30 



TRANSMISSION EXPERIMENTS IN ARIZONA 



14 16 18 20 22 24 02 04 06 08 10 12 14 16 18 20 22 24 02 04 06 08 10 



DECEMBER IS 



Figure 2. Soil and air temperatures at Datelan, December 16 to 17, 1944. 



the method of eahnihiting refractive index from mete- 

 orological data. 



When the modified index lapse rate is relatively 

 constant with altitude, a fairly simple transformation 

 makes the atmosphere nonrefracting and the effective 

 earth radius greater or smaller than the actual radius, 

 and ray tracing should then he valid in the interfer- 

 ence field. If no rays reach the receiver, the diffracted 

 field must supply all tlio energy received. 



The propagaticiu path extended from Datelan to 

 Gila Bend, Arizona, a distance of -47 miles over desert 

 terrain. A 3,200-mc transmitter was located on a tower 

 at a height of 53 ft alcove ground at Datelan, with the 

 receiver 35 ft above the ground in the control tower 

 at the Gila Bend airfield. There is a gentle rise of 

 ground from Datelan to Gila Bend with a total rise 

 in elevation of 40"J ft, or about 8.5 ft per mile. The 

 intervening terrain is remarkably uniform, without 

 trees or large irregularities, and there are no build- 

 ings except in the immediate vicinity of the trans- 

 mitter and receiver locations. 



Figure 1 shows the diurnal variation of surface air 

 temperature at Ajo, Gila Bend, and Phoenix for 

 December 16 and 17, 1944. These typical data show 

 the uniformity of conditions over that region and 

 the effect of radiation cooling on the air mass near 

 the surface. 



Figure 2 shows the variation of the soil tempera- 



ture with time at Datelan for the same period, as well 

 as temperature changes at 25, 50, 100, and 500 ft 

 above the earth. 



The general topography around Gila Bend in con- 

 junction with the diurnal variation in the prevailing 

 surface wind vector shows an interesting condition. 

 Hourly wind vector observations during November 

 and December 1944 showed that liy 1900 the prevail- 

 ing wind was downslope toward the lower elevations. 

 This flow of cold air into the area of the link may be 

 responsible for the overall cooling of the air up to 

 several hundred feet during the night. At present it 

 is not clear how much of this effect should be attrib- 

 uted to radiation and eddy diffusion of heat toward 

 the earth, although on nights with wind speeds from 

 calm to a gentle variable breeze it is difficult to at- 

 tribute the entire transport of heat to the latter 

 processes. 



Some pertinent data are tabulated in Table 1 show- 

 ing the time at which the signal was first detected and 

 completely lost and the general atmospheric and 

 ground conditions nearest these times. On the after- 

 noon of December 14, the sky was overcast, and the 

 signal was detected about an hour earlier than on the 

 other evenings. 



Figure 3 shows the field strength data for a typical 

 day plotted in decibels below free space. The maxi- 

 mum and minimum for half-hour intervals are shown 



