442 



TABLE 457A.— ATTENUATION COEFFICIENTS FOR VERY LOW FREQUENCY 



RADIO PROPAGATION 



For very low frequencies (100 kc and under), an empirical transmission formula of 

 the form 



377ju rr z 0- 



t - \d \ sin 6 X ' 



has been found useful (Austin-Cohen; Austin; Espenschied, Anderson, and Bailey), where 



F = received field intensity, in fi v/m 



h = effective height of transmitting antenna, in km 



/ = transmitting antenna current, in amp 



6 = transmission distance, in radians 



d= transmission distance, in km 



X = wavelength, in km 



Values of a and x were found to vary somewhat. 



Since theoretical justification for the Austin-Cohen value of x = i has been given by 

 Watson (Proc. Roy. Soc. London, A, vol. 95, p. 546, 1919), data furnished by the above 

 observers have been reevaluated, assuming validity of the relationship 



F = 



377 h 



\d 



\ sin 



Xe 



— ad 



and the resulting values of o presented in the accompanying table, together with their rela- 

 tive weights estimated from the number of observations used in their determination. 



a varies notably with frequency, time of day, and the type of ground along the trans- 

 mission path, and less definitely with season, solar activity, and the location of the trans- 

 mission path. The values presented here are for conditions where the entire transmission 

 path, at the height of the ionospheric reflecting layer, lies in daylight or in darkness. For 

 conditions of sunrise or sunset on the transmission path, a has generally been found to lie 

 between day and night values, but under certain circumstances, to far exceed these values. 



Transmission path 



Observations 

 by 

 Austin 



Espenchied, 



Anderson, Bailey 



Austin 



Austin 



Espenchied, 



Anderson, Bailey 



SMITHSONIAN PHYSICAL TABLES 



