Radar System Engineering

SEC. 3.8] RET UliN FltOM 1 W’O iSoTRoPl C TARGETS 73
When absorbent materials are applied to targets whose cross section
is mainly specula~, the cross section is reduced by the factor \R\~—aresult
that is confirmed by field tests.
The manner in which absorbent materials influence the diffraction
cross section is shown in Fig. 3,5. The diffraction pattern of a small
plate exhibits a strong maximum at an angle corresponding to that for
specular reflection, and at other angles fails off with the oscillations shown.
If the plate is covered with absorbing material of high refractive index,
the principal diffraction maximum is greatly reduced whereas the secondary
maximum is in general only slightly changed. Covering the plate
with magnet ic material of low refractive index not only reduces the
principal diffraction maximum, but also makes the secondary maxima
considerably lower. Generally speaking, material of high refractive
index can be expected to reduce the specular cross section while it leaves
the diffraction cross section substantially unaltered. Magnetic material
of low refractive index may, however, also effect a reduction in the
diffraction cross section.
COMPLICATED TARGETS
BY A. J. F. SIEGERT AND E. M.
PURCELL
Most targets are much more complicated than those dealt with in the
previous sections. A distinction will be made between ‘‘ complex targets”
and “compound targets” (Sec. 310). The latter term denotes targets
consisting of many independent elements (rain, vegetation) which
generally fill the volume illuminated by a “pulse packet” (Sec. 4.2)
completely. The former denotes complicated targets (such as ships, aircraft,
and structures) which are large, but still smaller than the illuminated
region. The power received from compound targets is thus dependent
on beamwidth and pulse length, since these govern the size of the pulse
packet; the signal from a complex target is not.
3.8. Return from Two Isotropic Targets.—The outstanding features
of the signal returned from complex targets—its fluctuations and its
wavelength dependence-can be studied by considering a simple model
consisting of two equal isotropic targets a distance 1apart. This distance
is assumed to be smaller than c7/2, where ~ is the pulse duration, so that
the signals overla~at least partly. The ratio of the received power
from the two targets to that which would be received from one of the
targets alone (at a distance large compared with 1) is given by
g = 1,%+3””’) +.%(’-;””’’ )12
“COS2(+’) ‘++4+ ’)1 ’23)