Antena Diedro

15.3 CORNER REFLECTOR
To better collimate the energy in the forward direction, the geometrical shape of the
plane reflector itself must be changed so as to prohibit radiation in the back and side
directions. One arrangement which accomplishes that consists of two plane reflectors
joined so as to form a corner, as shown in Figures 15.1(b) and in 15.2(a). This is
known as the corner reflector.Because of its simplicity in construction, it has many
unique applications. For example, if the reflector is used as a passive target for radar
or communication applications, it will return the signal exactly in the same direction
as it received it when its included angle is 90◦. This is illustrated geometrically in
Figure 15.2(b). Because of this unique feature, military ships and vehiclesare designed
with minimum sharp corners to reduce their detection by enemy radar. Corner reflectors
are also widely used as receiving elements for home television.
In most practical applications, the included angle formed by the plates is usually
90◦; however other angles are sometimes used. To maintain a given system efficiency,
the spacing between the vertex and the feed element mustincrease as the included
angle of the reflector decreases, and vice versa. For reflectors with infinite sides, the
gain increases as the included angle between the planes decreases. This, however, may not be true for finite size plates. For simplicity, in this chapter it will be assumed that
the plates themselves are infinite in extent (l =∞). However, since in practice the
dimensions must be finite,guidelines on the size of the aperture (Da), length (l), and
height (h) will be given.
The feed element for a corner reflector is almost always a dipole or an array of
collinear dipoles placed parallel to the vertex a distance s away, as shownin a perspective
view in Figure 15.2(c). Greater bandwidth is obtained when the feed elements
are cylindrical or biconical dipoles instead of thinwires. In many applications, especially
when the wavelength is large compared to tolerable physical dimensions, the
surfaces of the corner reflector are frequently made of grid wires rather than solid
sheet metal, as shown in Figure 15.2(d). One of the reasons for doing that is to reduce
wind resistance and overall system weight. The spacing (g) betweenwires is made a
small fractionof awavelength (usually g ≤ λ/10). For wires that are parallel to the
length of the dipole, as is the case for the arrangement of Figure 15.2(d), the reflectivity
of the grid-wire surface is as good as that of a solid surface.

Inpractice, the aperture of the corner reflector (Da) is usually made betweenon e
and two wavelengths (λ < Da < 2λ). The length of the sides of a 90◦ corner reflector
ismost commonly taken to be about twice the distance from the vertex to the feed (l
2s). For reflectors with smaller included angles, the sides are made larger. The feed-tovertex
distance (s) is usually takento be between λ/3 an d 2λ/3(λ/3 < s < 2λ/3). For
each reflector, there is an optimum feed-to-vertex spacing. If the spacing becomes too
small, the radiationresistan ce decreases andbecomes comparable to the loss resistance
of the system which leads to an inefficient antenna. For very large spacing, the system
produces undesirable multiple lobes, and it loses its directional characteristics. It has
been experimentally observed that increasing the size of the sides does not greatly affect
the beamwidth and directivity, but it increases the bandwidth and radiation resistance.The mainlobe is somewhat broader for reflectors with finite sides compared to that of
infinite dimensions. The height (h) of the reflector is usually takento be about 1.2 to
1.5 times greater thanthe total length of the feed element, inorder to reduce radiation
toward the back regionfrom the ends.
The analysis for the field radiated by a source in the presence of a corner reflector
is...