232 



REFLECTION AND TRANSMISSION OF RADIO WAVES 



10^ 



0.3 0.5 1 3 



X(cm) 



Fig. 4-49 The Variation of Radar Cross Section of Actual Rain-Filled Space with 

 Wavelength, for Various Rainfall Rates. 



The ionization processes which render flames conducting are still not 

 completely understood. A recent summary of the subject by Calcote^' 

 presents the status of the understanding of these mechanisms. He cites 

 experimental evidence from the older literature of ion concentrations of 

 10^^ cm~^ From the standpoint of radio wave attenuation, only the electron 

 density is of importance, since the conductivity due to a constituent ion of 

 a highly ionized gas is approximately inversely proportional to the mass of 

 the ion (see, for example, Guidance,'^'^ p. 121, Equation 4-19). 



The ion density varies greatly with the type of fuel. Furthermore, the 

 ion density is influenced markedly by small quantities of low-ionization 

 potential contaminants. For example, only trace quantities of the alkali 

 metals such as potassium and sodium are sufficient to increase greatly the 

 ion densities. 



Information on the quantitative attenuations to be expected from jet 

 flames can be pieced together from the literature. Adler'*^ measured the 

 attenuation in acid-aniline jet flames in a waveguide and found an atten- 

 uation of 0.033 db/m at 200 Mc. Since attenuation is approximately 

 proportional to/^'^, this is equivalent to 0.25 db/m at X band. Adler also 

 observed that the addition of slight amounts of sodium caused large and 

 erratic increases in the attenuation. It is probable that much higher 

 attenuations would occur in modern high-energy fuels. 



■•^H. F. Calcote, "Mechanisms for the Formation of Ions in Flames," Cotnbiistion and Flame 

 1, 385-403 (1957). 



^''F. P. Adler, "Measurement of the Conductivity of a Jet Flame," J. AppL Phvs. 25, 

 903-906 (1954). 



