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Anomalous atmospheric attenuation in the 7–10 cm−1 window
/n,frared
Phys. Vol. 23, No. 5, pp. 261~-262, 1983 Printed in Great Britain. AI1 rights reserved
ANOMALOUS
W20-089ij83 $3.00 f 0.00 Copyright Q 1983 Pergamon Press Lid
ATMOSPHERIC ATTENUATION 7-10 cm-’ WINDOW
IN THE
V. DE COSMO, N. GONZALES and M. PETRIZZELLI Department of Physics, IUPFAN, Apdo 1227, Maracay 2104-A, Venezuela (Received
6 June 1983)
Abstract-Measurements of atmospheric attenuation in the 7-IOcm-’ window, in the temperature range 296-309 K, were made. The results show that in this waveband the atmospheric attenuation increases with decreasing
temperature.
RESULTS
The 7-I 0 cm-’ atmospheric window is the most suitable near-millimeter window for astronomical and communication purposes. It is known that, in this window, there is disparity between the theoretical predictions and the experimental results. In particular, while the theoretical models predict that the attenuation decreases with decreasing temperature, the laboratory and field measurements have shown an increase of attenuation with the decrease of temperature.“) The possible cause of this effect can be the presence in the atmosphere of water dimers itl a non-equilibrium state. c2)The few field measurements of this effect are in the temperature range between 260-290 K. We are reporting here, the results of an investigation intended to show the temperature dependence of attenuation for very high atmospheric temperature. The measurements were carried out, at tropical latitudes, in the dry season, when the climatic conditions were very stable and the temperature range was between 295-3 10 K. The measurements were made using a mercury lamp at the focus of a 60 cm diameter searchlight mirror. The radiation was chopped mechanically, the receiver was a Golay cell detector coupled with a 10 cm Cassegrain telescope. The distance between the lamp and the receiver was of 40 m and the path height was 2m above the ground. The temperature was measured at the extremities and at the middle point of the path and the water vapor density was determined with the data of temperature and relative humidity, For isolating the 7-10 cm-’ window we used a narrow band filter centered at about 1200pm. The measurements were made in February and March 1983 under very stable meteorological conditions: temperature range 295-310 K, absolute humidity range 12-18 g/m3. On some days, in the afternoon, at a 3 hr interval, the variation of the temperature was of O.l”C and the absolute
Temperowe
I Kl
Fig. 1. Specific absorption coefficient (dB/km g-‘) vs atmospheric temperature T (K). The solid line is the best fit. 261
humidity practically constant. We observed that. in conditions 01‘ clear sky, at night. the output signal was without fluctuations. and during the day, the output signal is highly variable and is characterized by a time variability which cannot be explained by instrumental noise or variations in meteorological parameters. The results are shown in Fig. I. As has been shown. we can also see that in the temperature range between B-310 K. the atmospheric attenuation increases with the decrease of’ temperature. We note that. because we do not know exactly what the losses due to beam spreading are. the reported values of CYI could not be absolute values.
Phys. Vol. 23, No. 5, pp. 261~-262, 1983 Printed in Great Britain. AI1 rights reserved
ANOMALOUS
W20-089ij83 $3.00 f 0.00 Copyright Q 1983 Pergamon Press Lid
ATMOSPHERIC ATTENUATION 7-10 cm-’ WINDOW
IN THE
V. DE COSMO, N. GONZALES and M. PETRIZZELLI Department of Physics, IUPFAN, Apdo 1227, Maracay 2104-A, Venezuela (Received
6 June 1983)
Abstract-Measurements of atmospheric attenuation in the 7-IOcm-’ window, in the temperature range 296-309 K, were made. The results show that in this waveband the atmospheric attenuation increases with decreasing
temperature.
RESULTS
The 7-I 0 cm-’ atmospheric window is the most suitable near-millimeter window for astronomical and communication purposes. It is known that, in this window, there is disparity between the theoretical predictions and the experimental results. In particular, while the theoretical models predict that the attenuation decreases with decreasing temperature, the laboratory and field measurements have shown an increase of attenuation with the decrease of temperature.“) The possible cause of this effect can be the presence in the atmosphere of water dimers itl a non-equilibrium state. c2)The few field measurements of this effect are in the temperature range between 260-290 K. We are reporting here, the results of an investigation intended to show the temperature dependence of attenuation for very high atmospheric temperature. The measurements were carried out, at tropical latitudes, in the dry season, when the climatic conditions were very stable and the temperature range was between 295-3 10 K. The measurements were made using a mercury lamp at the focus of a 60 cm diameter searchlight mirror. The radiation was chopped mechanically, the receiver was a Golay cell detector coupled with a 10 cm Cassegrain telescope. The distance between the lamp and the receiver was of 40 m and the path height was 2m above the ground. The temperature was measured at the extremities and at the middle point of the path and the water vapor density was determined with the data of temperature and relative humidity, For isolating the 7-10 cm-’ window we used a narrow band filter centered at about 1200pm. The measurements were made in February and March 1983 under very stable meteorological conditions: temperature range 295-310 K, absolute humidity range 12-18 g/m3. On some days, in the afternoon, at a 3 hr interval, the variation of the temperature was of O.l”C and the absolute
Temperowe
I Kl
Fig. 1. Specific absorption coefficient (dB/km g-‘) vs atmospheric temperature T (K). The solid line is the best fit. 261
humidity practically constant. We observed that. in conditions 01‘ clear sky, at night. the output signal was without fluctuations. and during the day, the output signal is highly variable and is characterized by a time variability which cannot be explained by instrumental noise or variations in meteorological parameters. The results are shown in Fig. I. As has been shown. we can also see that in the temperature range between B-310 K. the atmospheric attenuation increases with the decrease of’ temperature. We note that. because we do not know exactly what the losses due to beam spreading are. the reported values of CYI could not be absolute values.