. "2 Basic Operation of Systems and Phenomenology." Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons. Washington, DC: The National Academies Press, 2007.
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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons
Resolution Versus Antenna Size
Another issue facing the application of RF techniques is that of resolution versus antenna size. When imaging in the far field of a circular aperture, the resolution R, in angle can be approximated by:
where R is the angular resolution in radians, c is the speed of light, F is the frequency of operation in hertz, and D is the diameter of the aperture of the imaging system.
Since resolution is inversely proportional to the frequency of the radiation, an imaging system would be desirable at as high a frequency as possible, given limitations in components and atmospheric propagation. Figure 2-1 shows the change in resolution across frequency for an imaging system with a 2 meter (m) diameter antenna or optical system. This linear curve shows the resolution for a passive imaging system. Passive system images are lower resolution because they are incoherent and have poor signal to noise at moderate scanning rates. Resolution can be increased by a factor of two by using an illuminator transmitting through the same aperture and thus focusing the transmitted resolution on the spot being imaged, a so-called confocal system.
FIGURE 2-1 Antenna resolution for an imaging system with a 2 meter aperture.
While a 2 m diameter antenna seems inordinately large, to achieve “eyeball” resolution—about 1 foot at 1 kilometer—an antenna at 100 GHz would have to be approximately 14 m in diameter. While it should be understood that eyeball resolution is not necessary for the detection of concealed objects or the identification of explosives, it is a convenient metric for examining relative aperture sizes of millimeter-wavelength/