KXI'KK I HUNTS AT AiNTIGl A 



71 



tweon the field strength and the number of modes 

 that, theoretically, are transmitted by the duet. On 

 the Other hand, there is a very pronounced inverse 

 correlation between the height of the inversion layer 

 and the strength of the received signal. This is just 

 what should he expected on the basis of reflection, as 

 distinguished from ray bending, from the elevated 

 layer of M inversion. The principle of this reflection 

 phenomenon has previously been outlined at the end 

 of Chapter 6, Section 6.7. Further study shows that 

 the rate of change of the field intensity and its varia- 

 tion with frequency are just of the magnitude re- 

 quired by the theory. Figure 15 shows a ray-tracing 

 diagram on which the paths of the reflected rays are 

 indicated. Summarizing the results of this experi- 

 ment, it may be said that the phenomenon of reflec- 

 tion from an elevated layer has been well established 

 qualitatively and, in some respects, quantitatively. 

 The meteorological conditions at San Diego are rather 

 singular, and so far such reflection occurring in a 

 systematic fashion has not been described elsewhere 

 though indications of similar effects have occasional- 

 ly been reported. 



Another transmission experiment was made by 

 the Navy Radio and Sound Laboratory in the 

 Arizona desert in December 1944. 188 The path was 

 nonoptical, 47 miles long, and the frequency used 

 was 3,200 mc. The desert air is extremely dry so that 

 the contribution of water vapor to the refractive 

 index is small and the change in M owing to changes 

 in humidity with height is nearly negligible. During 

 the clear nights a pronounced temperature inversion 

 develops from radiative cooling of the ground, a 

 ground-based duct thus being formed. The received 

 field strength varied in close correlation with the 

 formation and disappearance of the duct, with a 

 pronounced diurnal period. The overall results of 

 this, experiment are again in excellent qualitative 

 agreement with the predictions of the duct theory. 

 At the same time the experiment also furnished an 

 opportunity for studying the development over land 

 of low temperature inversions which are valuable 

 for radiometeorological forecasting. 



85 EXPERIMENTS AT ANTIGUA 



Operational experience in the Pacific Ocean led to 

 the conclusion that low ducts are very common over 

 the ocean surface in subtropical and tropical climates. 

 In order to study these ducts, an experiment was un- 



dertaken by the Naval Research Laboratory in the 

 spring of 1945. m The island of Antigua, one of the 

 Leeward Islands of the Lesser Antilles in the British 

 West Indies, was chosen as the site. The prevailing 

 winds there are northeasterly and the air has an over- 

 water trajectory of several thousand miles before 

 arriving at the island and is therefore considered 

 characteristic of large portions of the central Atlantic 

 and Pacific oceans. There is almost no diurnal and 

 only a limited seasonal variation in the air at the 

 lowest levels. 



Equipment for the transmission experiments was 

 comprised of S-band and X-band sets provided by 

 the Radiation Laboratory, MIT. The transmitters 

 with parabolic antennas were mounted on a ship at 

 heights of 16 and 46 ft. There were two parabolas for 

 each height and each frequency, one set pointing to 

 the stern and one to the bow, so that measurements 

 could be made on both the outward and inward runs 

 of the vessel. Receivers were located at heights of 14, 

 24, 54, and 94 ft on a tower at the edge of the water. 

 Monitoring and automatic recording were similar to 

 those used in the transmission experiments pre- 

 viously described. Records were obtained while the 

 ship was traveling away from the receiving station 

 and again on its return. Signals could usually be de- 

 tected up to 190 miles for some combination of 

 transmitter and receiver heights. Direction finding 

 equipment was used for keeping the ship on its course, 

 and fading of the signal caused by the ship's being 

 off course could be readily detected and rectified. 



An extensive program for measuring low-level 

 M curves paralleled the transmission measurements. 

 Since the weather conditions at Antigua are quite 

 steady there is little variation in these curves, as 

 shown by two typical ones illustrated in Figure 11 of 

 Chapter 7. The low-level duct indicated by these 

 graphs has been found present at all times in this 

 location. 



Typical field strength records for the S band and 

 the X band are shown in Figures 16 and 17, respec- 

 tively, the most outstanding feature being the varia- 

 tion of field strength with antenna heights. For the 

 S-band transmission, the field strength increases 

 slightly with increasing antenna height but not nearly 

 so fast as it would under standard conditions. For 

 the X band, on the other hand, the field strength, as 

 a rule, is increased by lowering the antennas. This 

 behavior can be explained on the basis of the mode 

 theory of duct propagation as outlined in Chapter 6. 

 For the shorter wavelength X band, we have genuine 



