Radar System Engineering

SEC.3.5] THE CORNER REFLECTOR 67
3.5. The Corner Reflector. -It is often desirable to make a compact
radar target with a large cross section. Aflat plate of dimensions large
compared to a w-avelength exhibits a large cross section when viewed
along its normal, because of specular reflection, but the cross section falls
off sharply in other directions [see Eqs. (7) and (8), Sec. 3.4]. The
problem of designing a target that will give strong specular reflection for
almost any direction of illumination has been solved by taking over into
microwave radar practice the corner reflector familiar in optics. The small
glass reflectors usedin highway markers work on this principle.
A corner reflector consists of three mutually perpendicular intersecting
planes (Fig. 3.2cL). If abeam is directed into the corner formed by the
planes, triple reflections occur which send it back in the direction from
(a)
(b)
FIG. 32.-Tl1e triangularCOI’UCV reflector.
which it came (Fig. 3.2b). The effective area for triple reflection depends
on the direction in \vhich the corner is viewed, but it is large over most
of the octant in which a single corner is effective. When the area for
triple reflection grows small, double reflection (from two planes whose
line of intersection is near]Y normal to the line of sight) and single reflection
(from a plane nearly normal to the line of sight) begin to make
important contributions to the radar cross section. A single corner will
be effective only for directions of illumination that cover one octant of a
sphere centered at the reflector, as has been remarked; but all directions
can be covered by makinga cluster of eight such corners (Fig. 3.3).
We can find the cross section for a corner by considering it equivalent
toa flat reflecting plate whose area is the effective area of the corner for
triple reflection. Equation (7) gives, for area .-l andwavelengthk, the
cross section
4TA ‘
‘= A’”
The maximum area for triple reflection ~villbe that afforded by the corner
tlhen it is ~-ieived along its axis of symmetry. This maximum area is
that of the wgular hexagon formed by cutting off the rorners of the
projection of the corner on its axis of symmetry; it is given by