SITING AND COVERAGE OF GROUND RADARS 93 
of the sonar head. 
In general the profile method should be used on 
long waves or on microwaves where only a few sites 
are being considered. It is well adapted for the 
estimation of nonstandard atmospheric effects. For 
air- or ship-borne radar the RPD or supersonic 
methods are convenient because of the large number 
of aspects involved. It may be noted that the latter 
two methods should not be considered more exact 
than the profile method, as the principle of similitude 
does not apply unless all elements including the 
wavelength are changed in proportion. The principal 
difficulty is to secure a source which has the same 
radiation characteristics as the antenna system. 
Prediction by Profile Method 
The profile method will be described in detail. The 
discussion will refer chiefly to VHF radars in a 
mountainous terrain, but the methods have general 
application. The principal requirements are topo- 
graphic maps of the surrounding area with a scale 
of 1 or 2 miles to the inch and a contour interval 
of 20 ft, although intervals up to 100 ft may be 
used. Maps with a scale of about 20 miles to the 
inch are needed for checking distant echoes. Regional 
aeronautical maps, with a scale of about 1 inch to 
16 miles and 1,000-ft contours, are suitable as the 
height of prominent peaks is indicated. 
From the maps, profiles are prepared for various 
azimuths about the radar station. The first mile or 
so should be plotted accurately, and at greater 
distances the critical points such as hills and breaks 
should receive the most attention. A convenient 
scale is 2 miles to the inch for range and 500 ft to 
the inch for elevation. The distances to which the 
profile should be plotted is a matter of judgment, 
but it should be extended to perhaps 20 miles, or 
further if there is doubt. 
On each profile is drawn the tangent line from the 
center of the antenna to the point on the profile 
which determines the shielding, as in Figure 36. 
eed 
Seed in 
RANGE IN M LES 
1000; 
FEET 
wo 
to) 
Figure 36. Typical profile. (Note:.Y in degrees). 
eee 
This is the line-of-sight curve; it is drawn for each 
azimuth, and the vertical angle 7 is marked on the 
profile. If the angle is below the horizontal it is 
negative, and caution must be used on high sites 
not to exceed the limiting shielding angle of the 
radar horizon. This is given by the expression 
y = —0.0108 V/ 2h , (45) 
where vy is the angle ‘between the effective horizon 
and the horizontal at the antenna in degrees and hy 
is the height of the center of the antenna in feet. 
The line of sight is actually curved, as explained 
in the section on visibility problems, but for ranges 
up to 10 miles the error in using a straight line is 
small. For longer distances the dip QX as computed 
from equation (5) should be considered. More con- 
venient for this purpose are the curves of the line 
of sight for various angles which are calculated from 
Figure 37. Standard refraction is taken into account 
by use of % a instead of a for the earth’s radius, 
5a = 1.33 X 3,960 = 5,280, 
d (46) 
he — hy = 5,280d tan y + 9 
with h; and he in feet and d in miles. Above 10°, or 
where the shielding is distant, equation (8) should 
be used. 
ANTENNA Was 
ee 
hy 
t) 
CENTER OF EARTH 
Figure 37. Line-of-sight geometry. 
These curves are plotted in Figures 38 and 39, and 
their use is illustrated in Figure 36. The center of 
the antenna is at 200-ft elevation, and the height 
of the shielding ridge 4 miles away is 400 ft. For a 
200-ft rise in 4 miles the angle is found from Figure 
38 to be 0.5 degree. This curve can then be used to 
determine the height of the shielded region at other 
ranges. Thus the range at which the shielded region 
is 4,000 ft high for the case considered is found from 
Figure 39 by using he — h; = 4,000 — 200 = 3,800 
ft for height and the 14-degree curve, giving 53 miles. 
It is desirable to be able to estimate diffraction 
effects in a simple fashion suited to the approximate 
nature of this kind of work. As shown in Section 
15.4.8 the field intensity varies in a rather compli- 
cated manner with the diffracting angle 0, and the 
distance of the shield d; [Figure 7 and equation (10) ]. 
In Figure 40 is plotted the relative field intensity 
compared to that obtained without a shield for 
