GROUND REFLECTION 



55 



principal problems to be discussed here. For micro- 

 waves the salt content in sea water makes little 

 difference, so that it may be assumed that the dielec- 

 tric constant and conductivity are the same over all 

 oceans at the same temperature. With increasing 

 temperature, the real part of the dielectric constant 

 diminishes roughly by one unit per 5° C. Figure 11 

 gives the dielectric constant e^ of ordinary sea 

 water at 17 C as a function of frequency. 



The dielectric constant also diminishes with in- 

 creasing salinity, but in the UHF-SHF region, normal 

 variations of salinitj^ have much smaller influence 

 than changes in temperature and frequency. The 

 imaginary part e; of the dielectric constant is, for 

 frequencies less than saj' 1,000 mc, related to the 

 conductivity a as follows : 



€i = -I- eoo-x 



(23) 



((7 in mhos per meter and X in meters). At 25 C the 

 average conductivity of sea water is usually given as 



Figure 12. Amplitude of the reflection coefficient p 

 versus angle of reflection * for sea water. (From 

 Radiation Laboratory Report C-11.) 



4.3 mhos per meter. The temperature dependence is 

 given by 



0- = (725 [1 + 0.02{f - 25)], 



where t is temperature in degrees centigrade. 



The conductivity of fresh water is much smaller. 



An average for inland lakes is o- = 10 ~'^ mho per 

 meter. For wavelengths shorter than about 10 cm, 

 the dielectric constant is influenced by the fact that 

 water is built up of polar molecules. The maximum 

 effect is found for wavelengths of the order of 1 cm. 

 For this region, there is no appreciable difference 

 between salt and fi'esh water. 



The probable run of Cj for sea water is shown in 

 Figure 11. The part of the ej curve extending from 

 0,000 mc to 30,000 mc corresponds essentially to 

 results obtained at Clarendon Laboratory, Oxford. 

 Other investigators gi^'e results which are markedly 

 different. This curve is thus affected by consider- 

 able uncertainties and should be used with caution. 



In Figures 12 to 15 are shown amplitude and 

 phase of the reflection coefficient for a smooth sea for 



0° 



Figure 13. Phase of the reflection coefficient (j) versus 

 angle of reflection * for sea water. Reflected wave E^ 

 lags incident wave E^ by (j). (From Radiation Labora- 

 tory Report C-11.) 



various frequencies. Figure 12 gives the amplitude 

 p of the reflection coefficient for both kinds of 

 polarization, for reflection angles up to 90 degrees, 

 and for 100 and 3,000 mc. Figure 13 gives the 

 phase (f) under the same conditions. The next two 

 figures give p and </> on a greatly enlarged scale of 

 i/', in the interval i/' = to i^ = 5.5°, which embraces 

 all cases of practical interest. 



