Report from
BELL LABORATORIES
A New Use for the Sun
Radio frequencies in the range of 10 to 40 GHz are of interest for satellite communications. But signals at these frequencies—including millimeter wavelengths at 30 to 40 GHz—are attenuated by rain, snow, fog, and other weather conditions. To study these effects, we needed a source of millimeter waves in the sky. What could we use? Putting up a research satellite would be difficult and expensive. But fortunately a millimeter-wave transmitter in the sky already exists ...the sun. By tracking the sun and
correlating the signal losses with atmospheric effects, we are gaining valuable insight into the problems and possibilities of maintaining reliable communications at millimeter-wave frequencies in all kinds of weather. Our apparatus includes a steerable, plane metal mirror that reflects solar energy into a stationary horn-reflector antenna (photo). The signals are processed and recorded in a temperature-controlled equipment house. The system automatically follows the sun on its daily
SUNSET
SUNRISE
30 GHZ ^ CLEAR WEATHER SIGNAL LEVEL TIME (HOURS)
I
U
-I 16 GHZ
- S U N CLEARS TREES
Intensity of solar radiation at two frequencies, recently observed at Bell Laboratories' installation at Holmdel, N.J. Graphs show one 9-hour period out of months of study. Note attenuation due to rain early in the day.
Bell Laboratories' station for studying atmospheric effects on solar radio energy. The flat mirror has two axes. One is parallel to the earth's polar axis—for daily rotation. The other is perpendicular—for seasonal variation. Experiment designer R. W. Wilson checks the horn antenna into which signals are reflected.
path across the sky by a clock mechanism which can be set for a full week's automatic observation. The sun emits radio signals at a great many frequencies, but the sun-tracker is tuned only to signals at 16 and 30 GHz. The received energy has two significant components: one due to the sun and one due to the atmosphere (which attenuates solar energy but also radiates energy of its own). To allow for the atmospheric component, we tilt our mirror away from the sun once each second, thus getting a sky-only reading. We subtract this from the sun/sky total and plot the difference. The equipment responds to and records signal changes as rapid as 30 dB in 15 seconds. The graphs (left) indicate that there are periods when rain will attenuate the received signal by 30 to 40 dB. Such attenuation would seriously impair communications signals from satellites. The problem can be solved by spacing several ground t e r m i n a l s far enough apart that at least one of them would always have a clear path to the satellite. This would be done at both receiving and transmitting terminals. Data from the sun-tracking experiments will help us determine how many t e r m i n a l s might be needed for communication systems operating at high frequencies, and how these terminals might be spaced to achieve maximum efficiency and economy.
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10
C&EN
Research and Development Unit of the Bell System
