110 TECHNICAL SURVEY 
A line is drawn at this angle, and a point is marked 
off. at a range of 53.46 miles. 
Lobe Diagrams of Medium Height Sites 
In dealing with radars at medium heights, say 
from 100 to 1,000 ft, a more involved treatment is 
required, owing to earth curvature effects. The 
procedure followed in this section is to compute a 
value of dma; for each lobe from which a sinusoid 
is constructed at the angle of the lobe. The envelope 
of the lobes is considered to be of principal interest, 
the lobe shape being of secondary importance. 
The strength of a wave is measured in miles, that 
is, the distance at which the standard target must 
be to give a standard signal response such as a 
signal-to-noise ratio of one. The distances corres- 
ponding to the direct and reflected waves are added 
to get lobe maxima and subtracted to get minima. 
The direct and reflected waves will therefore be 
computed separately. The phase shift due to reflec- 
tion will be taken as 180°, and the phase shift due 
to other causes than path difference will be considered 
negligible. This assumption greatly simplifies calcu- 
lations and is a good approximation for small angles 
and horizontal polarization. For vertical polariza- 
tion, especially. in the VHF band, it is a poor 
approximation. 
The direct wave is affected only by the modified 
antenna pattern. The reflected wave is affected by: 
1. Shoreline diffraction. 
2. The modified antenna pattern. 
3. Earth curvature. 
4. Coefficient of reflection. 
5. Divergence. 
Terrain effects such as reflection areas of limited 
extent, the shoreline, cliff edges, and obstacles involve 
diffraction. A simple, flexible method for solving such 
problems will be developed in the next section. 
Shoreline Diffraction 
Unfortunately sites of sufficient height are 
frequently some distance inland, and a considerable 
portion of the reflection surface is on land. The 
N, RADAR ANTENNA 
UY 
—Yy fly 
YW 
ULL 
DIFFUSE REFLECTION 
Uf] 
ROUGH LANDSS SN 
dy Se NS 
SHORE LINE 
yj 
poor reflecting qualities of land, especially when 
rough, cause. the high angle lobes due to nearby 
reflection to be reduced as much as 50 per cent in 
length. This is a common cause of poor high coverage 
‘so often experienced in field installations and the 
inability to detect high-level bombing attacks except: 
at perhaps 10-mile ranges. In this section will be 
developed a method of computing the vertical 
coverage pattern for the typical high site with part _ 
land and part sea reflecting surfaces. 
In most cases the profile of the land between the 
transmitter and the shore will be found to be too 
rough for coherent reflection, as may be determined 
from equation (16). If substantial regular areas or 
obstacles occur between the antenna and the shore 
line they should be treated as described in the ser 
tion on the modified antenna pattern on page 115: 
Sea Reflection 
with Diffuse Land Reflection 
The problem treated in this section will be that 
shown in Figure 57. The land in the foreground is 
so rough as to cause only diffuse reflection, and no 
regular areas exist which will affect the vertical 
pattern below 15°. 
The diffuse reflection from the land area has a 
random phase relation, and the field intensity in a: 
particular direction is relatively small. The effect of 
the land reflection on the interference pattern is 
therefore neglected. This is equivalent to termination 
of the reflecting surface at the shore line. 
in order to describe diffraction at a shore line a 
system of Fresnel zones for each lobe is considered 
to be formed on the sea with the reflection point of 
the lobe as their center. The zones will be ellipses 
because of the inclination of the rays. The influence 
of the shore line will be determined by the number 
of zones which are not interfered with by the shore. 
Thus a low angle lobe which has its central Fresnel 
zone far out to sea would be virtually unaffected by 
the limited reflection area, as numerous zones are 
FRESNEL ZONES FOR 
REGULAR REFLECTION 
| = 
Ss 
SE, 
Ficure 57. Fresnel zones on land and sea areas. 
