Radar System Engineering

276 ANTENNAS, SCANNERS, AND STABILIZATION [SEC.9.3
ray optics. Figure 9.10 shows two distortions of a paraboloid used for
this purpose. Another widely used method employs a cylindrical rejlector,
shaped like the blade of a snowplow or bulldozer, and having only single
curvature (Fig. 9“8). Such a reflector is illuminated by the radiation
from a linear feed rather than a point feed. In this case the linear feed
defines the beam sharpness in regard to azimuth and the reflector distributes
the energy in elevation. The ray diagram shown in l’ig. 9.10a
will serve again to illustrate the formation of the cosecant-squared fan.
FIG.9S.-A
scannerwith a 60-in.shapedcylindricalreflector,ventrally installedin a B-29.
In this figure the linear feed is imagined perpendicular to the plane of the
drawing. The feed is frequently the straight opening between two
identical parallel conducting sheets which are joined along their curved
backs by a parabolic reflector strip. Just inside the center of the opening,
at the focus of the parabolic strip, is located the open end of a waveguide
which irradiates the parabola, thereby setting up the propagation of
energy between the two sheets and straight out through the opening.
This assembly is called a ‘‘ pillbox.” Microwaves may be propagated
between the sheets with the electric field polarized either parallel or
perpendicular to the sheets. In parallel polarization the spacing must be
held accurately in order to prevent distortions of the wavefronts; slender
spacing posts are used since they have almost no effect on the waves. In