Radar System Engineering

282 ANTENNAS, SCANNERS, AND STABILIZATION [SEC. 98
with the conical scan produces the Palmer scan, the cycloidal motion of
the beam which results when a conical scanner slowly traverses the horizon.
The region of search is a horizontal rectangle with semicircular
ends. Palmer scan was used to accomplish an easy transition from the
search function to the gunlaying function of a fire-control radar. The
name derives from a familiar exercise in the Palmer system of calligraphy.
Other complex scans have been used mainly in experimental equipment,
Probably the most useful of these is a scan in which the elevation
(or azimuth) angle oscillates rapidly while the azimuth (or elevation)
angle oscillates slowly.
MECHANICAL SCANNERS
In most cases the entire radar antenna is put through certain angular
motions in order to make the beam scan. The antenna assembly, including
the mount that supports it and makes it move, is called a” mechanical
seamer. ” The t6rm “electrical scanner” is reserved for cases where the
beam is moved not by a motion of the antenna as a whole, but rather by
relatively subtle motions of the feed or other parts of the antenna. The
borderline between mechanical and electrical scanning is not well defined.
For instance, in denial of the foregoing definitions, the mechanical
category includes the conical scans that are mechanized by the circular
motion of a point feed around the focus of a fixed paraboloid. An electrical
scan is frequently used to produce a rapid sector scan of small angular
amplitude.
9“8. The Kinematics of Mechanical Scanners.-The intended use of
a radar fixes the type of scan, and the type of scan fixes in turn the
kinematic aspects of the scanner design. A circular scan is mechanized
by simply revolving the antenna on a horizontal turntable. If it is
desired to raise or lower the beam, the turntable must carry bearings so
that the antenna may be tilted about a horizontal axis at right angles to
the beam; the scanner may be des~ribed as a tilting antenna on a rotating
assembly on a fixed base. Airborne scanners of this type are usually
mounted on the under side of the fuselage, the antenna being below the
pedestal. Sector scanners are cinematically similar to circular scanners.
To enable the radar operator to control the antenna from a distance
the tilting is usually actuated by a motor. This motor may be located
on the turntable (in which case it must be powered through slip rings) or
else on the base (in which case a special mechanism must be provided for
transferring the mechanical power from the tilt motor to the revolving
antenna). Both methods are used in practice although only the former
is encountered in surface radars.
Conical scan may be effected by spinning the antenna about an axis
not quite parallel to the beam (AX/APS-6, Fig. 9. 17); or by spinning the