STANDARD PROPAGATION 1 
short wave transmission is that of diffraction by a 
straight edge. It is not necessary that the edge be 
perpendicular to the line connecting the transmitter 
and receiver but for, the validity of the theory it is 
necessary to suppose that the distances from the 
diffracting obstacle to the transmitter and receiver 
are large compared to the height of the obstacle, 
which means that the angles of diffraction are small. 
Figure 7 shows a nomogram from which the field 
strength in the shadow of a diffracting edge can be 
read in decibels below that of free space. The geo- 
metrical significance of the quantities used is illus- 
trated on the figure. 
Such experiments as have been made show a gen- 
eral agreement with theory, but it is difficult in prac- 
tice to realize conditions of transmission that ap- 
proach ideal ones, to which the Fresnel-Kirchhoff 
theory refers. When appropriate values are taken 
for the reflection coefficient of the ground and the 
four components of the resulting field are added 
vectorially, good agreement has been found between 
experiment and theory for selected terrain. (See 
Chapter 11 of this volume.) Sometimes the terrain 
conditions are often so complicated that they do 
not readily lend themselves to idealization by simple 
geometrical: models. For these reasons the Fresnel- 
Kirchhoff diffraction theory has been of only limited 
value in short wave radio propagation. 
A case which quite often can be described ade- 
quately by an idealized model is that of a sudden 
change of the dielectric properties of the ground, as 
at a coast line.?4°-346 If the land is rough while the 
sea surface produces full specular reflection, the 
coast line can be considered as a diffracting straight 
edge with respect to the image antenna, rays of which 
represent the field reflected by the sea surface. The 
straight edge serves.to cut off that part of the radia- 
tion from the image that would represent reflection 
from the land area. The geometrical conditions are 
shown schematically in Figure 8. For-the details of 
VERTICAL SECTION 
‘ PLAN VIEW 
Ficure 8. Diffraction by a coast line. 
the analytical treatment the reader is referred to the 
comprehensive report on standard propagation con- 
tained in Volume 3 of the Summary Technical Re- 
port of the Committee on Propagation. The disto1- 
tion of the coverage diagram of a radar set caused by 
this type of diffraction is often quite large and be- 
comes important operationally at frequencies of 100 
to 200 me. This is illustrated here by a computed 
coverage diagram shown in Figure 9. If diffraction is 
—_————WITH_ DIFFRACTION 
XN —— — WITHOUT DIFFRACTION 
HEIGHT 
——— 
—— 
DISTANCE 
Fiaure 9. Coverage diagram for coast line diffraction 
(relative field strength). (Heights exaggerated 3.5 to 1.) 
not taken into account the coverage pattern shows a 
constant amplitude through higher angular elevations 
reached only by the direct rays since the ground re- 
flection is negligible. At lower angular elevations 
rays reflected from the sea add to the direct rays. 
and the “lobe” type of pattern appears. It is clear 
that if the diffraction effect were neglected very 
serious errors of the estimated coverage would result. 
Similar methods can be used to treat diffraction 
caused by cliffs, edges of wooded areas, lakes, etc., 
but these cases are not so often of importance in 
radar practice. 
THE ELECTROMAGNETIC FIELD 
FieLp STRENGTH DIstTRIBUTION 
If a transmitter is erected over a plane, ideally re- 
flecting earth, the well-known lobe pattern results . 
FREE SPACE 
FIELD 
rt" EARTH 
FicurEe 10. Typical coverage diagram (lobes) over 
plane earth. 
(Figure 10), the curves being ones of constant field 
strength. The field is given by 
aaa | (22) 
E = Ey - 2 sin | —— 
0 sin ( Xd 
where h; and he are the transmitter and receiver 
heights, and d the distance from transmitter to 
receiver. The maxima and minima occur at the 
positions in space where 
