Radar System Engineering

280 ANTENNAS, SCANNERS, AND STABILIZATION [SEC. 95
by using sheet-metal ribs or channels formed by the hydropress method,
and boxed in at the rear as in Fig. 9“8, or by using a stamped outer frame
made by the drop-hammer method, as in the 8-ft reflector shown in
Fig. 9.12. These waffle-shaped frames require no ribs for stiffening
because the mating section conforms to the contour of the back of the
reflector face. An aluminum-alloy supporting bracket can be riveted to
I .A
FIG.9.12.—Rearview showingthe constructionof an e-ft airborneparaboloidtrimmed to a
3-ft height.
the structure; or, if the elevation angle of the beam must be adjustable,
the ends of the structure may be supported by ball-bearing trunnions.
Surface-based antennas are usually not housed, and therefore are
subject to wind forces. These forces can be reduced by as much as 50
per cent if the reflector is a grille (Figs. 9.14 and 9.28) or a perforated
sheet (Fig. 9.13). Fortunately such perforation does not impair the
electrical performance if the holes are not larger than about one-eighth
wavelength in the direction of the magnetic vector.
RADAR SCANNING PATTERNS
We have seen that although a few radars have been designed with
radiation beams that are fixed in direction, the great majority of radar
beams are made to scan. The motion of the beam maybe thought of as
the motion of a point on a sphere that is centered at the scanner. The
beam sweeps over a certain region on the sphere determined by the operational
function of the radar; often it does so periodically, and in a set
geometrical pattern termed the “scan.” The great variety of scans may
be divided into two categories, simple and complex. A simple scan is one