FIRE-CONTROL RADARS FOR NAVAL VESSELS 29 



that the beam position depends upon the rclati\c phase of the excitation 

 appHed to the radiating elements of the array. If all elements are excited 

 in phase, as in Radar Mark 1, the beam will be normal to the line of the 

 array, while gradually increasing phase diiTerence across the array will 

 result in displacement of the beam. For small angles of beam shift, en- 

 tirely satisfactory results may be obtained by shifting the phase of excitation 

 applied to one-half of the array with respect to the other, and this expedient 

 results in a much simpler phase shifting mechanism than would be required 

 to obtain uniform phase change. This system was used in Radar Mark 

 3 and its application is illustrated schematically in Fig. 26. It will be 

 seen that this array is identical to that used for Radar Mark 1 except for 

 the central section of transmission line in which a lobe switching unit has 

 been added. In this unit the phase of excitation to one-half of the array 

 is retarded with respect to the other half by connecting a capacitive re- 

 actance alternately across one feed line or the other to obtain the two beam 

 positions. Switching is accomplished by the use of a motor driven rotary 

 capacitor shown in Section A-A. The rotor is a semicircular aluminum 

 casting w'hich is maintained at substantialh' ground potential by very close 

 spacing to the grounded metal housing. The two slators are small metal 

 plates which interleave with the rotor during approximately one-half 

 revolution and are connected through half-wa\clcngth coaxial lines to the 

 antenna transmission lines. The purpose of the half-wavelength stub hnes 

 is to avoid physical limitations which would otherwise be encountered in 

 connecting the rotary capacitor to the lines. Allowance is made in these 

 stubs for end-loading caused by stray capacitance of the stator plates and 

 supporting insulators. It will be seen that during nearly one-half revo- 

 lution of the rotor one of the stators is engaged to shift the antenna beam in 

 one direction while during the other half revolution the other stator is 

 engaged to produce the other lobe position. The switching occurs during 

 the small interval in which both stators are engaged by the rotor. Signals 

 received during this interval are blanked out in the indicator. The rotor of 

 the lobe switcher is driven at about 30 RPS by an induction motor mounted 

 within a weatherproof housing. The motor shaft also carries cam operated 

 contacts to produce image spacing on the indicators, control signals for the 

 Train Meter, and blanking during the lobe switch interval. The entire unit 

 is gas tight and is filled with dry gas through the transmission line. 



The value of the lobe switching capacitor and its position along the feed 

 line must satisfy two conditions: first, the phase shift must be such that the 

 antenna beam will be displaced by the desired amount; and second, the 

 impedance at the feed point must be such that equal division of power will 

 be obtained in the two halves of the array. In the first Radar Mark 3 

 antenna (6 ft. by 6 ft. parabolic array) a beam displacement of about 3.0 



