Radar System Engineering

SEC.13.1] ELECTRICAL PROPERTIES OF CATHODE-RAY TUBES 477
approximately half of the total voltage across a gap in the Aquadag at a
point beyond the plates. The two parts are then spoken of as the
“second anode” and “third anode.”
Any variation of the mean potential between either pair of plates has
a marked effect on the focus; consequently if good focus is important it is
necessary to use push-pull deflection. I Furthermore, it is best that the
mean potentials of the plates be close to that of the second anode. All
(a)
(b)
(c)
FIQ.1.3.2.-Deflection coils. (a) Air-core deflection coil with lumped winding; (b) airaore
deflection coil with semidistributedwinding; (c) iron-core deflection coil with pie
windings; (d) motor-stator type of deflection coil.
these potentials are often made the same, but in many tubes it is profitable
to provide an adjustment for making the first pair of plates slightly
more negative.
Magnetic deflection is accomplished by passing currents of the desired
waveforms through a coil or combination of coils surrounding the tube
neck (Fig. 13”1). The deflection due to each coil is proportional to the
current through it, and the combined deflection is the vector sum of the
individual ones. A few of the coil geometries used are described below.
A single coil’ is used in a rotating-coil PPI. Typical air-cored and
1Forthe sake of simplicity this is not usually indicated in block diagrams.
1A coil usually consists of at least two separate windings symmetrically placed
with respect to the tube. They maybe connected either in seriesor in parallel.
(4