126 TECHNICAL SURVEY 
Analysis of Test Data 
INCOMING MEDIUM BOMBER ~ 12000 FEET 
Cas 
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beens et MILES 
SIGNAL /NOISE RATIO 
FicureE 76. Typical signal-to-noise test data. 
In Figure 76 is shown a signal-to-noise graph for 
a station similar to Example 20. The noise is set at 
a relative height of 1, and the signals are read in 
proportion as the plane comes in. The weak signals 
at medium ranges are due to shore line diffraction. 
The peaks correspond to lobe maxima and ranges at 
S/N = 1 to the locations .of the lobe contour at 
12,000 ft. The receiver in this case is of the ‘linear’ 
type, and the lobe maxima may be obtained by 
extrapolation. Along a line of constant path difference 
such as the maxima of the lobes the signal-to-noise 
ratio varies as the inverse square of distance. Thus 
the fourth lobe has a peak S/N ratio of 6 at 68.5 
miles, and the lobe length is L = 68.5+/6 = 167.5 
miles. 
In practice the length computed in this manner 
would be compared to those obtained from tests at 
other altitudes. Notes made during the test and other 
factors would be considered and the data weighted 
accordingly. For example, at 90 miles the S/N ratio 
is 2 and the percentage error is probably greater than 
on the reading at 68.5 miles. The location of points 
on the lobe cannot be read with accuracy from Figure 
76 at S/N = 1 since this is threshold data which 
may be in considerable error. 
To determine the maximum free space range F, 
the lengths of the lobes obtained from the test data 
may be listed along with the site factors from equa- 
tion (99) or equation (107). A value of F is then 
selected which will most nearly fit the test data. 
Variations in performance of the equipment affects 
the lobe lengths in proportion. Variations from the 
standard atmosphere assumed will shift the position 
of the lobes, particularly at low altitudes. 
Where better accuracy is desired or the receiver is 
nonlinear, the calibrated receiver method is required. 
Such data are recorded as gain voltage; range, and 
time. For each gain voltage, the equivalent receiver 
input voltage is read from a calibration curve such: 
as Figure 74. The equivalent value of the noise 
voltage of this set is 30 wv. Dividing the equivalent 
receiver signal voltages by 30 gives the S/N ratio 
which is plotted against range in Figure 77. The 
lobes are identified by reference to a lobe angle 
diagram. The extrapolated lobe lengths may be listed 
as follows: 
Height, feet Length of lobes, miles 
1 2 3 4 
20,000 243 196 235 
10,000 159 212 169 162 
5,000 156 216 163 158 
The 20,000-ft data were taken last and indicate the 
effect of certain equipment adjustments. The ability 
to maintain this performance is one of the questions 
to be considered in arriving at a weighted average 
value of lobe lengths. Comparison -of these lobe 
lengths with the computed lobe factors will indicate 
a fair value to be used for the free space maximum 
range. 
SIGNAL/NOISE RATIO 
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HY 
Rea ais 
5000 FEET 
80 a 
MILES 
Ficure 77. Test flight data from a calibrated receiver. 
