Radar System Engineering

50 THE RADAR EQUATION [SEC.212
The effect of the interference described by Eq. (28) is to break up the
original radiation pattern of the source at A into a lo’be structure, as
sketched in Fig. 2.11; at least this is an appropriate description of the
effect at a considerable distance from the source. The lowest maximum
in Fig. 2.11 occurs at an elevation angle of A/4hl radians. Below this
angle the field strength diminishes until, at the reflecting surface itself,
it vanishes. X’o modification of the above argument is required for a
diwcti~e antenna at A, unless the directivity is high enough to affect the
relative intensity of the waves traveling along All and AM (Fig. 2.9)
respectively. In any case, the width
of an interference lobe will be less
than the width of the primary radiation
pattern since MI will certainly
be greater than the vertical aperture
_~ of the antenna.
_&=,,,/, ------
A
‘- A,/4hl
------
As the geometry of the problem
is not altered if we interchange
I:IG. 2.1 I.—Lobes causedby interference transmitter and target, the interferondirectandreflectedwares.
ence must affect the return of the
radar echo in the same degree as it affects the pulse transmission. Therefore
the required modification of the radar equation is obtained by multiplying
by F’ the expression given in Eq. (3) for the received signal po~ver
S; that is,
ll=P~,16sin4 * (29)
()
To discuss the practical consequences of this we first distinguis!l
between the problems of “ high coverage” and “ low coverage, ” In
the former case we are concerned with targets so high or so close that
2rh,hZ/kR > 1; that is, with targets lying well up in, or above, the
lowest lobe of Fig. 2.11. Evidently the detection of such a target will
depend critically on its range and altitude. lf an airplane flies in toward
the radar at constant altitude hZ we must expect the signal to vanish
and reappear repeatedly, as the lobe structure is traversed. The smaller
x and the larger h,, the more finely divided is the lobe pattern. A
welcome change, on the other hand, is the additional factor of 16, effective
in Eq. (29) for directions of maximum constructive interference. This
factor should double the maximum range of detection, compared to freespace
conditions. Something like this increase is actually observed,
although it should not in every case be taken for granted, in view of the
idealized model upon which it is based.
“Low coverage” refers to a situation in which the target lies well
below the lowest maximum in Fig. 211, in the region where