Radar System Engineering

316 ANTENNAS, SCANNERS, AND STABILIZATION [SEC. 923
so far as the aerodynamics is concerned, but limits the rearward view.
This space, too, is desirable for the location of certain crew members and
of the bombsight. Interference with other installations is usually less in
an aft location, but the space is often limited by the size of the fuselage
at this point and the forward view may be cut off by the forward portion
of the plane.
9.23. Electrical Transmission.-The ultimate shape of the radome is
generally limited by electrical design considerations. The retraction of
the antenna is controlled by edge interference, for an edge can cause
blanking and diffraction of a radar beam. The result is a distortion and
deterioration of the antenna pattern. Any discontinuity such as that
caused by reinforcement of the radome or by a piece of metal may cause
diffraction if it intercepts the radar beam.
The permissible range of incident angle is the other electrical factor
limiting the shape of the radome. The range of satisfactory incident
radiation transmission varies with different materials and radome wall
constructions. The properties of the materials affecting the transmission
are the dielectric constant and the loss tangent; the latter is an index of
the absorption and the former is a factor determining the reflection. For
optimum combinations to date, satisfactory transmission can be obtained
over a range of 0° to 70°. Beyond this value the transmission decreases
very rapidly, reaching zero transmission at 90° incidence.
Having determined a reasonable shape for the radome, it is then necessary
to check the angles of incidence by means of an antenna ray diagram.
Typical ray diagrams for cosecant-squared antennas are shown in Figs.
9.2, 9.10, and 9“37. If the antenna is stabilized, the angles will have to
be checked throughout the range of stabilization. When the angles of
incidence exceed the limiting value the contours of the radome must be
adjusted until the upper limit of angle of incidence is not exceeded.
9.24. Structural Design of Radomes.—The thickness and construction
of the radome wall can be determined on the basis of structural considerations
within limits depending upon the allowable reflection. The
reflection is a cyclic function depending upon the dielectric constants,
thickness of the material, and angles of incidence of the radiation.
There are two basic types of construction: the single plastic or laminated
wall, and the so-called “sandwich” construction involving two or
more materials. The sandwich construction consists of high-strength
skin surrounding a core of lower density, which results in a high-strength,
low-density construction. The sandwich construction also provides a
greater variety of wall construction for high strength and efficient electrical
transmission.
The approach to the structural design problem will depend upon the
shape of the radome and the pressure distribution. These two factors