Radar System Engineering

74 PROPERTIES OF RADAR TARGETS [SEC. 38
‘—-m—–-——” “ Because of interference, the received power varies
,-:
LA ~
I between O and 4 times the po~ver received from
one of the targets.
A beautiful example of the change of signal
power with 1, as predicted by this formula, is
given in Fig. 3,6, which sho~vs sucmssive frames
...
?’LJ_
0
,g
0
a-—l
IO ...
1-
1.4 .:
,*
“k,
o’ . .
.- —.. .-J
FIG. 3.6.—Successive
frames of a photographic
recording of an A-scope
trace at +sec intervals.
of a photographic recording of an A-scope trace
at +T-sec intervals. These recordings \rere obtained
on Deer Island in Boston Harbor, The
signal on the right-hand side is received from
the antenna towers of Radio Station WBZ. The
.,
c+)~ ? signals received from these two towers overlap
~L_.JL in the center and there show the effects of destructive
and constructive interference. In the
fd
last frame (No. 0311 ) and in the enlarged frame
(No. 0479, Fig. 37) the parts of the signal \vhere
there is no overlapping are separated by a gap of
zero power due to destructive interference in the
overlapping region. In Frame h-o. 0303 and in
Frame No. 0538, Fig. 37, we have full constructive
interference. The deflection of the A-scoue .
used w-as not linear in the power, so that the
highest deflection with constructive interference
is only 3 times that of the individual signals,
whereas the power itself is 4 times the power
received from the towers individually. The
change in power is due to changes in the distance
1 arising from a swaying motion of the towers.
The other possible cause, change of wavelength
due to change in frequency or refractive index,
has been ruled out.’ Correlation was actually
observed between the wind velocity and the rate
of fluct uat ion. The towers would have to move
only one inch relative to each other to change
from constructive to destructive interference.
In Eq. (23) we note further that the return
power from a complex target consisting of two
equal isotropic scatterers a fixed distance apart
depends upon k, and that the change in signal
for a given wavelength change increases with the
value of 1. This fact has been applied in a
18ee Propagation of Short Radio ~~aves, Vol. 13, Sec.
6.17, Radiation Laboratory Series.