SITING AND COVERAGE OF GROUND RADARS 95 
diffracted wave is virtually constant for a given 
angle when the distance from the edge is large. 
Equation (22) may then be written in the form 
v= — (47) 
where @, is in radians (1 radian = 57.3°) measured 
from the geometrical shadow line (Figure 7) and 
d; is in the same units as \. This equation is approxi- 
mate, and the error is of the order of a/b. 
Where the shield consists of several ridges close 
together, an equivalent shield is used instead of 
successive shields. The height and distance of the 
equivalent shield is found by constructing a triangle 
between the radar and the reflecting object which 
encloses the shielding ridges. The apex of this triangle 
is then treated as though it were the diffracting 
edge. In Figure 41 H and d;, are the quantities to 
be used in equations (10) and (47). 
The general procedure.to be followed in preparing 
— Tut aN 
SS 
——_ 
TO DISTANT 
en OBJECT 
Figure 41. The equivalent shield. 
a prediction of permanent echoes will now be out- 
lined. By examining a topographical sheet the azi- 
muths are determined at which profiles should be 
prepared. This will normally be about every 10 
degrees. Where the shielding is obviously good the 
interval may be 20 degrees, but where the terrain 
is questionable such as a region of low hills the 
profiles should be taken at 5-degree intervals. The 
profiles are prepared and the angle of the line of 
sight determined as described above. 
The next step is to make an overlay of a map of 
scale 1 to 1,000,000. The principal features as coast- 
line, towns, and rivers are sketched in to aid in 
reading the completed chart. On this is drawn a 
polar coordinate system with azimuths marked every 
10 degrees and range circles every 10 miles out to 
the full range of the indicator. 
On the overlay are now drawn the coverage 
contour lines. These lines represent the limits of 
the heights of the shielded regions. Targets or 
mountains below these coverage contours will not 
be visible except by diffraction, and targets above 
the contours are in line of sight and receive direct 
radiation. For each azimuth and the corresponding 
angle of sight (Figure 36) the ranges are plotted for 
various contour heights as 1,000, 5,000, 10,000, and 
15,000 feet. Where these coverage contour lines are 
close together the shielding is good but the coverage 
is poor; where the lines are widely separated, the 
shielding is weak, and toward the sea there is no 
shielding except by the horizon. 
With the coverage contour diagram superimposed 
on a map, the peaks exposed to radiation may be 
noted. The extent of the echoes due to these peaks 
depends on the horizontal radiation pattern and 
pulse width. The horizontal beam width is only a 
very rough measure of the width of an echo, and 
some other angle usually between the half-power 
points and the nulls will determine the echo width. 
The angle may be estimated by considering the range 
and size of the peak. The extension of the echo in 
range will be at least as great as the pulse width in 
miles, which as it appears on the indicator is about 
0.1 mile per psec. Actual echoes are usually much 
wider than this, as all of the exposed hill sends back 
an echo. 
The echoes are then sketched in, based on inspec- 
tion of the profiles. The plotter’s judgment is a very 
important factor, but the following rules may be 
used as a guide. 
1. Shade in a circle for the main pulse several 
miles wide, depending on the pulse width and local 
return. 
2. Consider each profile in turn and for each peak 
or hillside in front of the shielding plot an echo on 
the main and all sidelobes. 
3. A series of sharp hills within the shielding 
region should be plotted as a single echo rather than 
a number of echoes. 
4. The inner edge of an echo should be at the same 
range as the hill, and its extension depends on the 
slope of the hill and the pulse width, which may be 
several miles with some sets. 
5. In case of doubt plot the echo. 
6. Peaks beyond the shield may be in the diffrac- 
tion region and the relative intensity of the radiation 
at these peaks will then be obtained from Figure 40 
as described above. 
7. If the mountain is large enough to intercept 
several lobes, the interference effects may be ignored. 
The echo strength may be estimated roughly as 
proportional to the cross-sectional area of the moun- 
tain, the relative intensity of the radiation from 
Figure 40, and the inverse square of the distance. 
For side and back lobes an additional factor is 
required. 
8. The 1 to 1,000,000 scale map should be carefully 
checked to make sure that no peaks are missed in 
between the azimuth vonsidered or at extreme ranges. 
In the above method much is left to the judgment 
of the plotter but it will be found that with experience 
a reasonably good estimate of permanent echoes may 
be made from a map. 
Example 8.,Profile Method. A detailed example of 
a difficult site will be worked out, and comparison 
will be made with the actual recorded echoes. The 
site selected is that of Figure 35. The characteristics 
of the SCR-270B radar-are given in Table 2. 
