Radar System Engineering

84 PROPERTIES OF RADAR TARGETS [SEC. 310
essential point of difference:l the rate of fluctuation of the receiver output
is determined by the bandwidth of the receiver, whereas the rate of
fluctuation, in time, of the echo from a given element of volume in the
rain is fixed by the time required for the drops in the volume to assume
a new configuration. Thk is ordinarily so long that several successive
radar pulses find the drops disposed in nearly the same way, that is, with
changes in relative positions amounting to a small fraction of a wavelength
only. A quantitative formulation of this statement is derived in
Vol. 24, Sec. 6.2.
The latter effect makes the detection of a desired target echo within
the rain clutter even more difficult, for the very persistence of the target
echo on successive pulses helps greatly to distinguish it from noise
(Sec. 2. 10). Such help is of no avail against rain unless the target echo
is received over a time long compared to the rain fluctuation period
discussed above.
To distinguish a target in the midst of rain clutter, we must make
use of some peculiar feature of a raindrop as a radar target. One such
feature, perhaps the only one unique to rain, is that the raindrops are
round; thus the intensity and phase of the reflection from a single drop
do not depend on the direction in which the incident beam is polarized.
This cannot be said of most radar targets, which, being complicated
objects usually not rotationally symmetrical about the line of sight,
show very large variations, with polarization, of the total (complex)
reflection coefficient. An experimental verification of the theory that
this property of symmetry which is peculiar to raindrops can be used
to distinguish a target signal in the midst of rain clutter has been carried
out.
J“ery briefly, the principle is this: if a sphere is struck by a circularl,v
polarized plane wave, formed by passing a linearly polarized plane wave
through a quarter-~vave plate, z the scattered wave observed in the back-
‘ Thereareother minor differences: the length in range of a “noise spot” is deterrniaedby
the receiverbandwidthand the cathode-ray-tubespot size. The leugthin
rangeof a “rain spot” dependson the pulselengthas well. The width, in azimuth,
of a noisespot dependsonl’yon cathode-ray-tubespot size, or sweepinterval,whichever
is larger,whereasthe azimuthalwidth of a rain spot dependson the antenna
beamwidth.
‘ Quarter-waveplates and half-waveplatesfor radar, entirelyanalogousto those
familiar in optics, can be made. l~hat is necessaryis to make the phase velocity
of the \va\,eas it passesthroughthe plate dependupon its directionof polarization
with respectto some directionin the face of the plate. This is done by making the
plate of a stack of parallel metal sheets spaced an appropriatedistance from one
another. l~-hena linearly polarizedradar wa~-eencountersthe edges of the sheets
forming the stack, the component\vhoseelectricYectoris normal to the edge passes
through unaffected. The component whose electric vector is parallel to the edge,
however,findsitselfin a waveguideof greatheightbut of finitewidth, Its wavelength