MEASUREMENTS OF THE REFI.ECTION COEFFICIENT OF LAND 



147 



Table 9. Reflection coefficient of ground ridged along or across the direction of transmission. X = 9 cm. 



9 3 MEASUREMENTS OF THE REFLECTION 

 COEFFICIENT OF LAND AT CENTI- 

 METER WAVELENGTHS, CARRIED 

 OUT AT NATIONAL PHYSICAL 

 LABORATORY" 



Experiments have been made on the reflection and 

 absorption of radio waves in the S-band of wavelengths 

 by workers in the National Physical Laboratory in 

 England. The reflection coefficient has been measured 

 at angles of incidence to the vertical, of 80°, 68°, 54°, 

 and 44° on level ground, fresh water, sea water, uneven 

 ground, ground covered with vegetation, and ground 

 covered with Va in. mesh wire netting. The absorption 

 in soil, fresh water, sea water, and Va in. mesh wire net- 

 ting has also been measured by a laboratory method. An 

 interim report^ gave some of the salient results 

 obtained on ground reflection. 



The main conclusions which have been drawn from 

 the results obtained are given below. 



1. Specular reflection can be obtained only from a 

 very level surface, with little or no vegetation on it. 

 The electrical constants of such surfaces are given in 

 Table 10, from which the coefficient of specular reflec- 

 tion can be deduced for the angle of incidence and 

 state of polarization concerned. 



°By W. Ross, British Central Scientific Office, Washington, 

 D.C. 



3. The reflection coefficient decreases with uneven 

 ground and is reduced to a value of about 0.3 by in- 

 equalities of level of about one wavelength. This con- 

 clusion is based mainly on a series of experiments in 

 which the ground was raked into a series of ridges 

 resembling waves, which could be either in, across, or 

 at an angle to, the direction of transmission. Similar 

 results were obtained when a large sheet of wire net- 

 ting was similarly disposed in a series of waves. 



3. Vegetation reduces the reflection coefficient, in 

 general, and when about 3 ft high causes a reduction 

 in reflection coefficient to a value of about 0.3. An in- 

 teresting exception was found when level ground was 

 covered with vegetation less than half a wavelength 

 high (about 1% in.) when the reflection coefficient 

 with vertical polarization increased slightly with high 

 angles of incidence over that obtained with level 



ground. 



Table 10 



