32 



TRANSMISSION EXPERIMENTS IN ARIZONA 



so that the fading range is apparent. The meteorologi- 

 cal data for the period are given in the form of modi- 

 fied refractive index curves relative to a fictitious earth 

 radius of 4a/3, the time of the sounding being given 

 on each curve. 



The diurnal variation in field strength is quite pro- 

 nounced and regular. The maximum range of fields 

 measured vas around 46 db, the maximum field gen- 

 erally occurring at times when the inversion layer was 

 thickest. There is no significant correlation between 

 strong fields and the amount that j\I decreases at some 

 elevation above the antennas. In fact, at times such 

 as 0900 on December 17 the field strength is quite 

 high and yet il/ shows little indication of trapping. 

 In most cases strong fields occur at times when the 

 4ff/3 modification of the index of refraction gradient 

 is near zero or varying slowly with altitude. 



The general results of this experiment may be sum- 

 marized in the following way. Over the desert loca- 

 tion a ground-based temperature inversion was found 

 each night due to radiation cooling of the under- 



lying surface. This temperature inversion produced a 

 strong index of refraction gradient in the first few 

 hundred feet above the earth. 



The 10-cm nonoptical link showed a marked diur- 

 nal variation in field strength in close correlation 

 with the building up and intensification of the tem- 

 perature inversion. The strongest fields generally ac- 

 companied modified index gradients approaching zero 

 in the first few hundred feet above the earth's surface. 



The trapping criterion most widely accepted here- 

 tofore specifies that, at some elevation above the 

 transmitter and receiver antennas, M should be less 

 than at the antennas. The data herein reported seem 

 to indicate that this criterion is neither necessary nor 

 sufficient to insure strong fields below the optical 

 horizon. The strongest fields observed at 10 cm ap- 

 proached the flat earth value, assuming a reflection 

 value of unity for the earth.'' 



''Section 5.4 will be of interest in connection with this 

 chapter. 



