374 PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
figures give p and ¢ on a greatly enlarged scale of 
y, in the interval y = 0 toy = 5.5°, which embraces 
all cases of practical interest. 
Overland Transmission 
Conditions over land are very different from those 
found over the sea. Land as a reflecting surface has 
larger irregularities and their effect is more pro- 
nounced. Therefore in selecting a radar site, it is 
preferable to choose a location which is surrounded 
by relatively smooth ground. The electrical proper- 
ties of the earth vary considerably for different 
localities, so that it is necessary to study the ground 
conditions for each particular case. Experimental 
data concerning reflection of very short waves from 
ice- or snow-covered ground seem to be lacking. 
Precise information might be of operational interest, 
particularly in Arctic regions. Laboratory exper- 
iments indicate that ground covered by ice or snow 
will influence the propagation of short waves some- 
what in the same way as very dry ground. 
Conductivity of Soil 
Extensive investigations have been made on the 
conductivity of different types of soil, particularly 
on low and medium frequencies. For 10 mc, the 
observed values range from 6 - 10°* mhos per meter 
for chalk to 0.13 mhos per meter for blue clay. The 
conductivity increases with increasing moisture 
Ficure 16. Amplitude of the reflection coefficient for 
moist and dry soil. 
content, so that marked seasonal changes may be 
anticipated for a given locality. It also varies with 
frequency. Under field conditions it will not be 
possible to measure the conductivity in individual 
cases and one will have to assume a value of about 
10°? mhos per meter for poorly conducting ground 
like chalk or very dry soil and take a value of about 
107* for good conductors like blue clay or water- 
bogged marshy land. Fortunately, the amplitude 
of the reflection coefficient is not very sensitive to 
minor changes in conductivity when the frequency is 
sufficiently high, say 200 mc or higher. Then the 
real part of the dielectric constant is the most 
important factor. 
Dielectric Constant of Soil 
It is not possible to give a standard table of 
dielectric constants of various types of soil, because 
the variation with the moisture content is consider- 
able. For very dry ground ¢, is likely to be about 4, 
but this value may rise to 25 when the ground is 
thoroughly soaked with water. The dielectric con- 
stant of ground will normally decrease with increas- 
ing frequency. 
Above 200 me, the dielectric constant will dom- 
inate the conductivity term, and for field conditions 
the ground may be assumed to be a pure dielectric. 
This is illustrated in Figure 16 for ¢, = 7; ¢;=3 
Ficure 17. Phase of the reflection coefficient for 
moist and dry soil. 
and e; = 0; and for ¢, = 25, e¢; = 19 and e; = 0. 
Except for values close to the Brewster angle, the 
zero conductivity curves give a usable approxima- 
tion. 
In Figure 17, the phase, ¢, of the reflection coeffi- 
cient corresponding to the above values of ¢, and e¢, 
is also given. 
The Divergence Factor 
The preceding considerations apply only to 
reflection from plane surfaces. For reflection from a 
