SITING AND COVERAGE OF GROUND RADARS 89 
Figure 34. Edge effects. 
field point is at Q, lying at a large angle of diffraction, 
or at R, close to the edge. 
Near the diffracting edge, a certain amount of re- 
flection occurs, especially near R. This reflection is 
divergent and decreases rapidly in intensity as one 
recedes from the edge. When the edge is blunt or 
has a large radius of curvature, the amount reflected 
is increased and the field is affected over a greater 
distance. Since the angle is near grazing, the nature 
of the reflecting surface is not important. If Fresnel’s 
theory is applied to spheres and cylinders, the results 
may be only approximate. 
When the edge and the electric vector are parallel, 
the theory gives good results. When the electric vec- 
tor is perpendicular to the edge, the field strength in 
the shadow region may be several times larger than 
that obtained with the electric vector parallel, and 
the theory should then be used only for small angles 
of diffraction. : 
Other discrepancies are due to ignoring the ob- 
liquity factor and the effect of the inclination of the 
wavelets with respect to each other, The theory does 
not give the correct phase angle for the diffracted 
wave. 
The same objections may be raised for apertures 
and obstacles whose dimensions are of the order of 
a wavelength. 
It should be noted that 
1. Fresnel’s theory is valid when the wavelength is 
small compared to the dimensions of the diffracting 
object (as in optics). 
2. Fresnel’s theory should not be used: 
a. For large angles of diffraction. 
b. Close to the diffracting edge. 
c. For apertures or obstacles of the order of a 
wavelength. 
d. When the diffracting edge is not parallel to 
the direction of polarization of the wave. 
In spite of these shortcomings the theory is useful 
because it provides simple solutions for the majority 
of the diffraction problems encountered in the field, 
and, considering the difficult nature of the general 
problem, it is still the most manageable treatment 
that has been developed. 
PERMANENT ECHOES 
Introduction 
Permanent echoes are due to reflection from terrain 
features such as mountains, islands, or even smooth 
surfaces near the antenna (ground clutter). Nearby 
hills and surfaces produce strong echoes which 
obscure the indicator and widen the main pulse so 
that the minimum range of detection is increased. 
More important are the distant hills, especially those 
in the operating sector which obscure areas of tactical 
importance. Permanent echoes are a prime considera- 
tion in siting, as many otherwise excellent sites are 
rendered worthless by excessive fixed echoes. A care- 
ful analysis of the terrain will enable an approximate 
prediction of such echoes. In this section is presented 
a systematic method of preparing: permanent echo 
predictions so that the suitability of sites may be 
determined without actual field tests. 
Several factors combine to make permanent echoes 
more troublesome than might be expected on first 
thought. 
1. Hills and land surfaces are so much greater in 
extent than the target which the equipment is 
designed to detect, that strong echoes may be 
obtained from distances where an ordinary target 
would give an echo far below normal detection levels. 
2. The low elevation of the land surfaces places 
them in regions most subject to nonstandard propa- 
gation effects where extreme ranges and large 
responses are frequently obtained. 
3. Side lobes of the horizontal pattern of the 
antenna cause permanent echoes to appear at several 
other azimuths in addition to that of the main lobe. 
Although the signal intensity of the side lobes is 
much reduced, the echoes may still be strong enough 
to obscure targets. 
4. Strong permanent echoes causing considerable 
trouble may be obtained from distant mountains in 
the rear as a result of back radiation. Again, the 
weakness of the radiation and distance of the moun- 
tains are often compensated for by the large extent 
of the reflecting surface. 
5. Antennas with wide beams cause permanent 
echoes to be much wider than the object that 
produces them. 
6. Diffraction over intervening ridges is often 
sufficient to nullify their screening action so that 
objects behind the ridge are visible. 
Permanent Echo Diagrams 
The permanent echoes associated with a radar 
station may be plotted on a chart and their extent, 
location, and strength represented. Permanent echo 
diagrams should be prepared for each unit of a radar 
system using a standard procedure for the taking 
and presentation of data. These diagrams are very 
