764 



BELL SYSTEM TECHNICAL JOURNAL 



commonly employed in radar receivers employing electrostatic deflection, 

 and the second anode is maintained at the average potential of these de- 

 flection plates. Astigmatism, the selective focussing in one direction on the 

 screen at the expense of focus in the other, results from the mechanical 

 limitations whereby the electric fields of the two pairs of deflection plates 

 cannot be made effective at the same point within the tube. Some im- 

 provement can be realized in this respect by operating the pairs of deflecting 

 plates at slightly different average potentials. 



The limitation in deflection response at high frequencies is a function 

 of the total deflection circuit capacitance. To eliminate the blocking con- 

 densers with their considerable parasitic capacitance to ground it is common 

 radar indicator practice to operate the deflection plates directly from the 



COILS 



FLUORESCENT 

 / SCREEN 



CATHODE-. 

 HEATER 



CONTROL GRID 



r^s 



CENTERING FOCUS DEFLECTION 

 I 



1 r i_ 





I I 

 I I 



HlGH-VOLTAGE 

 ANODE 



Fig. 39. — Schematic diagram of a magnetic type cathode-ray tube. 



plates of the sweep amplifier tubes and then accordingly operate the cathode 

 of the cathode- ray tube at a high negative potential. 



Magnetic Deflection Type 



A typical form of a magnetic-type cathode-ray tube is illustrated in Fig. 

 39. In this tube the electron gun structure comprises a heater, cathode, 

 control grid, and second or screen grid which, together with the second 

 anode, usually formed by an aquadag coating within the glass envelope and 

 which is maintained at a high positive potential, roughly delineates the 

 beam. The second grid in this case serves to shield the control grid from 

 the high potential second anode and results in an improved control grid 

 characteristic. The beam of electrons is focussed through the use of a mag- 

 netic field located as shown in Fig. 39, and produced by direct current flow- 

 through a coil or by a |)crmanent magnet structure. The centering of the 

 beam u{)on the screen under no deflection conditions is accomjilishcd by the 

 use of a distorting field commonly introduced as a part of the deflection coil 

 and fo( ussing assembly. The deflection of the beam for a rectangular coor- 



