ABSORPTION OF K-BAND RADIATION BY WATER VAPOR 



175 



tiou Laboratory using a K-band radar set in an air- 

 plane. For tliis pur^jose, the set was provided with 

 fixed attenuators which coukl be switched in or out 

 of the system. Both r-f and i-f attenuators carefully 

 calibrated were used. The experiment consisted in 

 flying a straight level course away from a known 

 target and determining the maximum range to which 

 the target could be seen with and without attenuation 

 in the system. The maximum ranges involved were of 

 the order of 30 miles. From the results a value for 

 the attenuation in the atmosphere can be calculated, 

 assuming free space propagation, and this value in 

 turn correlated with the meteorological data. The 

 latter were obtained from radio-sonde flights at MIT. 



After making allowance for the rather small oxygen 

 effect, the results are best represented by a figure of 

 0.02 db per nautical mile for 1 g/m^ of water vapor. 

 In several of the flights the target was an accurately 

 made 4-ft corner reflector. This provides an indei^end- 

 ent upper limit to the attenuation, since all system 

 parameters (antenna gain, S/N", etc.) were known, and 

 one can calculate how far the corner should have been 

 seen with any supposed amount of atmospheric atten- 

 uation. The up23er limit estimated in this way is about 

 0.04 db per nautical mile for 1 g/m^ of water vapor. 



An entirely difl'ereiit method for measuring attenu- 

 ation in the atmosphere has been develojaed at Eadia- 

 tion Laboratory. It is possible to measure the apparent 

 radiation temperature of any matched r-f load, includ- 

 ing an antenna, with great precision ( — •! C). In the 

 case of an aiitenua, the temperature measured is the 

 temperature of whatever the antenna is looking at, 

 that is, the temperature of whatever would absorb the 

 energy emitted from the antenna if the antenna were 

 transmitting. When the antenna is pointed at the sky, 

 the temperature measured is some mixture of the tem- 

 perature of outer space and the temperature of the air, 

 the influence of the latter being determined in a direct 

 and simple manner by the absorption coefficient of the 

 air layer. From measurements of the apparent tem- 

 peratiri'e of the sky at various elevation angles, the 

 total absorption in decil>els for a vertical path through 

 the entire atmosphere can be deduced. The data which 

 luive 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 Aalue 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. 



i»^ 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 exj)eriment consists in the measurement of the 

 change in Q of a large copj)er 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 A) copper box of 

 15.8 eu 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 707 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 cm 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 

 box, and the fan previously nientioned 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 



"By J. M. B. Kellogg, Columbia University Radiation 

 Laboratory. 



