Handout (114.7 kB)
Millimeter-wavelength (MMW) cloud Radars operating at W-band (95 GHz) and Ka-band (35 GHz) are popular atmospheric research tools because they are compact, have low prime power requirements and are highly sensitive to small hydrometeors. Fast ferrite MMW transmit/receive switches also allow measurement of targets at typical minimum altitudes of 100 to 200 m. In order to maximize sensitivity, ground-based systems use large diameter high-gain antennas. However, these antennas have substantial far-field distances that are much larger than the minimum altitude for such systems. The far-field distance is defined as rf=2 D2/l where D is antenna diameter and l is radar wavelength. The region r > rf is referred to as the far-field while the region r < rf is referred to as the near-field. The far-field is characterized by constant gain and by constant antenna pattern shape. Both of these characteristics vary with distance in the near-field.
Classic meteorological forms of the radar equation assume far-field antenna characteristics. A correction factor is needed to modify this relationship to correct for the antenna near-field response. A correction factor is derived from a numerical model of an aperture antenna. Results are parameterized as a function of r/rf and are independent of other radar characteristics. While the focus of this work is MMW systems the correction is applicable to any radar system using a dish or lens antenna.
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