SITING AND COVERAGE OF GROUND RADARS 
IMAGE ANTENNA 
111 
= 
Za 
Fiaure 58. Shore line diffraction. 
formed on the sea. This is indicated by A in Figure 
57, which represents the reflected wave. At B, a 
higher angle lobe, there are only two zones intact, 
and the reflected wave is weak. Had only one zone 
been complete, the reflected wave would have been 
stronger than A. At C only portions of outer zones 
are formed on the sea, and the reflected wave is 
negligible. 
The effect of the reflecting surface may be repre- 
sented by an image antenna located in the earth 
under the radar antenna at a depth /; below the 
surface as in Figure 58. The nonreflecting land surface 
then acts precisely as a straight diffracting edge for 
the image antenna and indirect ray. A general 
formula will be developed which gives the situation 
of any Fresnel zone of any lobe for a given radar 
station. From this formula and the distance to the 
shoreline it may be determined for each lobe which 
zone is intercepted by the shore. In the graph in 
Figure 58 is plotted the relative intensity of the 
reflected ray as a function of m, the number of the 
- zone touching the shore. In the illuminated region 
at large angles, as A, the relative intensity is close 
to unity. Approaching the shore it oscillates about 
unity, reaching a maximum of 1.18. In the shadow 
region, the intensity drops to low values. Thus, 
knowing m, the effect of shoreline diffraction on the 
reflected ray may be obtained. The derivation will 
be developed for a plane reflecting surface, since, as 
‘it has been explained on page 102, for lobe angles 
corrected for standard earth curvature, the effect of 
earth curvature may be taken into account by using 
hy’ [equation (59)] instead of h:. In most cases dy will 
be small and h; may be used with little error. 
General Formula 
for the Reflection Area 
In Figure 59 is shown an image antenna 7” sending 
radiation through a plane of indefinite extent. In 
TO TARGET 
SEA SURFACE 
Ficure 59. Fresnel zones on the reflecting surface. 
order to simplify the calculations it will be assumed 
that the distance from the reflection point to the 
target is large, so that the rays from the Fresnel 
zones may be considered parallel. With regard to 
