74 



TRANSMISSION EXPERIMENTS 



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. 10 ' 182 



Figure 19 is a schematic view of the receiving 



T 



20 FEET 



1 



6 INCHES WIDE (15 HORIZONTAL BEAM) 



ANTENNA SWINGS 

 ±.75 " 



Figure 19. Sharp-beamed antenna for angle-of-arrival 

 measurements. 



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 



A ONLY (RECORD 1) 



| \ UP / \D0WN / \ UP / \00WN / \ UP / \ 



• v7v v7 v V7\ 



MO SEC-I 



A + B (RECORD 2) 



Fiquke 20. Typical record of angle-of-arrival measure- 

 ments. Top, direct ray only. Bottom, direct and ground- 

 reflected ray. 



set to include the largest variations of the angle of 

 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 0.46 degree above the standard, it 

 was apparently being trapped and no reflected ray 

 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 ground-reflected ray 

 was too small to be observed. 



The Evans Signal Laboratory analyzed some 

 low-level meteorological records which were made 

 simultaneously and in the near vicinity of these 

 transmission experiments. The angles of arrival were 

 determined by ray-tracing methods and were in 

 satisfactory agreement with the observations. The 

 difference between angles in standard atmospheres 

 of various climates was also analyzed theoretically, 

 and the results show that the maximum angular 

 deviations are less than the tolerance of present-day 

 fire control equipment. 



For early warning 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. 



Measurements of the angle of arrival by BTL 

 were continued in 1945, and the results, though not 

 yet published, give more details and corroborate the 

 previous observations. During the second half of 

 1945, the Electrical Engineering Department of the 

 University of Texas has embarked on a program 

 to study the angle of arrival. 



