terahertz sensors versus optical sensors. Figure 2-2 shows the diameter of an antenna required to achieve the resolution of the human eye versus frequency.
The atmosphere attenuates millimeter-wave radiation at frequencies determined by molecular absorption by water vapor, oxygen, and other atmospheric molecules. The atmospheric attenuation characteristics must be accounted for in any system design.
Figure 2-3 shows the atmospheric attenuation under various environmental conditions from 10 GHz to 10,000 GHz. The conditions are typical for what may be experienced outdoors in various locations. The “clear” condition, which represents the U.S. standard atmosphere, is typical for a climate like that of the mid-Atlantic states in the springtime, while the curve labeled “humid” represents what would be expected in the same region in August (hazy, hot, and humid) when the atmosphere may contain up to five times the water vapor contained by the standard atmosphere. This is an example of a worst-case condition. The data labeled “dust” are predicted on the basis of a dust model3 that has been validated at 10 GHz and represent the attenuation from a storm that has a visibility of 10 m.
The minima in Figure 2-3 clearly show the atmospheric windows that are used to define the normal frequencies of operation for these systems. While systems tend to operate around specific frequencies, both for historical reasons and because of requirements of the Federal Communications Commission, the minima where the attenuations are lower are somewhat broad. In the millimeter-wave region they are typically 26 to 40 GHz, 70 to 110 GHz, 140 GHz, and 220 GHz. In the submillimeter-