Radar System Engineering

58 THE RADAR EQUATION [SEC.2.15
not require total reflection at CD to get appreciable guiding for a considerable
distance. Nor does the source of radiation have to lie within
the duct to permit a portion of the energy to be partially trapped in the
duct, although it may not be too far above it. The variation of field
strength with distance from the transmitter and height above the surface
is very complicated, and to cover this type of propagation by a mere
modification of the radar equation is entirely out of the question, as the
reader who pursues this subject into Vol. 13, where it is treated at length,
will learn. It is perhaps best here to summarize the aspects of superrcfraction
which have a significant bearing on radar planning and design.
1. The guiding of microwaves by refractive anomalies of the duct
type appears to be the sole means by which coverage beyond the horizon
can be obtained. Extensions of range up to several times the horizon
distance have often been observed.
2. The most prevalent, the best understood, and probably the most
important example of the type is associated with the surface evaporation
duct, which seems to exist most of the time over large areas of the oceans
of the world.
3. Short wavelengths are required to take advantage of the guiding
effects of ducts, and, for surface ducts, low antenna (and target) heights.
This is especially true of the evaporation duct.
4. Substandard as well as superstandard radar ranges can be caused by
refractive anomalies, if the transmission path is nearly horizontal.
5. Propagation at angles steeper than a few degrees with respect
to the horizontal is not affected by the refractive anomalies here discussed.
2.15. Attenuation of Microwaves in the Atmosphere.—The earth’s
atmosphere, excluding the ionosphere, is for all practical purposes
transparent to radio waves of frequency lower than 1000 llc/sec. Even
over a transmission path hundreds of miles long no appreciable fraction
of the energy in the radio wave is lost by absorption or scattering in the
atmosphere. With the extension of the useful range of radio frequencies
into the microwave region we have at last entered a part 01 the electromagnetic
spectrum to which the atmosphere is not wh 011y transparent.
Indeed an upper limit to frequencies useful for radar, imposed by the
properties of the atmosphere, is now within sight. The radar engineer
must therefore acquaint himself with certain phenomena falling heretofore
within the exclusive province of the molecular spectroscopist.
Broadly speaking, there are two ways in which energy can be dissipated
from a radar beam: (1) by direct absorption of energy in the
gases of the atmosphere; (2) through absorption or scattering of energy
by condensed matter such as water drops. All such processes lead to an
exponential decrease of intensity with distance from the source, superimposed
on, and eventually dominating, the iu~erse-square dependence.