Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/48

 The truncated paraboloid reflector may be used in height-finding systems if the reflector is rotated 90 degrees, as shown in view C of figure 2-42. This type of reflector produces a beam that is wide horizontally but narrow vertically. The beam pattern is spread like a horizontal fan. Such a fan-shaped beam can be used to determine elevation very accurately.

Orange-Peel Paraboloid

A section of a complete circular paraboloid, often called an ORANGE-PEEL REFLECTOR because of its shape, is shown in view D of figure 2-42. Since the reﬂector is narrow in the horizontal plane and wide in the vertical, it produces a beam that is wide in the horizontal plane and narrow in the vertical. In shape, the beam resembles a huge beaver tail. This type of antenna system is generally used in height-finding equipment.

Cylindrical Paraboloid

When a beam of radiated energy noticeably wider in one cross-sectional dimension than in the other is desired, a cylindrical paraboloid section approximating a rectangle can be used. View E of figure 2-42 illustrates this antenna. Aparabolic cross section is in one dimension only; therefore, the reﬂector is directive in one plane only. The cylindrical paraboloid reﬂector is either fed by a linear array of dipoles, a slit in the side of a waveguide, or by a thin waveguide radiator. Rather than a single focal point, this type of reﬂector has a series of focal points forming a straight line. Placing the radiator, or radiators, along this focal line produces a directed beam of energy. As the width of the parabolic section is changed, different beam shapes are produced. This type of antenna system is used in search systems and in ground control approach (gca) systems.

CORNER REFLECTOR

The corner-reflector antenna consists of two ﬂat conducting sheets that meet at an angle to form a corner, as shown in view F of figure 2-42. This reﬂector is normally driven by a half-wave radiator located on a line that bisects the angle formed by the sheet reflectors.

BROADSIDE ARRAY

Desired beam widths are provided for some vhf radars by a broadside array, such as the one shown in figure 2-43. The broadside array consists of two or more half-wave dipole elements and a flat reﬂector. The elements are placed one-half wavelength apart and parallel to each other. Because they are excited in phase, most of the radiation is perpendicular or broadside to the plane of the elements. The flat reflector is located approximately one-eighth wave-length behind the dipole elements and makes possible the unidirectional characteristics of the antenna system.

Figure 2-43.—Broadside array.

ELECTRONICS SCAN

Electronic scan type of radiation is becoming more prevalent in naval use. The earliest radar systems used arrayed or multiple element antennas with long wavelengths. Due to technological developments in the 1930s, shorter and shorter wavelength systems were developed. These systems used reﬂector-type antennas to focus the beam due to technical limitation at the time. Technological advances in higher frequency solid-state devices and computers during the later part of the twentieth century renewed interest in research of array antennas for short wavelengths. See figure 2-44.

A phased array antenna (fig. 2-44) is comprised of many sub arrays that are either fixed or alternating groupings of individual elements. The elements trans- 2-26