768 BELL SYSTEM TECHNICAL JOURNAL 



tions during the past war. The deflection coils illustrated include both air 

 and permalloy core structures as used in rectangular and polar types of 

 displays. Where the radar system involves extremely short time interval 

 sweep wave forms, the maximum inductance which can be employed in the 

 deflection coil is limited by the power supply voltages available, and in these 

 indicator designs the air-core type of deflecting coil is usually employed. 

 Where the sweep wave form is relatively slower the permalloy core types 

 have been extensively employed with an effective improvement in deflection 

 sensitivity. For the PPI form of display a toroidal coil structure has been 

 devised which contains two distributed windings connected in an opposing 

 sense. The internal leakage flux of such a structure is essentially uniform 

 and, therefore, satisfactory for magnetic cathode-ray deflection purposes. 

 The usual PPI type coil structure is arranged to mount within a large ring 

 ball bearing to enable rotation around the neck of the tube with provisions 

 being included on the coil housing for slip rings to afford connection to the 

 deflection coil proper. 



With the increased emphasis on extremely accurate radar presentations, 

 which developed during the later war years especially in connection with the 

 radar bombing program, the design and manufacturing tolerances allowable 

 in connection with the large scale production of these magnetic deflection 

 coils were severely reduced. Figure 41 illustrates the constructional details 

 of a deflection coil as employed in the AN/APQ-7 radar bombing equipment. 

 The presentation in this instance is of the GPI type employing rectangular 

 coordinate deflection and extremely fast sweep wave forms. This deflection 

 coil structure employs accurately formed open air-core windings which are 

 initially adjusted and cemented to concentric phenol plastic cylinder forms. 

 This design features a vernier rotation adjustment of the horizontal and 

 vertical pairs of coil assemblies to meet a manufacturing scanning require- 

 ment of 90° ± 0.5°. 



Two examples of focussing and centering structures for magnetic-type 

 cathode-ray tube radar indicators are included in Figure 40. The focus coil 

 consists of a simple winding located axially about the neck of the tube with a 

 shielding magnetic structure containing an annular air-gap which restricts 

 the external field to a region including the cathode-ray tube electron beam. 

 This structure is designed to produce a uniform magnetic field distribution 

 in the complete area of the beam to avoid defocussing effects. In certain 

 early-design airborne radar cquii)ment applications where the equipment 

 was subjected to extreme variations in ambient temperature over short 

 periods of operation some difiiculty in maintaining optimum focus was 

 experienced. This defocussing, due to the change in coil resistance with 

 ambient temi)erature and in part to dissipation in the winding proper, is 

 minimized in the designs shown by the introduction of a varistor element 



