

 operating at 100 or 200 megacycles for this service. The reason for this 

 can best be uuderstood by reference to Fig. 15 whicli illustrates what happens 

 when a radio beam is directed horizontally over water. The beam breaks 

 up into an interference pattern of several rays due to reflection from the 

 surface; the position of the lowest ray depending only upon the height of 

 the antenna measured in wavelengths above the water. Since the mount- 



RADIATION PATTERN IN PRCSENCEI OF PERFECT REFLECTING SURFACE 



ANTENNA 



J 



e=2SIN (27?— SIN e) X FfREE SPACE ANTENNA PATTERN | 



WHERE H = ANTENNA HEIGHT 



A= WAVE LENGTH IN SAME UNITS AS H 

 = ELEVATION ANGLE IN DEGREES 

 C- RELATIVE FIELD STRENGTH 

 Fig. 15— Effect of surface reflection on elevation beam 



Table I 



Operating Frequency Tunable 680-720 mcs. 



Antenna Dipole array of 8 half-wave radiators, reflector 6' x 6' 



beam width 12 degrees, gain 22 db. 



Transmitter Pulse Power Approximately 2 kw. 



Pulse Repetition Rate 1640 PPS 



Pulse Duration Variai)le in 5 steps from 1 to 5 microseconds. 



Receiver-Superheterodyne 1 mc bandwidth, 30 mc IF frequency. 



Receiver Noise Figure Approximatelv 24 db. 



Range Cahbration '. . Electronic marks at 10,000 and 2,000 yard intervals. 



ing height available aboard ship is fixed, the use of shorter wavelengths 

 made it possible to keep the lowest ray more nearly horizontal where it 

 could intercept a target's superstructure at greater distance. 



The principal characteristics of the CXAS Radar as set up for operation 

 at 700 megacycles are given in Table I. 



This equipment gave useful results on surface targets at ranges of 10 

 miles or more (depending on the size of the target) and the range accuracy 



