Custom Search
|
|
  |
|
|
RADAR ANTENNAS In this section, we will briefly review the requirements of radar antennas. Antenna characteristics are discussed in detail in NEETS, Module 10, Introduction to Wave-Generation, Transmission Lines, and Antenna and in Module 11, Microwave Principles. A review of these modules would be helpful at this point to prepare you for the following radar antenna discussion. Antennas fall into two general classes, OMNIDIRECTIONAL and DIRECTIONAL. Omnidirectional antennas radiate rf energy in all directions simultaneously. They are seldom used with modern radars, but are commonly used in radio equipment, in iff (identification friend or foe) equipment, and in countermeasures receivers for the detection of enemy radar signals. Directional antennas radiate rf energy in patterns of LOBES or BEAMS that extend outward from the antenna in one direction for a given antenna position. The radiation pattern also contains minor lobes, but these lobes are weak and normally have little effect on the main radiation pattern. The main lobe may vary in angular width from one or two degrees in some radars to 15 to 20 degrees in other radars. The width depends on the system's purpose and the degree of accuracy required. Directional antennas have two important characteristics, DIRECTIVITY and POWER GAIN. The directivity of an antenna refers to the degree of sharpness of its beam. If the beam is narrow in either the horizontal or vertical plane, the antenna is said to have high directivity in that plane. Conversely, if the beam is broad in either plane, the directivity of the antenna in that plane is low. Thus, if an antenna has a narrow horizontal beam and a wide vertical beam, the horizontal directivity is high and the vertical directivity is low. When the directivity of an antenna is increased, that is, when the beam is narrowed, less power is required to cover the same range because the power is concentrated. Thus, the other characteristic of an antenna, power gain, is introduced. This characteristic is directly related to directivity. Power gain of an antenna is the ratio of its radiated power to that of a reference (basic) dipole. Both antennas must have been excited or fed in the same manner and each must have radiated from the same position. A single point of measurement for the power-gain ratio must lie within the radiation field of each antenna. An antenna with high directivity has a high power gain, and vice versa. The power gain of a single dipole with no reflector is unity. An array of several dipoles in the same position as the single dipole and fed from the same line would have a power gain of more than one; the exact figure would depend on the directivity of the array. The measurement of the bearing of a target, as detected by the radar, is usually given as an angular position. The angle may be measured either from true north (true bearing), or with respect to the bow of a ship or nose of an aircraft containing the radar set (relative bearing). The angle at which the echo signal returns is measured by using the directional characteristics of the radar antenna system. Radar antennas consist of radiating elements, reflectors, and directors to produce a narrow, unidirectional beam of rf energy. A pattern produced in this manner permits the beaming of maximum energy in a desired direction. The transmitting pattern of an antenna system is also its receiving pattern. An antenna can therefore be used to transmit energy, receive energy, or both. The simplest form of antenna for measuring azimuth (bearing) is a rotating antenna that produces a single-lobe pattern. The remaining coordinate necessary to locate a target in space may be expressed either as elevation angle or as altitude. If one is known, the other can be calculated from basic trigonometric functions. A method of determining the angle of elevation or the altitude is shown in figure 3-16. The slant range is obtained from the radar scope as the distance to the target. The angle of elevation is the angle between the axis of the radar beam and the earth's surface. The altitude in feet is equal to the slant range in feet multiplied by the sine of the angle of elevation. For example if the slant range in figure 3-16 is 2,000 feet and the angle of elevation is 45 degrees, the altitude is 1,414.2 feet (2,000 X .7071). In some radar equipments that use antennas that may be moved in elevation, altitude determination is automatically computed. Figure 3-16. - Radar determination of altitude.
A SPHERICAL WAVEFRONT spreads out as it travels and produces a pattern that is neither too sharp nor too directive. On the other hand, a PLANE wavefront does not spread out because all of the wavefront moves forward in the same direction. For a sharply defined radar beam, the need exists to change the spherical wavefront from the antenna into a plane wavefront. A parabolic reflector is one means of accomplishing this. Radio waves behave similarly to light waves. Microwaves travel in straight lines as do light rays. They may be focused and/or reflected just as light rays can. In figure 3-17, a point-radiation source is placed at the focal point F. The field leaves this antenna with a spherical wavefront. As each part of the wavefront reaches the reflecting surface, it is shifted 180 degrees in phase and sent outward at angles that cause all parts of the field to travel in parallel paths. Because of the shape of a parabolic surface, all paths from F to the reflector and back to line XY are the same length. Therefore, all parts of the field arrive at line XY the same time after reflection. Figure 3-17. - Parabolic reflector radiation.
If a dipole is used as the source of radiation, there will be radiation from the antenna into space (dotted lines in figure 3-17) as well as toward the reflector. Energy that is not directed toward the paraboloid has a wide-beam characteristic that would destroy the narrow pattern from the parabolic reflector. This occurrence is prevented by the use of a hemispherical shield (not shown) that directs most radiation toward the parabolic surface. By this means, direct radiation is eliminated, the beam is made sharper, and power is concentrated in the beam. Without the shield, some of the radiated field would leave the radiator directly. Since it would not be reflected, it would not become a part of the main beam and thus could serve no useful purpose. The same end can be accomplished through the use of a PARASITIC array, which directs the radiated field back to the reflector, or through the use of a feed horn pointed at the paraboloid. The radiation pattern of a parabola contains a major lobe, which is directed along the axis of revolution, and several minor lobes, as shown in figure 3-18. Very narrow beams are possible with this type of reflector. View A of figure 3-19 illustrates the parabolic reflector. Figure 3-18. - Parabolic radiation pattern.
Truncated Paraboloid View B of figure 3-19 shows a horizontally truncated paraboloid. Since the reflector is parabolic in the horizontal plane, the energy is focused into a narrow horizontal beam. With the reflector truncated, or cut, so that it is shortened vertically, the beam spreads out vertically instead of being focused. Since the beam is wide vertically, it will detect aircraft at different altitudes without changing the tilt of the antenna. It also works well for surface search radars to overcome the pitch and roll of the ship. Figure 3-19. - Reflector shapes.
The truncated paraboloid reflector may be used in height-finding systems if the reflector is rotated 90 degrees, as shown in view C. Because the reflector is now parabolic in the vertical plane, the energy is focused into a narrow beam vertically. With the reflector truncated, or cut, so that it is shortened horizontally, the beam spreads out horizontally instead of being focused. Such a fan-shaped beam is 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 3-19. 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. When a beam of radiated energy noticeably wider in one cross-sectional dimension than in the other is desired, a cylindrical paraboloidal section approximating a rectangle can be used. View E of figure 3-19 illustrates this antenna. A parabolic 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 obtained. This type of antenna system is used in search and in ground control approach (gca) systems. Q.11 Which of the two general classes of antennas is most often used with radar? |
 
|
|
|
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
|
