3-16
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
