Radar System Engineering

SEC 3.11] EXTENDED SURFACE TARGETS 85
ward direction will be circularly polarized. However, the sense of
rotation of the vector which represents the field of the scattered wave
is such that if the scattered wave passes back through the original
quarter-wave plate it WN emerge as a wave whose polarization is perpendicular
to that of the initial linearly polarized wave (Fig. 3“13).
It will therefore not enter the antenna which was the source of the
original linearLy polariaedwwre.
Raindrop
Experimental results are shown
Quarter-wave plate
in Fig. 3.14. Theaverage intensity
of the rain echo at a given range
was reduced approximately 26 db,
while the ground targets (buildings) JW’
which were being observed at the
KA”tenna ~
same time suffered a reduction of
4 to 8 db. Imperfections of the
quarter-wave plate, as well as the
FIG. 3 13,—Scheme for reduction of rain
echo.
slight ellipticity of falling raindrops, prevent complete cancellation of the
rain echo.
3.11. Extended Surface Targets.’—In many cases the individual
scatterers in a compound target are confined to a relatively thin layer,
which can be treated as an extended surface target. Nearly all of the
signals received by airborne radar are from targets that fall into this
classification. One example is vegetation, which covers most of the
land over which an airplane is likely to fly. Another is the diffuse return
from irregularities on the surface of the sea.
Let us examine briefly the processes involved in the reception of a
radar signal from a layer of scatterers, such as the layer of vegetation
on the ground. The radar set transmits a pulse of duration ?, which
travels from the airplane toward the target at the velocity c. A particular
scatterer will be illuminated for a time equal to the pulse length,
as the advancing pulse goes by, and will send a reflected pulse of the
same duration back to the receiver. The signal received at a time t,
measured from the moment the transmitter begins to radiate its pulse,
will consist of contributions from all those scatterers Which lie within
the antenna beamwidth a and within a range interval AR, where
R’ – AR = ~c(t – r), (27)
R{ = ~ct.
in thk guide is greaterthan the free-spacewavelength (Sec. 11.3), and the phaseof
this component can be advanced over that of the other component by any desired
amount,simplyby choosingthe widthof thesheetsalongthe directionof propagation
appropriately. SeeW. E. Kock, ‘‘ Metal PlateLensesfor Micro waves,” BTL Report
hlM-45-160-23.
1By R, E. Clapp.