443 



TABLE 457B.— ATTENUATION IN HIGH FREQUENCY PROPAGATION OVER 



LONG DISTANCES 



At high frequencies and distances where the radiation is chiefly received by means of 

 sky-wave transmission, reference is given to the methods for calculation of received field 

 intensities presented in Chapter 7, National Bureau of Standards Circular 462, "Ionospheric 

 Radio Propagation." 



For long transmission paths (over 4000 km), 



F = F + i log P - So J Q K d 



where F = log of the received field intensity, in — 



m 



Fa = log of the ionospherically unabsorbed field intensity, in 



effective radiated power 

 = 1.6 — 1.44 [log d — 3.60] 

 d = transmission distance, in units of 1000 km 

 P = effective radiated power, in kw 

 log 5*0 = 0.502 - 1.916 (log / — 0.477) 

 / = frequency, in Mc 

 Q = 1 4- 0.005 R 

 R_ = sunspot number 

 K = average K for the transmission path 

 K= 0.142 4- 0.858 cos * 

 \p = solar zenith angle 



Kd = 0.142 D' + (Ki + K 2 — 0.284) tan -^ 



1 K 



f-V 



for 1 kw 



where D' = the length of the path in the region where K is not equal to zero, in 



units of 1000 km 

 Ki and Ki= values of K at transmitting and receiving stations 

 R = radius of the earth in units of 1000 km 



J= seasonal variation factor. / has the values 1.0, 1.3, 1.15, respectively, if both ter- 

 minals of the transmission path lie in summer, winter, or equinoctial regions. If one ter- 

 minal lies in a summer region, the other in winter, / = 1.15. 



TABLE 458.— E-LAYER MAXIMUM USABLE FREQUENCIES IN Mc FOR 

 2,0004<m TRANSMISSION DISTANCE 



June 



For December, use reversed latitudes. 



Equinox 

 00 04 08 12 16 



Sunspot number = 



9.7 



Sunspot 



11.2 



20 



SMITHSONIAN PHYSICAL TABLES 



