Radar System Engineering

476 THE RECEIVING SYSTEM—INDICATORS [SEC.13.1
In electrostatically focused tubes a “focus electrode” (first anode)
between the second grid and the high voltage anode is maintained at an
intermediate potential. Thevarious electrodes aresoshaped that strong
electron lens actions are produced by the fieldsat the two gaps. Adjustment
of focus is made by controlling the potential of the focus electrode.
Magnetic focusing is accomplished by means of a longitudinal magnetic
field of circular symmetry which increases in intensity fromthe
center to the edge of the tube and thus has radial components that help
provide the focusing action. External coils or permanent magnets are
used to produce this field.
Fmus electrode
Ancde
Heater
f
Cathode
Cathode
Heater
FOCUScoil
Grid
Electrostatictube
———————__
Magnetic tube
/
High voltage
connection
Persistent phosphor~
J!l
Blue
Glasstube
“$:;:~o~ phosphor =
Cascadescreen
FIG. 13.1.—Elements of cathod~ray tube,
Electrostatic deflection is accomplished by passing the beam between
two orthogonal pairs of deflecting electrodes or plates. The deflection
due to each pair is accurately proportional to the potential difference
between its members. The individual deflections due to the two pairs
of plates add vectorially. The deflection sensitivity at the screen
depends upon the geometry of the deflecting plates themselves, their
distance from the screen, and the velocity of the electrons as they pass
the plates. The two pairs are similar in geometry, but, since they are
at different distances from the screen, their deflection sensitivities are
unequal.
In the electrostatic tube shown, the electrons achieve their final
velocity before reaching the deflecting plates. In some tubes designed
for high-voltage use, deflection sensitivity is increased by applying