Radar System Engineering

SEC 13.2] CATHODE-RAY TUBE SCREENS 479
wasteful of power—for example, when a steady deflection is to be added
to a high-frequency sweep that requires a coil of low inductance and therefore
low sensitivity. It is then worth while to use two separate coils or a
permanent magnet and a single coil. If two coils are used, they are
usually placed together (on the same yoke when iron is used), but in
extreme cases they must be separated to avoid interaction.
A rotating coil and a fixed coil can be used in combination—for
example, to produce an off-center PPI. The fixed coil is placed outside
of the rotating one, which must then contain no iron. The outer unit
may contain a single coil that can be manually oriented, or it may have
two orthogonal coils that can be separately excited. If the amount of
off-centering is to remain fixed, a permanent-magnet arrangement can
replace the outer coil.
In the specific design of the coils many factors must be considered.
They should be as economical of over-all power as possible. The magnetic
fields must be so ihaped that they produce a linear deflection and
cause no harmful effects on focus. When multiple coils are used, it rn.ust
be possible to prevent harmful effects from interaction.
The current sensitivity of the coils should be made as high as practicable
since this minimizes power losses. For a given geometry, the
sensitivity is directly proportional to the number of turns, which should
therefore be as high as practicable. However, for a given rate of deflection
(i.e., a given rate of change of flux) the voltage induced in the coil is
directly proportional to the number of turns. This usually sets an upper
limit to the number of turns that can be used within the bounds of a
reasonable power supply voltage.
These and other matters pertaining to coil design are discussed at
length in Vol. 22 of the series.
13.2. Cathode-ray Tube Screens. Phosphorescent Screens.—The
important characteristics of the screen are its decay properties, its efficiency,
and the manner in which it integrates or “builds up” on repeated
signals.
When scanning interrupts the picture, the screen must have sufficient
persistence (’( afterglow”) to permit observations and measurements on
the echoes and in so far as possible to furnish a continuous picture. On
the other hand, the image must not persist so long as to cause confusion
on a changing picture. In terms of the scanning rate, three cases may
be considered:
1. Cases in which little or no persistence is needed, either because the
frame time is less than the retentivity time of the eye (about ~ see)
or because observations are made in the absence of scanning.
Screens incorporating the green willemite phosphor used in ordi-