Radar System Engineering

SEC.215] ATTENUATION OF MICROWAVES 59
The effect is therefore truly an attenuation in the sense in which the word
is applied to transmission lines, and is properly measured in decibels per
kilometer. We shall discuss first the absorption by the gases of the
atmosphere.
Of the three abundant gases of the atmosphere—nitrogen, oxygen,
and water vapor—the latter two are intrinsically capable of interacting
with and absorbing energy from a radio wave by virtue of the permanent
electric dipole moment of the water molecule and the permanent magnetic
dipole moment of the oxygen molecule. We know, however, that
molecules absorb radiation in more or less well-defined absorption lines,
or bands, and we have to inquire whether either of these molecules
exhibits absorption lines in the microwave range—that is, at frequencies
much lower than those usually associated with molecular absorption
spectra. It has been found that both oxygen and water vapor do in fact
dkplay such absorption. Although the effects observed would be
classed as very weak %y a spectroscopist, radar involves transmission
over such long paths that very serious attenuation is encountered in
certain parts of the spectrum. In Fig. 2.16 are plotted curves showing
the course of the water vapor absorption and oxygen absorption, as a
function of wavelength. The absorption is measured by the rate of
attenuation in decibels per kilometer.
The most prominent feature of the water-vapor absorption is a single
“line” which appears as a broad maximum centered about 1.3-cm
wavelength, superimposed on the residual effect of a multitude of far
stronger li~es located at much shorter wavelengths. The soiid part of
the curve is based on extensive direct measurements. These confirmed
the main features of the theoretical predictions,l on the basis of which
the remainder of the curve has been sketched in; one cannot, however,
rely on the quantitative accuracy of the dotted part of the curve. The
curve is plotted for an atmosphere containing 10 g of water vapor per
cubic meter. This corresponds to a Felative humidity of 66 per cent
at a temperature of 18°C, for example. Over the range of absolute
humidities normally encountered in the atmosphere one may assume
that the attenuation is simply proportional to the absolute humidity.
The rapid rise of the curve below 3 mm is evidence of the powerful
absorption displayed by water vapor throughout the far infrared. No
further transparent regions are to be found until we reach a wavelength
of the order of 15 microns (0.0015 cm).
The oxygen absorption rises to a high peak at 5-mm wavelength;
this has been quantitatively verined by direct measurement. At longer
lJ. H. VanVleck, ``FurtherTheoreticalInvestigationsof the At,mosphericAbsorption
of Microwaves,” RL Report No. 664, March 1, 1945. See also Vol. 13 of
thk series.