516 



BELL SYSTEM TECHNICAL JOURNAL 



Effective Earth Radius 



Air Mass Type Summer 



Tropical Gulf— r^ 1.53 X i? 



Polar Continental— Pc 1.31 X i? 



Superior— 5 1.25 X R 



Winter 



1.43 X R 

 1.25 X R 

 1.25 XR 



" R" = actual earth radius 



The boundaries between different air-mass types furnish discon- 

 tinuities adequate for radio reflections. The greater the stabiUty 

 of the boundary, the more abrupt it is Hkely to be. In general, when 

 "5" air overlays either "Tg" or "Pc" air, the resulting boundary is 

 stable. Possible discontinuities, for the three types discussed, may be 

 summarized in the following table. Here the positive sign means that 

 the radiation originates in the more refractive medium. For stability 

 the lower medium is the denser though not necessarily the more 

 refractive. 



Concluding Remarks "^ 



The characteristics of this seventy -mile circuit indicate that for 

 ultra-short-wave transmission it rates as a long distance one. If we 

 assume that the air refraction is on the average such that the effective 

 earth's radius is 4/3 the actual one, then the receiving station lay 1400 

 feet below the line of sight from the transmitter. This is equivalent to 

 0.57° below the horizon. The reception, using high effective-height 

 antennas, was good; there was, however, very little lee-way left, above 

 set noise, for reception with simple doublet antennas. Any longer 

 circuit will require to be terminated on elevations such as to keep the 

 intermediate horizon height down. The fading was too slow to be 

 noticeable on amplitude modulated speech unless a deep minimum or 

 drop-out occurred. 



The circuit was probably unusable for television, most of the time. 

 A system adhering to the R.M.A. standard ^^ of 441 Ungg on an inter- 



