Radar System Engineering

96 PROPERTIES OF RADAR TARGETS [SEC. 3.14
antenna pattern is discussed in Sec. 2.5, where an expression for the
ideal antenna pattern is derived from an approximate expression for u:
u = (Ra)(L) sin 0. (34)
In Sec. 25 it was assumed that L wasa constant, and a comparison of
Eq. (34) with Eq. (33) shows this to be approximately true, since sec o
differs appreciably from unity only for very short ranges. Based on
Eq. (34), theideal antenna pattern is
G(o) = GO CSC28. (35)
A more exact expression, based onEq.
(33), is
G(o) =GOcsc20~0. (36)
The effect of using an antenna whose pattern fits Eq. (35) rather than
Eq. (36) is to increase the strength of the signals at close ranges above
those at medium and long ranges. This increase is not large, amounting
only to 3 db at 60°; its main effect is to make the altitude signal stronger
than the succeeding ground signals. Radar experience indicates that the
specification of what curve the pattern should follow is less important
than the requirement that the pattern be smooth.
In Fig. 3.20 the diffuse bright rings at the center of the picture are
the result of an imperfectly shaped antenna pattern. These intensification
ringsl make it difficult to distinguish irregularities on the ground,
but they are not as objectionable as the black rings or “holes” which
would have appeared if the antenna pattern, instead of being too strong,
had been too weak in those regions.
Equations (30) and (33), combined, give only the average level S
of ground return. Particular signals vary widely. On the PPI, with a
medium gain setting, ground return can be seen to consist of many
bright signals. Among these bright signals are weaker signals, some
of them too faint to show on the screen. The resulting stippled or
“ beaded” texture of ground return is clearly visible when fast sweeps
are used, as in Fig. 3.25. If the receiver gain control is set high, most
ground signals rise to saturation and ground return takes on the more
flattened texture seen in Fig. 317.
3.14. Mountain Relief .—The presence of hills and mountains in the
area covered by the radar presentation is indicated by the bright returns
from the mountain sides facing toward the aircraft and by the shadow
regions on the far side of the crests. These conditions follow directly
from the geometry of the illumination, and produce a very realistic
I ‘l_he ratherdiffuseringsof intensificationshouldnot be confusedwith the range
markers,which are narro~verand appear at equal radial intervalsout to the edge of
the picture.