100 TECHNICAL SURVEY 
ALTITUDE IN FEET 
SITE DATA 
hy= S00 FEET BHORE LINE = 3 MILES 
£2200 MS ANTENNA—FIG. 63 
< =1 TARGET- MEDIUM BOMBER 
Ket 
0"_I9 
Ficure 46. Vertical lobe diagram. 
angles and numerous lobes. The need for good rein- 
forcement and tactical considerations lead to the 
use of the sea as a reflecting surface where feasible. 
‘The general high-sited radar problem will be ana- 
lyzed in detail, and the use of approximate, simplified 
methods of calculation will be described where 
applicable. 
The Vertical Coverage Diagram 
The object of test flights and field intensity 
calculations is the construction of the vertical cover- 
age diagram. A typical diagram for a long-range, 
early warning, VHF radar is shown in Figure 46. 
'The contours or lobes on this diagram represent the 
locus of all points in space along a particular azimuth 
where an incoming plane of standard type, usually 
a twin engine medium bomber, will produce a mini- 
mum detectable signal. A minimum detectable signal 
is ordinarily taken to be one that has a signal-to-noise 
ratio of unity. This may also be expressed in other 
terms such as field intensity or voltage at the 
receiver terminals. For other types of planes, or a 
number of planes, or different aspects of the same 
plane, the lobe pattern has a different size. 
It will be noted that the vertical scale is nearly 
10 times as great as the horizontal scale, causing a 
marked distortion in angles and crowding of angles 
above 10 degrees. The lines of constant altitude are 
parabolas, owing to the curvature of the earth. Their 
shape is given by the equation 
d? - 5,280 
Pig 
Here y = the ordinate measured from the horizontal 
line through zero; 
GS (48) 
h = the height of the curve at zero range, in 
feet; 
distance along the earth in miles; 
radius of the earth in miles; 
ka = equivalent earth radius. 
aa 
ll 
TO DISTANT TARGET 
Ficure 47. Flat earth ray diagram. 
For standard conditions k is taken as %. At 40° 
latitude the radius of the earth is 3,960 miles. Sub- 
stituting in equation (48) gives the convenient 
relation: 
d? 
- (49) 
y=h 
with y and h in feet, and d in miles. 
Thus in Figure 46 a medium bomber coming in 
at 5,000 ft would first be detected at 108 miles, the 
signal would increase in strength, reaching a maxi- 
mum around 96 miles, and then decrease and be lost 
at 84 miles. In the null region between 84 and 77 
miles there would be no detection. Similar regions 
of detection and nulls would be encountered as the 
plane came in closer. The nulls do not come into 
the origin when the direct and reflected rays are 
