TABLE 459.— TRANSMISSION FACTORS (concluded) 445 



Latitudes and local times are those of the ionospheric reflection points. The F 2 -layer 

 zones (IV, I, and E) are those chosen for practical description of longitude effect by the 

 International Radio Conference of April-May 1944. The W and E zones are centered on 

 70° W. and 110°E. longitude, respectively; the two / zones are intermediate between these. 



Values are presented for sunspot numbers of and 125. Since both critical frequencies 

 and maximum usable frequencies show approximately linear variation with sunspot num- 

 ber, values for any other sunspot number, X, may be obtained by interpolation. 



[World-wide charts of predicted MUF, three months in advance, for both E and F 2 

 layers, are regularly published in Central Radio Propagation Laboratory Series D 

 reports, "Basic Radio Propagation Prediction."] 



E-Layer ordinary-wave critical frequencies. — These may be obtained by dividing 

 the £-layer 2,000 km MUF by 4.78, since the minimum virtual height of reflection is 

 nearly constant for this layer. 



Extraordinary-wave critical frequencies, /* (or zero-distance MUF). — The or- 

 dinary-wave critical frequency f°, the extraordinary-wave critical frequency f, and the 

 gyrofrequency /* are related by the equation 



U°y=U' ±h)f 



The gyrofrequency, fh, varies with the intensity of the earth's magnetic field, H, and is 

 given by 



, e H 

 2inn c 



where c and m are, respectively, the electronic (or ionic) charge and mass, c the velocity 

 of light in free space, and H is given in gauss. 



Ion density. — The number of ions per cm 3 at the reflection point may be obtained from 

 the value of the ordinary-wave critical frequency, /", by the equation 



N 



e 2 (/°) 2 

 where m and e are, respectively, the ionic mass and charge. 



Minimum virtual heights of reflection. — The maximum usable frequency at any 

 transmission (except for those nearly equal to zero) is equal to 



MUF — f seep 



where <p is the angle of incidence of the wave upon the ionospheric reflecting layer. 

 <t> is approximately given by 



sin \ 6 



• = tan -1 



1 + (h/R) — cos i e 



where 6 is the angular distance of the transmission path, h the virtual height of reflection, 

 and R the radius of the earth. (Cf. Smith, N., Proc. Inst. Radio Eng., May 1939, p. 232.) 



Maximum usable frequencies for other transmission distances. — These may be 

 obtained from the MUF of Table 461 by using the factors and procedure presented in 

 Table 462. 



Skip distances. — The MUF for a given distance is the frequency for which that dis- 

 tance is the skip distance. 



TABLE 460.— ATTENUATION OF MICROWAVES BY WATER VAPOR IN THE 

 ATMOSPHERE (in db/km) 151 



Measured at 45 °C at atmospheric pressure 



154 Adapted from Becker and Autler, Phys. Rev., vol. 70, p. 303, September 1946. 

 SMITHSONIAN PHYSICAL TABLES 



