118 TECHNICAL SURVEY 
The smooth land causes land lobes to be formed 
as in Example 17 which furnish high angle coverage. 
The sea lobes are computed using the method of 
Example 18 for the direct and reflected rays. If the 
cliff top were tilted down, the land lobes would be 
tilted by the angle of the land. Speculation about 
complex sites yields many unusual patterns, but in 
practice the results are usually disappointing. Com- 
plex sites seldom have horizontal symmetry, and 
gaps in the coverage pattern may be expected. 
Attempts to reinforce the pattern in a particular 
direction by siting back from the cliff edge generally 
cause poor coverage at other angles. Best all-round 
CHL operation results from siting on cliff edges and 
exclusive use of the sea as a reflector. 
Earth Curvature Effect 
on Lobe Lengths 
The effect of earth curvature on lobe angles was 
described on pages 102-104. The angles to be used 
with the modified antenna pattern of the image 
antenna are affected by earth curvature, and there- 
fore the strength of the reflected wave is also affected. 
In Figure 68 is shown a radar antenna at height h; 
above the earth’s surface, with the center line of the 
antenna pattern parallel to GH, the horizontal at 
the base of the antenna. Because of diffraction at a 
cliff edge the modified antenna pattern f(y) is unsym- 
metrical as in Example 18. The lines GH are parallel 
to the horizontal at the antenna. The line CE is 
horizontal at the reflection point and makes an angle 
6 with GH. The target is at an angle 7 with respect 
to GH. The incident and reflected rays make the 
angle y — @ with CE. It will be noted that the 
direct ray makes the angle y ~ 6 with the centerline 
of the antenna pattern, and the reflected ray makes 
the angle y — 20. 
Coefficient of Reflection 
The coefficient of reflection of, the reflecting surface 
is in general complex. That is, both the magnitude 
and phase of the reflected wave are affected. The 
reflection coefficient varies with the conductivity and 
dielectric constant of the reflector and with the 
frequency, polarization, and angle of incidence. 
Careful consideration should be given to the rough- 
ness of the surface, and a substantial reduction in 
the coefficient should be made when the height of 
roughness is comparable to that computed from 
equation (16). In general the reflection obtained 
with microwaves is of minor importance. 
The magnitude and phase angle of the reflection 
coefficient are plotted as functions of the angle of 
reflection, Y in Figures 69 and 70. Curyes are given 
for horizontal and vertical polarization and for the 
extreme conditions of sea water and dry soil. For 
dry soil the reflection coefficient is not sensitive to 
frequency changes, and the 100-me curve may also 
be used for 3,000 me. 
For most purposes the reflection coefficient for 
horizontal polarization may: be taken as unity, and 
the phase angle as 180°. The use of these values 
simplifies computations. 
The coefficients of reflection and phase angle for 
vertical polarization vary rapidly with frequency 
vi IN DEGREES 
p REFLECTION COEFFICIENT 
(*) 0.04 0.08 0.12 0.16 
SoC 
cote 
ae 
=a 3000 MG 
_— 
———— 
_—_ 
VP -500 MC 
VP-3000MG 
€.281 o=1 MHO/METER 
——— DRY SOIL 
&.210 0.002 MHO/METER 
0.20 0.24 0.28 0.32 0.36 
¥W ANGLE- OF REFLECTION IN RADIANS 
Figure 69. Reflecting coefficient curves. 
