Radar System Engineering

SEC. 15.8] CHOICE OF BEAM SHAPE 603
provided by addition of a third system, but the additional complexity
was thought to be operationally not worth its cost.
Reflection of radiation from the earth’s surface greatly influences
the radar beam characteristics in elevation angle, and such effects must
be given careful consideration. Over water, the reflection coefficient is
high for all wavelengths of interest for air surveillance, and for very small
angles of incidence it can be assumed to be 100 per cent. Over land,
the reflection coefficient varies markedly with detailed surface shape and
with the composition of the terrain, and in general increases with increasing
wavelength. Adequate data are not available on the reflection
coefficient, but at wavelengths of 10 cm or less it is assumed to be normally
negligible, and at a few meters it is normally substantial.
~
ẓ
2
::
5 c .- 1
s
2
g
g
<
0
Rance
in miles
FIG. 15.5.—Calibrated vertical-coverage diagram of actual radar using 7-in. B-scope
with antenna rotating at 2 rpm. Effective antenna height is 100 ft. Target is a single
B-24.
The detailed effects of surface reflection which influence coverage are
very complex. Some of the more important effects and conclusions
are summarized below (see also Sec. 2.12).
1.
2.
3.
4.
If surface reflection is high, the lowest possible angle of coverage is
raised from the horizon to an angle a = k/2h, where a is in radians
and h is the height of the antenna above the surface.
If surface reflection is low, radiation striking the surface is lost.
It is clearly desirable to minimize this loss by beam-shaping.
Over an angular region in elevation where reflected intensity
equals direct intensity, R~,, at the center of an interference lobe
is doubled, but midway between two lobes R~.X = O.
At long wavelengths (one meter or greater), the use of reflected
radiation reduces requirements on antenna size, but the nulls