38 TECHNICAL SURVEY 
different slopes in the curves of power versus dis- 
tance. This probably indicates that two different 
modes predominate in these two regions. The pattern 
shown in Figure 16 can occur if for some distance 
near the ground the height-gain function of the 
second mode is greater than that of the first mode. 
The second mode, however, is attenuated more 
rapidly with distance than the first. At moderate 
distances from the transmitter the second mode 
prevails, but at greater distances it will become 
smaller than that of the first which decreases less 
rapidly with distance. 
Finally Figure 18 shows a set of curves for attenua- 
tion versus distance of the target for an X-band radar 
on Antigua. Again it is evident that, on the whole, 
the lowest elevation of the radar gives the largest 
signal strength. 
RADAR ECHO STRENGTH VS RANGE 
DECIBELS 
5 10 15 20 25 30 35 40 45 50 
RANGE NAUTICAL MILES 
Ficure 18. Radar echo strength as function of range. 
X band, Antigua experiments. Target is a PC boat. 
ANGLE-OF-ARRIYVAL MEASUREMENTS 
Because the effects of nonstandard propagation 
are most pronounced at great distances from the 
transmitter, they are most important for early warn- 
ing radar and communication work. These effects 
were investigated earlier than the question of the 
deviation of the angle of arrival from that prevailing 
in a standard atmosphere. This deviation, though 
small, may nonetheless be significant for fire control 
radars operating in the microwave band. The angle 
of arrival may vary by several minutes of arc because 
of ducts, and this effect was first studied systema- 
tically by BTL in 1944.19 182 
Figure 19 is a schematic view of the receiving 
antenna used for such measurements. This antenna 
is a section of a parabolic cylinder arranged so that 
its beam, at the center of swing, is directed toward 
the transmitter, this being the angle at which waves 
arrive on a day with standard propagation. The 
antenna measures the vertical angle of arrival and 
a duplicate antenna rotates about a vertical axis 
and measures the horizontal angle. The antennas 
are periodically swung through an angle which is 
set to include the largest variations of the angle of 
6 INCHES WIDE (I5° HORIZONTAL BEAM) 
3 
20 FEET ae 
ANTENNA SWINGS 
+.75~ 
Ficure 19. Sharp-beamed antenna for angle-of-arrival 
measurements. 
arrival. Figure 20 shows a typical record of received 
field strength versus time for a periodic swing, the 
upper record representing the presence of a direct 
ray only, and the lower indicating both a direct and 
a ground-reflected ray. 
Observations near New York during the summer 
of 1944 were made on two optical paths 24 and 12.6 
miles long with a common receiving antenna. These 
measurements are estimated to be accurate to 0.04 
degree, and they indicate that the greatest variation 
of the horizontal angle of arrival is 0.10 degree. 
Fluctuations within this magnitude, however, are 
quite common. The maximum in the vertical angle 
for the long path was 0.46 degree above the standard 
for the direct ray and 0.17 degree below the standard 
for the reflected ray. No correlation between depar- 
tures from the standard of the direct ray and the 
ground-reflected ray has been observed. When the 
direct ray was 046 degree above the standard, it 
was apparently being trapped and no reflected tay 
was observed. The greatest spread observed between 
the direct and reflected rays was 0.75 degree, as 
compared to a standard of 0.35 degree. The variation 
of vertical angle over the short path was less than 
over the long one, the greatest change in angle being 
an increase of 0.28 degree over the standard for the 
direct ray while that of the groun/-reflected ray 
was too small to be observed. 
For early warnimg radars where the target is 
perhaps 75 to 100 miles away, the difference in 
bending of the rays between standard atmospheres 
of moderate and warm climates becomes appreci- 
able. In this case differences in estimated height 
vary by as much as 2,000 ft, if the target height is 
determined by the first signal in the lowest standard 
lobe. 
A_ONLY (RECORD 1) 
SIGNAL AMPLITUDE 
A+B (RECORD 2) 
kio SEC 
TIME ——> 
Figure 20. Typical record of angle-of-arrival measure- 
ments. Top, direct ray only. Bottom, direct and ground- 
reflected ray. 
