124 TECHNICAL SURVEY 
TaseE 14. The general lobe formula. (Example 21.) 
oy VY ° Ya 
” radians radians Z £ » radians g 
0  —.00600 1.061 2.86 1.000 —.00589 0 
.00421 +.00110 0.920 
.00712  .00492 0.897 
01000 .00835 1.039 
01295 .01163 1.158 
01595 01485 1.158 
.01897  .01802 1.067 
.02200 .02119 0.930 
5.27 0.640 +.00141 165.0 
3.04 0.792 .00527 48.5 
7.56 0.862 .00858 180.5 
2.75 0.908  .01206 36.3 
5.04 0.9388 .01557 178.5 
10.88 0.955 .01894 52.7 
15.00 0.963 .02225 155.6 
02500 .02428 0.779 15.00 0.969 .02544 66.5 
02810 .02744 0.648 11.00 0.973  .02860 137.0 
10 .03110 .03051 0.500 0.00 0.982 .03161 989.1 
11 .03420 .03365 0.408 17.18 0.984  .03453 126.4 
12 .03720  .03669 0.332 43.0 0.987 .03731 96.6 
13 .04050 04005 0.267 74.4 0.991 .04024 120.6 
14 .04330 .04288 0.222 +1089 0.991 .04253 100.7 
15 .04640 .04600 0.190 +1546 0.992 .04493 117.7 
16 .04950 .04912 0.165 +206.2 0.993 .04894 103.0 
17 .05250 .05216 0.143 +263.3 0.993 .05010 116.5 
18 .05560 .05528 0.129 +326.2 0.994  .05264 104.0 
19 .05870 .05840 0.114 +4123 0.995  .05479 115.6 
20 .06170 .06142 0.104 +4926 0.997  .05694 104.5 
21 .06480 .06454 0.094 +590.0 0.998  .05909 114.6 
22 .06780 .06755 0.089 +687.0 0.998  .06127 105.6 
23 .07090 .07067 0.081 +813.0 0.998 .06486 114.2 
COINAEMARWNHO 
0 1 0 tharsrar tf 
+4-+ 
The vertical coverage diagram is shown in Figure 
73. The lobe maxima and minima and the 90-degree 
points have been sufficient for sketching the lobes 
except on the first lobe where a few additional points 
have been computed. When the net angle is 60 
degrees, the field strength at the bottom of the first 
lobe is equal to the free space field. At ranges shorter 
than this the reflected wave opposes the direct wave. 
Directly under the antenna the contour passes near 
‘the surface so that the waves are very nearly in 
opposition. Because of the variation in Dpz with y 
the maxima will not occur exactly when the cosine 
is unity, but this effect is generally negligible. 
CALIBRATION AND TESTING 
One should not infer from the foregoing that a 
reliable coverage diagram can be obtained by calcu- 
lation alone. Under field conditions it is necessary 
to make test flights and other checks before equip- 
ment can be depended upon to meet a calculated 
performance. On the other hand it is seldom possible 
or desirable to obtain a satisfactory coverage diagram 
from tests alone. Best results are attained when tests 
and analysis supplement each other. 
Test flights are arduous, expensive in personnel 
and materials, and time consuming. In most theater's 
a number of agencies become involved, and careful 
planning and organization are required to achieve a 
useful result. For these reasons the amount of test 
flying should be held to a minimum by intensive 
analysis and equipment tests before and after the 
test flights. 
Equipment Tests 
It is difficult to overemphasize the. importance of 
proper equipment maintenance. An unfortunate ten- 
dency of inexperienced personnel is to maintain on an 
emergency basis, rather than as a matter of system- 
atic routine. In most cases the need is for a careful 
check of all elements and restoration to as-good-as-new 
condition, rather than a brilliant intuitive process 
known as “trouble shooting.” One survey of a large 
number of systems disclosed an average reduction 
from optimum performance of 13.5 db. This corre- 
sponds to a maximum range of 50 per cent of normal. 
Careful tests have shown the use of “standard tar- 
gets” to be very misleading in many cases. Large 
changes in’ the maximum ranges of small targets 
were found without appreciable changes in the 
strength of the permanent echoes used for checking 
purposes. 
Full use of test. instruments available should be 
made in checking the equipment. Orientation should 
be completed and the accuracy of range and azimuth 
indicators checked. Tuning and modifications should 
be done before the test flights are made, unless the 
tests indicate poor performance. A great handicap 
in this work is the lack of absolute measures of power 
output, but much may be done with echo boxes and 
field intensity meters. 
Signal Measurements 
Several methods are used for recording signal 
strengths, and these determine the type of receiver 
calibration required. Estimation of signal-to-noise 
ratios by means of scales on the face of the scope 
requires some means of specifying the gain setting. 
The means used, such as height of noise, position of 
gain dial, and so forth, should be calibrated with a 
signal generator so that there is an assurance of 
adequate sensitivity and a way of checking the 
measurements. The saturation line on the scope is 
assigned a height of 10, and the signal and noise 
heights are read in proportion. Ratios in excess of 
10 are usually read as 10+. This method requires 
considerable skill on the operator’s part and is 
limited in scope. In Figure 74 is shown a calibiation 
curve on a typical ‘square law” receiver. In the circle 
is represented a signal on an A scope which would 
commonly be read as a signal-to-noise [S/N] ratio 
of 8. Actually the ratio of receiver inputs correspond- 
ing to the signal and noise heights is 8.5/3.25 or 2.6. 
A considerable improvement over the above 
method may be obtained as follows. An index line 
is drawn on the face of the A scope about an inch 
from the baseline. To measure a signal it is brought 
to the index line by adjustment of the gain control, 
and the gain control voltage is recorded. The gain 
voltage required to bring the noise to the index line 
is also noted occasionally during the test. A calibra- 
tion curve is made using a pip signal generator or a 
modulated signal generator connected to the receiver 
input. The gain voltage required to bring the signal 
to the index line is méasured for various inputs. 
Gain voltage readings on the test target and noise 
are converted by means of the curve to equivalent 
