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.
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/