40 BELL SYSTEM TECHNICAL JOURNAL 



selected by the range operator. This was suppHed in the form of an ex- 

 ternal unit which controlled the gain of the receiver IF amplifier to reduce 

 signal fluctuations produced by fading. 



The first production Mark 3 Radars were delivered to the Navy in October 

 1941, and the first two installations were completed on the main battery 

 directors of the U.S.S. Philadelphia at the Brooklyn Navy Yard that month. 



Radar Mark IV 



During the development work on Radar Mark 3 the Navy pointed out the 

 need for a fire control radar for use with the 5-inch Naval guns against 

 enemy aircraft. The Bell Telephone Laboratories was therefore requested 

 to further modify the radar design to meet this need. The anti-aircraft 

 equipment was first designated FD, later becoming known as Radar Mark 

 4. 



For antiaircraft fire control a new coordinate had to be added to the 

 target-locating system; namely, elevation angle. Again it was desired that 

 the additional information be obtained from the single antenna with a 

 precision equal to that already obtained in azimuth. This problem was 

 approached in a manner similar to that used for the Mark 3 antenna and is 

 described below. 



Two Plane Lobe Switching 



In considering two plane lobe switching methods it appeared that the 

 desired result could be obtained by mounting two 3 ft. x 6 ft. parabolic 

 arrays one above the other. This arrangement was tried and resulted in 

 the array shown in Fig. Zi. It provided two plane lobe switching with an 

 antenna only slightly larger than the 6 ft. x 6 ft. antenna used before and 

 had comparable gain and beam width (see Table II). 



A schematic diagram of the array is shown in Fig. 34. Here it will be 

 seen that there are two horizontal dipole arrays, each mounted along the 

 focal line of a cylindrical parabola. The dipoles are in four groups and the 

 interconnecting harness is criss-crossed and joined to the feed line at the 

 center. Symmetrically placed around the feed point are four stub lines 

 connected to the lobe switcher stators. Here again a semi-circular rotor is 

 used for the lobing shifting capacitor. It will be observed that during each 

 quarter turn of the rotor two stator plates are engaged, and the sequence is 

 such that the beam shifts left, up, right, and down during one rotation. 

 A separate Indicator was provided for the Pointer (elevation operator). To 

 avoid signal confusion on the two Indicators it is necessary to show only 

 left-right signals on the Trainer's oscilloscope and up-down signals on the 

 Pointer's oscilloscope. This is accomplished by means of cam operated 

 contacts in the lobe switcher which blank the indicators during the required 



