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DIELECTRIC CONSTANT, ABSORPTION AND SCATTERING 293 
the influence of the latter being determined in a direct 
and simple manner by thé absorption coefficient of the 
air layer. From measurements of the apparent tem- 
perature of the sky at various elevation angles, the 
total absorption in decibels for a vertical path through 
the entire atmosphere can be deduced. The data which 
have been collected in this manner show good internal 
consistency ; assuming that the MIT radio-sonde data 
give the total water vapor in the atmosphere correctly, 
a value of 0.04 db per nautical mile for 1 g/m® is ob- 
tained for the water vapor attenuation. This is larger 
than the other value quoted above. The reason for the 
discrepancy is not yet known. 
ABSORPTION OF K-BAND 
RADIATION BY WATER VAPOR? 
An experiment to determine the location and shape 
of the water vapor absorption line in the K-band 
region of the electromagnetic spectrum is in progress. 
The experiment consists in the measurement of the 
change in Q of a large copper box when water vapor is 
introduced. From this change in Q the loss by absorp- 
tion in the water vapor can be determined and hence 
the attenuation of K-band radiation in water vapor. 
The experimental setup consists of an approximately 
cubical (but irregular in terms of X) copper box of 
15.8 cu m volume. Energy from a pulsed magnetron 
is fed into this box through a wave guide which ter- 
minates in a matched horn facing a rotating copper 
fan placed in the roof of the box. The purpose of this 
fan is to stir up the standing wave pattern in the box. 
Throughout the interior of the box are’ placed strings 
of Chromel-constantan thermocouple junctions sealed 
in 70% glass tubing. Alternate junctions are coated 
with a mixture of polystyrene and iron powder. In all, 
there is a total of 220 painted or “hot” junctions in the 
box. Provision is made for introducing water vapor 
into the box and for circulating the air. The tempera- 
ture is maintained at 45 C during all runs, and the 
pressure is atmospheric (760 + 15 mm, depending on 
conditions). An aperture of area 400 sq em which may 
be opened or shut by means of a sliding copper door is 
located in one side of the box. Radiation entering the 
box is absorbed by the walls, by the paraphernalia in 
the box, by the gas, by the apertures (if any), and by 
the thermocouple junctions. The coated thermocouple 
junctions absorb more energy than the uncoated junc- 
tions and a net emf is produced. A single junction 
would give an emf proportional to the value of the 
square of the electrical field at its position, but the 
reading would be very sensitive to the location of the 
couple and, even if this were held fixed, would be sen- 
sitive to small deformations of the walls. The large 
number of the couples actually used averages the value 
of the square of the electric field, E?, over the entire 
°By J. M. B. Kellogg, Columbia University Radiation 
Laboratory. 
box, and the fan previously mentioned assists in this 
averaging. The Q of the box and its contents is, for 
constant’magnetron power output, proportional to E? 
and thus to the emf of the thermocouples. 
Since the couple emf is also proportional to the 
power output of the magnetron, changes in the output 
power will show up in the results in the same way as 
changes in Q. Original difficulties arising from this 
cause, which were encountered because of variation in 
the a-c line voltage and the modulator voltage, have 
been largely eliminated by the use of stabilizing trans- 
formers and a magnetron load current stabilizing cir- 
cuit. Furthermore, a method of taking data was de- 
vised which only required the power output to be 
maintained constant for a few minutes at a time. 
The Q of the water vapor, Qy, is given by 
1 
Be (72) 
where y is the attenuation in db per nautical mile, » 
is in centimeters, and K is a constant. In order to 
obtain absolute values of the attenuation, it is necessary 
to introduce into the system a known Q in terms of 
which the other Q’s may be evaluated. For this purpose, 
the aperture, which acts as a perfect absorber, is used. 
Lamb has derived a formula for the Q of an aper- 
ture, Qa, and this is 
Am S See (73) 
where A is the area of the aperture and V is the volume 
of the box. 
The @Q of the whole ensemble may now be written 
down. 
1 1 1 1 
O7 @> v Qa’ @ 
where Qs takes account of all losses (including the 
losses in oxygen) other than those in the vapor and 
the aperture. Inserting values, and using 1/y as the 
proportionality constant connecting the emf, €, and 
Q, one then has 
1 1 A 
en(dten+ 24). cs) 
For constant conditions of humidity, wavelength, and 
magnetron power output, measurements are now made 
of the emf €,, with A = 0, and emf €4, with A = A. 
Using these measured values of emf, equation (75) 
can be written in the form 
(En 8rV ( 1 ) 
=—-(\~— + kya ])=F. 
Er=eli a AR NORE a os) 
The humidity is then changed and the measurement 
repeated until enough points have been obtained to 
provide a curve of F as a function of p, the water vapor 
density, for constant. wavelength. 
