Radar System Engineering

54 THE RADAR EQUATION [SEC.213
small compared with the size of the obstacle, to put it very crudely.
There is some spreading of this sort, of course, and it is a phenomenon
upon which a good deal of theoretical effort has been expended. Methods
have been developed for calculating the intensity of the radiation in the
“diffraction region,” that is, beyond the horizon (Vol. 13, Chap. 2). In
this region, however, the field strength normally diminishes so rapidly
with increasing range that any additional radar coverage thus obtained
is of little value. From the point of view of the radar designer, targets
over the horizon might as well be regarded as totally inaccessible under
‘ 1standard” conditions of propagation.
D,stancein males
FIG,2.14.—Coverage diagram for 2600Me/see,transmitterheight 120ft. Solid curve for
totally reflecting earth. Dotted curve for nonreflectingearth.
.
As for regions well within the horizon, the curvature of the earth at
most complicates the geometry of the interference problem discussed in the
preceding section. Naturally, we have no right to apply Eq. (29), as it
stands, to targets near the horizon. We need not concern ourselves here
with these complications, which are adequately treated in Vol. 13.
Methods have been worked out for rapidly calculating the field strength
over a curved reflecting earth. The radar designer usually prefers to
display the results in the form of a coverage diagram, which shows contours
of constant field strength plotted in coordinates contrived to show directly
the effect of the curvature of the earth. One such contour, shown in
Fig. 2.14, is calculated for an omnidirectional antenna transmitting at
2600 Me/see from a height of 120 ft above a totally reflecting earth.
The dotted curve is the corresponding contour with a nonreflecting earth.
Both contours would usually be modified by the directional pattern of the