Radar System Engineering

SEC.2.14] SUPERREFRAC TION 55
radar antenna and it is not difficult to take this into account in the
calculation. Note the exaggeration of the vertical scale.
A contour of constant field strength is a contour of constant power
intensity as well; moreover, the intensity of the signal received from a
radar target of given cross section, located anywhere along such a contour,
is the same.1 Thus, iftheproper contour is chosenit will represent, as
nearly as any curve can, the boundary of the region in which a given
target can be detected. The reader’s earlier introduction to some of the
statistical factors involved in radar should prepare him for the warning
that such coverage diagrams arenotto betaken too literally. They are,
nevertheless, useful intheplanning and design of long-range search radar.
2,14. Superrefraction.-As we have seen, the effect of the normal
vertical gradient of refractive index in the atmosphere is to introduce a
slight downward curvature in the path of light and of microwaves. Were
this curvature only a few times greater, it would equal the curvature of the
earth itself, and it would be possible for a ray to bend around the earth
without leaving the surface; in other words, there would be no horizon.
Whatever misgivings we may have about the use of the word “ray” in
this connection, it would not be surprising if some interesting departure
from standard microwave propagation were to manifest itself under such
conditions.
Refractive index gradients of the requisite strength (5 parts in 10s per
ft) can be produced under some conditions by temperature gradients
alone. For example, if land heated by the sun cools by radiation at
night, a fairly thin layer of cold (therefore dense) air may be formed just
above the ground, which results in an unusually rapid decrease of refractive
index with height, the index of the lowest layer being abnormally
great.
A more widespread cause of strong vertical gradients in refractive
index, and therefore of excessive bending of rays, is the refractive effect
of water vapor mentioned earlier. Over most of the surface of the ocean
the region above the water is not saturated with water vapor, whereas
the layer directly in contact with the water must be very nearly saturated.
There is, in other words, a continual evaporation of water from the sea
10ne must be careful not to confuse the directional pattern of an antenna with
the plot of contours of constant intensity in the field of the antenna, which it occasionally
superficially resembles. If we plot the gain of an antenna as a function of
angle, in polar coordinates, we have an antenna pattern that has meaning, strictly)
only if the antenna is isolated in space. On the other hand, contours of constant
intensity, the coordinates of which refer directly tO pOsitiOnsin sPace, can be used tO
describe the radiation field no matter what the surroundings or type of propagation
involved. The fact that a signal of the same intensityis receivedfrom a target at
any position on one such contour does not depend on the inverse-squarelaw or any
otherlaw of propagationbut only on the Reciprocity Theorem.