Radar System Engineering

274 ANTENNAS, SCANNERS, AND ST ABILIZA TIO.V [SEC.9.3
tally, the beamwidth can be measured by exploring the intensity of the
radiation to the right and left of the center of the beam or, alternatively,
above and below the center of the beam. It is an important fact that
the beamwidth as measured vertically depends, according to Eq. (l), upon
the vertical dimension of the cut paraboloid, and the horizontal beamwidth
depends upon the horizontal dimension. Thus, in order to realize
high resolution in azimuth, the antenna must be wide but not necessarily
tall, whereas a height-finding radar, affording accurate measurement of
the angular elevation of an airplane, must have a tall antenna which need
not be wide. These two types are exemplified by the scanners shown
in Figs. 9.15 and 9.14 respectively.
FIC. 95.-Two-dipole
feed radiating towarrl the left.
We have not mentioned the “mattress” type of antenna, which is
perhaps the most familiar to the public because it is commonly seen on
naval ships. These antennas are rectangular arrays of radiating elements
so phased as to produce a broadside beam. They are widely used in
radars of wavelength over 1 m. With the trend toward shorter wavelength,
the mattress antennas are giving way to the types discussed above.
9.3. Fan Beams.—One of the functions for which airborne radars are
designed is aiding air navigation, and an extremely important type of
airborne navigational radar presents a map of the terrain around the
aircraft. In order to map the ground the transmitted energy must be
directed toward the ground, and rather than having the energy beamed
like a searchlight, a “fan beam” must be employed as shown in Fig. 9.6
for complete coverage. A uniformly intense map is desired, which will
display ground objects that lie at depression angles between, for example,
5° and 70° below the horizontal. The energy in the fan beam must therefore
be properly distributed to give adequate illumination of the most
distant objects while not overilluminating those which are at greater
depression angles. It can be shown, subject to certain simplifying
assumptions, that the energy should be distributed as the square of the
cosecant of the depression angle (see Sec. 2.5). Figure 9.7 shows how
well such a pattern is realized in practice, with an antenna 12 in. in height
(shown in Fig. 9.8) used at the 3-cm band. In this polar diagram the
square root of the power is plotted rather than the power itself, since the
desired distribution is then given by a straight line as shown. Figure 99
is a photograph of the PPI display, showing the performance of this
antenna.